John Sayers' Recording Studio Design Forum Archive

There really should be (or at least it would be nice if there was) a thread where people can ask questions about studio treatments, especially some from this forum

So my question is: What happens if you take a slat resonator (like this one in the attached pic) and instead of leaving a cavity, you fill the sucker up with insulation. Will it extend to lower frequencies?

There really should be (or at least it would be nice if there was) a thread where people can ask questions about studio treatments, especially some from this forum

That's sort of related to design, so it's fine to ask questions about that here.

So my question is: What happens if you take a slat resonator (like this one in the attached pic) and instead of leaving a cavity, you fill the sucker up with insulation. Will it extend to lower frequencies?

That would actually kill the device completely! It would not work at all, because there would be no resonance in the cavity: it would be completely damped, 100%. No resonance = no effect.

That's a great question, actually, since it's a common issue. On other forums I have seen people trying to do exactly what you say: stuffing the entire cavity with insulation, then not getting any Helmholtz effect. They still get some pure absorption, yes, and some light diffusion maybe from the slats, but no resonance.

You actually don't need much insulation at all behind the slats to get good absorption. 1" is usually plenty. There's even a noticeable effect from just having fabric across the slats. you can have a little bit of insulation at the rear of the cavity, to damp the resonance slightly. That will have the effect of lowering the Q of the device: make it work over a broader range of frequencies, but with less total effect (lower coefficient of absorption at the peak).

Here's a thread where I designed a Helmholtz device for the rear corner, and you can see how it was done, and what the results were.

http://www.johnlsayers.com/phpBB2/viewt ... =2&t=21368


- Stuart -

Soundman2020 wrote:On other forums I have seen people trying to do exactly what you say: stuffing the entire cavity with insulation, then not getting any Helmholtz effect. They still get some pure absorption, yes, and some light diffusion maybe from the slats, but no resonance.

hmm Interesting. So what benefit does the resonance provide that cannot be achieved by not having the cavity at all since absorption is what we are after anyway isn't it? The resonance in the slat resonator is to absorb a frequency/range of frequencies so if filling it absorbs too, I can't think of what the difference is.

So what benefit does the resonance provide that cannot be achieved by not having the cavity at all since absorption is what we are after anyway isn't it?

Not necessarily. Absorption is only the goal if that's what the room needs! And even then, you would only want to use absorption that specifically deals with the problem, while NOT dealing with other "problems" that don't exist in your room

The resonance in the slat resonator is to absorb a frequency/range of frequencies so if filling it absorbs too, I can't think of what the difference is.

You answered your own question: The difference is FREQUENCY.

A Helmholtz resonator is a TUNED device, that is designed to absorb one specific frequency very aggressively, with maximum effect, while ignoring other frequencies. You could, for example, tune it to the frequency of a very stubborn modal problem, as I did in that thread I linked you to. Not sure if you followed that, and looked at the graphs. I think they show, very convincingly, what a tuned resonator can achieve.

On the other hand, if you just have a bunch of insulation in a box, then by definition it is NOT tuned: it remains broadband. It absorbs all frequencies pretty much equally, across a very wide range, even the ones that you DON'T want absorbed. If you then put slats across the front of that box (ie,. make it into a slot wall that has no resonance), all you achieve is a large reduction in efficiency: it still absorbs broadly, but at a much lower level, and the absorption range is skewed toward the low end... but WITHOUT being tuned. In some cases, that might be exactly what you need for a room, but it would not be a true slot wall, since it would not be carefully tuned.

A slot wall works on the principle of Helmholtz resonance, which you can demonstrate to yourself by blowing across the top of an empty glass bottle, such as beer bottle of glass coke bottle. The "slug" of air trapped in the neck of the bottle acts like a single mass, and the air trapped inside the main body of the bottle acts like a spring. The "mass" bounces up and down on the "spring" at a very tightly controlled frequency (high Q) that is determined by the dimensions of the device. That "bouncing mass" produces a single tone that rings out loud and clear as you blow over the bottle top. The tone is fairly pure: a single note, and it is rather loud, considering how small the neck of the bottle is. It is a very efficient resonator. In a slot wall, each gap between a pair of slats acts in exactly the same way: it is tuned to a specific frequency, and will resonate sympathetically whenever it "hears" the note that it is tuned to.

If you take any type of Helmholtz resonator and put an acoustic damper on the slug of air (not the cavity!), then the device now becomes an ABSORBER of that specific frequency: as the slug moves in and out of the device, it's motion is greatly damped (very much like the shock absorber works on your car's suspension), and that removes energy from the system at that specific frequency. But this ONLY works if you put the damper on the slug of air! And ONLY if you do not over-damp it. That leaves the resonant cavity in tact, so the device is still tuned to it's correct frequency, and the cavity still acts as a spring for the slug, so resonance still happens. But the motion of the slug is damped, and that's the trick. If you put a tiny wisp of cotton wool inside the neck of your beer bottle, it will no longer play the note as you blow over the top, or bets case you will hear a very soft, muffled note. It will now ABSORB that note, if you happen to play it on some instrument nearby.

Helmholtz resonators are useful devices in the acoustic designer's toolbox, and can be used very successfully to treat certain problematic notes, without also treating other notes that don't need treating. However, they are hard to tune, they must be placed at the location in the room where there is a pressure node for that specific problem, and each device can only treat one single frequency, or at best a very small number of frequencies. So they do have their uses. They can also be "de-tuned" in several different ways, to cover a broader but still tight and carefully controlled range of frequencies, unlike broadband absorption which treats very wide chunks of the spectrum all at once.

I guess the real question here is: why are you looking at Helmholtz resonators? What problem do you have in your room (or project having in the room, if it hasn't been built yet) that you think you'd need a Helmholtz resonator to solve? These are precision tools that are usually only used to deal with very specific issues: if you don't have that issue on your room, then you don't need a Helmholtz resonator to solve it. Arrays of Helmholtz resonators, such as slot walls and perforated panels, can be used to treat other issues, and that's more common, but you still need to know what specific problems you have, in order to tune the device correctly. What issues do you have in your room, or are concerned that you might have, where you think that a slot wall will be the correct solution?

- Stuart -

Soundman2020 wrote:A Helmholtz resonator is a TUNED device, that is designed to absorb one specific frequency very aggressively, with maximum effect, while ignoring other frequencies. You could, for example, tune it to the frequency of a very stubborn modal problem, as I did in that thread I linked you to. Not sure if you followed that, and looked at the graphs. I think they show, very convincingly, what a tuned resonator can achieve.

It isn't always a tuned device though. Over on this page http://johnlsayers.com/Recmanual/Titles/Acoustics2.htm there is an angled ceiling slat resonator. It says: Because the distance from the front to the back is varying from 300mm to 100mm or around 12 degrees, the wall becomes a broadband absorber. This is when I got confused because a broadband fiber absorber with slats also achieves this so what is the difference?

By the way, I found that thread a few times when searching for info on slat resonators so yes i have read it. Thank you!

Soundman2020 wrote:I guess the real question here is: why are you looking at Helmholtz resonators? What problem do you have in your room (or project having in the room, if it hasn't been built yet) that you think you'd need a Helmholtz resonator to solve? These are precision tools that are usually only used to deal with very specific issues: if you don't have that issue on your room, then you don't need a Helmholtz resonator to solve it. Arrays of Helmholtz resonators, such as slot walls and perforated panels, can be used to treat other issues, and that's more common, but you still need to know what specific problems you have, in order to tune the device correctly. What issues do you have in your room, or are concerned that you might have, where you think that a slot wall will be the correct solution?

- Stuart -

I was recommended using broadband absorption covered by slats in my live room kind of like in this room http://thebunkerstudio.com/studio-b/ studio so was just trying to understand what the difference between the two. I do understand that tuned devices are used to solve a specific frequency problem so I might need them after testing but for now just learning about them trying to understand when each of these is used. It also says on that page:

As you can see a slat wall like this can break up parallel walls thus stopping standing waves

and since I will have parallel walls, might be good to use these devices

Hi Stuart. Seems like a lot of people are going to disagree with you about what you said about the slat resonator. Apparently, a slat resonator filled with fiber DOES function as a slat resonator even though it is full of fiber so long as the fiber is light fiber. I just thought you (and other readers on this site) might be interested in a couple of other threads on the topic here.

https://www.gearslutz.com/board/studio- ... st13253663

A Helmholtz resonator is a TUNED device, that is designed to absorb one specific frequency very aggressively, with maximum effect, ...

It isn't always a tuned device though.

Yes it is. Sorry, but I think you need to do some research on Helmholtz resonators and how they work. It only stops being tuned if you specifically do something to it that forces it to loose its tight tuning.... such as filling it with porous absorption....

Because the distance from the front to the back is varying from 300mm to 100mm or around 12 degrees, the wall becomes a broadband absorber.

Right. Read it again, carefully, and do the math. Try to understand things properly before commenting on them. If you work your way through the math, you will see that one of the parameters that defines the resonant frequency of a Helmholtz resonator is the depth of the air cavity behind it. Deeper cavty = lower frequency, as you can see from the equation itself:

fo = 2160*sqrt(r/((d*1.2*D)*(r+w)))

r = slot width
d = slat thickness
1.2 = mouth correction
D = cavity depth,
w = slat width
2160 = c/(2*PI) but rounded
c = speed of sound in inch/sec.

Take a close look. The parameter "D" is the DEPTH of the cavity. Work through it: that parameter is BELOW the division line, therefore INCREASING the depth causes a DECREASE in frequency.

Now think about that for a second: If you have a slot resonator where the depth varies across the slot, so that one side of the cavity is deeper than the other side, then obviously, the resonant frequency for the side with the shallowest depth will be higher than for the side with the deeper depth. ie, what John said is perfectly correct: it is now to tuned to a broader RANGE of frequencies, rather than one single frequency, which would be the case if the depth was constant. Hence, it is now a tuned broad-band absorber: It is TUNED to a RANGE of frequencies.

If you have several such slot resonators arranged vertically up a wall, each tuned to a different RANGE, then you can cover an even broader range of frequencies, .... but the device will still be TUNED, since you can control the range that it will absorb by adjusting the slot width, slot depth and cavity depth. Thinner slots = lower frequency, thicker slat = lower frequency, greater depth = lower frequency.

This is when I got confused because a broadband fiber absorber with slats also achieves this so what is the difference?

Careful there! A broadband absorber with slats is NOT necessarily a slot resonator! Sometimes we studio designers add wood slats in front of pure absorption simple to return some of the high-end to the room, so it won't all be absorbed by the insulation behind. But that does not make the device into a tuned slot resonator. To do that, you must have a sealed cavity behind the slats. If the cavity is NOT sealed, then it is not a resonant cavity, so there is no Helmholtz effect that could be tuned. The DEVICE can still be tuned, to a certain extent, by varying the slat width (narrow slats only reflect high frequencies, broad slats reflect lower frequencies: the broader it is, the lower it goes), but it is NOT tuned by resonance. Different effect. Differ tool. Different uses. You can also de-tune a slot wall that DOES have a sealed cavity, simply by making the slots so wide that no resonance can happen, or by using a depth that is outside the range where the Helmholtz effect occurs efficiently, ... or by fully stuffing the cavity with insulation, which kills the resonance.

I was recommended using broadband absorption covered by slats in my live room kind of like in this room

Take a close look at those walls: That is very not a tuned slat wall. Even assuming that the cavity behind is sealed (no way of knowing that from the photo) the slots appear to be too wide to be creating a useful Helmholtz effect. So that very probably is not a tuned slat wall at all, and therefore the diagram you showed above does NOT apply, and neither does the equation.

Frequently it is impossible to draw any valid conclusions about how a room is treated by just looking at what you see on the surface. Unless you know what is BEHIND that surface, and HOW it was built, you can't say much about it. Very different treatment devices can look identical in the surface, especially to someone with little background in acoustics. This is the reason you often see home studios with carpet on the walls: because the person who did that once went to a movie theater, saw carpet on the walls there and noticed that the room acoustics were great, so they figured that they must also need carpet on their wall.... then they can't understand whey their room is so boomy, muddy, and unpleasant! What they did NOT know is that the "carpet on the wall" in the movie house was NOT the acoustic treatment! It was merely aesthetic, acoustically transparent, and covering the REAL treatment that was hidden behind the carpet...

Same here: wood slats over fabric doesn't tell you anything at all about what that device is, or what principle it is working on. The only clue that suggests it isn't Helmholtz based, is the large gap between the slats. If I had to guess, I'd say that it is simply slats over insulation, not tuned resonantly. Some of them might be tuned: the ones that are closer together. But there's no way to be sure of that, unless you know what is inside the wall, how it was built, and WHY it was built that way.

Which gets back to the question: Why was that recommended for YOUR studio? What issues do you expect to have in your studio where the best solution is slats over insulation? There must be a reason for that recommendation: Some type of acoustic analysis? Somebody did some calculations, based on the room dimensions and your goals for the room? Recommendations like that based on nothing but wild guesses would be highly questionable....

I do understand that tuned devices are used to solve a specific frequency problem

Not necessarily! Did you take a look at the link I gave you, were we are in the process of tuning right now? Did you notice that I used one single tuned device to solve SEVERAL problems at once, some of which are frequency specific, while others are not? Did you follow the discussions and explanations there? Did you see the slat wall that he is building today, right now, as we speak, and testing at each point? Can you look at the photos of that wall and tell me what it is doing, just from the photos and without asking me why I designed it like that? Can you tell which parts of that wall are tuned Helmholtz devices, which are tuned only by slat size not be resonance, and which are untuned? Can you tell me what frequency range that wall will deal with, both as a complete unit (the entire wall all at once), and also the individual parts of it? There are MULTIPLE things going on at once with that device. It is tuned to many frequencies at once. It is partly tuned broadband, and partly tuned specifically. It deals with velocity based problems, and also with pressure based problems. It deals with phase issues, frequency issues, and time issues all at once. But there's no way you could know that just by looking at the face. The only way you'd know it is if I told you how I designed each part of it, and why....

As you can see a slat wall like this can break up parallel walls thus stopping standing waves

Well, that's just pure garbage! Absolute trash. It's one of the most persistent myths that continue to propagate around the internet. To start with, the walls of the room are STILL parallel, regardless of what you put in front of them! People don't seem to get this, but it's very simple. Room modes (standing waves) are low frequency problems (they still occur at higher frequencies, but those are NOT a problem, for various reasons. In fact, they are a good thing....). Low frequency = very long wavelength. One of the basic concepts of acoustics is that any object can only reflect waves that are smaller than its own dimensions, and can only defract waves that are similar in size to its dimensions. So a thin wood slat, just a few inches wide, cannot by itself reflect a wave that is dozens of FEET wide. Go find a nice stretch of beach, where there's plenty of good waves rolling in. Now go stand in the water, where the waves are. Does your body have any effect at all on the waves? Nope! Nada. Zip. Nothing. Your body is way too small, since it is just a couple of feet wide, and the waves are hundreds of feet long. Now go get an ocean liner, and park it across the beach, broadside on. Does it have an effect on the waves? You bet it does! Downstream from the ship, the wave action is hugely reduced. Because the ship is similar in size to the wavelength. Take it one step further: take a walk down the beach to a protected marina, where's there a stone wall a mile long around the yachts. Do any waves get into the marina? Nope! None at all. Because that stone wall is much longer than then longest wavelength.

This is simple to understand, and simple to demonstrate by this type of experiment. Yet for some reason, the myth persists.

In reality, the low frequency standing waves (room modes) will STILL be there in that room, regardless of the slats (I'm assuming that the slats are not tuned in this case, since you said it was a broadband absorber, not a low frequency tuned Helmholtz absorber, and the slots are too wide to be tuned for deep bass, even if it is resonant). That room will still have all of its original modes. Some of them might be damped in one way or another, but the actual modal response is defined by the walls of the room, not by the slats in front of them. Room modes are always calculated to the hard, solid, rigid, dense boundary walls of the room. The modal response can be changed by treatment, yes, and it can be damped by treatment, yes, but wood slats will not "stopping standing waves".

Even more curious, it's a really BAD idea to "stop standing waves" anyway! What a lot of people don't realize is that the problem with small rooms is NOT that they have too many standing waves (room modes), but the exact opposite: they don't have enough! I could go into the explanation for that if you want, but if you think about it, it's obvious. The very way we try to deal with room modes shows that! We do NOT try to get rid of them, since we NEED them! Rather, we try to control them, by spreading around evenly the very few that we have. To understand this better, you should do some research on the Schroeder frequency of a room, why it is important to have that as low as possible, and why "getting rid of room modes" would make it go higher, not lower...

and since I will have parallel walls, might be good to use these devices

This room has parallel walls, yet it does not have those devices: Since it does not have those wood slats on the walls, I guess it must sound awful, right? No professional musician would want to rehearse or record an album in there! Obviously! Parallel walls, no wood slats, just a few flexible absorbers hanging every now and then on the walls, .... disgusting acoustics, surely! Except that the room in the photo is the main live room at Abbey Road Studios, arguably one of the best rooms on the planet! Pretty much every musician anywhere would die to have the opportunity to record in there! Yet, there are no wood slats, and the walls are parallel...

Seems like a lot of people are going to disagree with you about what you said about the slat resonator.

Only those who don't understand them, and have never built them...

Apparently, a slat resonator filled with fiber DOES function as a slat resonator even though it is full of fiber so long as the fiber is light fiber.

True.

From that thread, Jason gave you the correct answer in his first two responses: "Slot resonators are a type of helmholtz. Pressure based traps that are tuned to specific frequencies. They need to be sealed. - Slatted broadband panels are broadband absorbers that reflect highs to keep the room ffom being too dead " Yup. Spot on. I don't see how that conflicts with what I have been telling here.

He then adds "They are very efficient, but very specific to tunings and placement. They are like icing on the cake when you know your problem frequencies ". Yup. Very true.

You then post two images over there, with the comment "Ok but look at these two. They are both sealed Helmholtz devices? So why use one over the other since they are both supposed to be tuned to specific frequencies. " No, that's not true. Only ONE of those is a sealed Helmholtz device. They are both sealed, and both tuned, but only one is Helmholtz.

Dan Dan then says much the same thing, and admits that he's not sure about how perforated tuned devices work. Honest man.

Jens then says the same thing: "The Q will naturally be much lower than less fill,". Yup. He then follows up later with the following advice: " make sure to place the wool close to the openings (not at the boundary)." Very true!

The problem with Jens is that he knows his stuff, but gives very short, abbreviated answers that don't always go into all the necessary detail. For example, at one point he says "Perforation percentage is what matters most. " Very true! But he doesn't explain WHY it is so important, or WHAT happens with low open area percentages, medium open percentages, or high open percentages....

Jason then gave you a link to one of the best resources for figuring out performance of tuned slot devices, but you didn't like it, and said it was wrong... "Thanks I tried that but it seems wrong. Look here:" Apparently, you didn't like it because it asks you for both "thickness" and "width"? I'm not sure why you would assume it was "wrong" from that!!! How else would it be able to calculate the tuning, if you don't give it both the slat thickness and also the width? It would only be "wrong" if it did NOT ask you for both of those. I use that tool regularly, and I know it does give valid results, because that's what I used to tune his devices, and they ARE working exactly as shown by that tool. I went ahead and plugged in some values, to show you that it does work:
Exact same parameters in both cases: 25mm thick slats, 80mm wide, 8mm slots, over 225 mm sealed air cavity. The ONLY difference is that for the blue curve, there's very thin layer of absorption, just 5mm thick, directly behind the slats, while for the blue curve, it is filled with the same absorption (GFR similar to OC-705). So which of those would you rather use to treat a modal issue at 180 Hz? Would you prefer the high Q blue curve, that is fairly sharply tuned to that frequency, and has a coefficient of absorption of 1.0? Or would you prefer the low Q green curve that absorbs everything from 50 Hz to about 500 Hz, with a coefficient of absorption of around 0.6?

The calculator is not wrong, as you claim: it works perfectly.

In your next post over there, you arrive at the wrong conclusion, and lament that you can't understand the answers to the question:

that is a pity, because then the OP question remains unanswered because we only have data on a slatted panel full of fiber and not a "slat resonator" which only has one sheet of fiber glued to the back of the slats so are not able to compare the two.

However, that is NOT because there is no answer, as you assume, but because you are not understand the underlying acoustic principles! Once you get your head around that, then it will all become clear.

As Jens said in that thread: "GFR and density (density to a larger degree in terms of fc) will affect the Q and fc of a Helmholtz resonator." Very true. I'm not sure why you think I said something different!

I have read through that entire thread, and did not come across the part where you say " a lot of people are ... [disagreeing]" with me! All I see is some people who have a good background in acoustics (Jason, Dan Dan, Jens) actually AGREEING with what I have been saying!

The problem is that you are not yet understanding what Helmholtz resonance is, and you are confusing devices that are based on the principle of Helmholtz resonance with devices that are NOT based on the Helmholtz resonance, but are still tuned. A panel trap is not based on Helmholtz resonance, but you think it is. A membrane trap is not based on Helmholtz resonance, but you think it is. A porous absorber with slats over it is not necessarily based on Helmholtz resonance, although it could be if it is built right, yet you think that is is ALWAYS a Helmholtz device.

I'd suggest that you you do some research on Helmholtz resonance to understand exactly what it is, which devices uses it, which don't, how to tuned them, and what results you can expect. Devices that CAN be Helmholtz devices include slot walls and perforated panel, as well as simple sealed cavities with a tuned port, but they don't HAVE to be tuned Helmholtz devices, unless they have been specifically designed and built that way. Even speakers with bass reflex ports are Helmholtz devices. But dipole speakers and sealed speakers are not. After you have learned how Helmholtz resonance actually works, you'll be able to understand the difference.

Apparently, a slat resonator filled with fiber DOES function as a slat resonator even though it is full of fiber so long as the fiber is light fiber.

No it does not! If you fill it with insulation, then it stops functioning as a slat resonator, and starts functioning as a tuned broadband absorber! See the graph above...

I just thought you (and other readers on this site) might be interested in a couple of other threads on the topic here.

I'm always interested in learning new stuff that I don't already know, but that thread was not enlightening at all. Nothing new, and certainly nothing earth-shattering, as you seem to think. Nobody there is disagreeing with what I said, as you claimed.

I'd also suggest that you read the Bible! Not THE bible (although reading that is very recommendable too), but rather the "bible of acoustics", also known as "Master Handbook of Acoustics" by F. Alton Everest That will give you the background in acoustics that you need to be able to design a studio. It explains all of the above concepts and many others too.

- Stuart -

Soundman2020 wrote:Yes it is. Sorry, but I think you need to do some research on Helmholtz resonators and how they work.

That is what I am trying to do and that is why I posted this because on the one hand you are saying that a Helmholtz Resonator is tuned, but then on this same site it is said to be either tuned or broadband (when angled). You said in your earlier post that filling a slatted trap with fiber also makes it more broadband so naturally I was interested in the difference between the two.

Soundman2020 wrote: It only stops being tuned if you specifically do something to it that forces it to loose its tight tuning.... such as filling it with porous absorption....

Then why do these results look so similar? Also in the other thread on GS I posted to it has been said several times that a Helmholtz resonator is still a Helmholtz resonator when filled completely with fiber

Soundman2020 wrote:Right. Read it again, carefully, and do the math. Try to understand things properly before commenting on them. If you work your way through the math, you will see that one of the parameters that defines the resonant frequency of a Helmholtz resonator is the depth of the air cavity behind it. Deeper cavty = lower frequency, as you can see from the equation itself:

fo = 2160*sqrt(r/((d*1.2*D)*(r+w)))

r = slot width
d = slat thickness
1.2 = mouth correction
D = cavity depth,
w = slat width
2160 = c/(2*PI) but rounded
c = speed of sound in inch/sec.

Take a close look. The parameter "D" is the DEPTH of the cavity. Work through it: that parameter is BELOW the division line, therefore INCREASING the depth causes a DECREASE in frequency.

Now think about that for a second: If you have a slot resonator where the depth varies across the slot, so that one side of the cavity is deeper than the other side, then obviously, the resonant frequency for the side with the shallowest depth will be higher than for the side with the deeper depth. ie, what John said is perfectly correct: it is now to tuned to a broader RANGE of frequencies, rather than one single frequency, which would be the case if the depth was constant. Hence, it is now a tuned broad-band absorber: It is TUNED to a RANGE of frequencies.

If you have several such slot resonators arranged vertically up a wall, each tuned to a different RANGE, then you can cover an even broader range of frequencies, .... but the device will still be TUNED, since you can control the range that it will absorb by adjusting the slot width, slot depth and cavity depth. Thinner slots = lower frequency, thicker slat = lower frequency, greater depth = lower frequency.

You say read carefully before posting and it sounds condescending/presumptuous like I have not read it or have not understood it. I can assure you that I have read that page 20 times and for long enough to come up with the question I have asked you that apparently have not been asked before. Don't know why you think I woulkd ask these questions without reading it. I am trying to question you to be able to learn from you Stuart not to annoy you. I know you know much more than I do but I will put up my hand if something is not clear. Hope that is ok.
So the slat wall that is ANGLED is broadband in nature because each slat is a different height from the back of the panel and the non-angled broadband absorber with slats on the front is broadband because it is fill of fiber. Did I understand you?

Soundman2020 wrote:Sometimes we studio designers add wood slats in front of pure absorption simple to return some of the high-end to the room

Exactly! Isn't this also the idea with an angled slat resonator?
From this website:
If the gaps vary say 5mm, 10mm, 15mm,20mm and the wall is angled as shown below, a broad band low mid absorber is created that still keeps the the high frequencies alive.

Soundman2020 wrote:Take a close look at those walls: That is very not a tuned slat wall. Even assuming that the cavity behind is sealed (no way of knowing that from the photo) the slots appear to be too wide to be creating a useful Helmholtz effect. So that very probably is not a tuned slat wall at all, and therefore the diagram you showed above does NOT apply, and neither does the equation.

Frequently it is impossible to draw any valid conclusions about how a room is treated by just looking at what you see on the surface. Unless you know what is BEHIND that surface, and HOW it was built, you can't say much about it. Very different treatment devices can look identical in the surface, especially to someone with little background in acoustics. This is the reason you often see home studios with carpet on the walls: because the person who did that once went to a movie theater, saw carpet on the walls there and noticed that the room acoustics were great, so they figured that they must also need carpet on their wall.... then they can't understand whey their room is so boomy, muddy, and unpleasant! What they did NOT know is that the "carpet on the wall" in the movie house was NOT the acoustic treatment! It was merely aesthetic, acoustically transparent, and covering the REAL treatment that was hidden behind the carpet...

Same here: wood slats over fabric doesn't tell you anything at all about what that device is, or what principle it is working on. The only clue that suggests it isn't Helmholtz based, is the large gap between the slats. If I had to guess, I'd say that it is simply slats over insulation, not tuned resonantly. Some of them might be tuned: the ones that are closer together. But there's no way to be sure of that, unless you know what is inside the wall, how it was built, and WHY it was built that way.

I agree, I never said it was a slat resonator. I always thought it to be an absorptive wall with slats on the front but I did need to understand what the difference is between using this or an angled slat resonator wall would be. That is the very purpose of this thread.

Soundman2020 wrote:Which gets back to the question: Why was that recommended for YOUR studio? What issues do you expect to have in your studio where the best solution is slats over insulation? There must be a reason for that recommendation: Some type of acoustic analysis? Somebody did some calculations, based on the room dimensions and your goals for the room? Recommendations like that based on nothing but wild guesses would be highly questionable....

To control low mids while diffusing high frequnecies. Isn't that desireable in most rooms or do we need to measure the room first and then decide to use it? I record a lot of guitar/cello/hand percussion and the low mids are where most of the energy is so I thought this device a good option for these instruments

Soundman2020 wrote:Not necessarily! Did you take a look at the link I gave you, where we are in the process of tuning right now? Did you notice that I used one single tuned device to solve SEVERAL problems at once, some of which are frequency specific, while others are not? Did you follow the discussions and explanations there? Did you see the slat wall that he is building today, right now, as we speak, and testing at each point? Can you look at the photos of that wall and tell me what it is doing, just from the photos and without asking me why I designed it like that? Can you tell which parts of that wall are tuned Helmholtz devices, which are tuned only by slat size not be resonance, and which are untuned? Can you tell me what frequency range that wall will deal with, both as a complete unit (the entire wall all at once), and also the individual parts of it? There are MULTIPLE things going on at once with that device. It is tuned to many frequencies at once. It is partly tuned broadband, and partly tuned specifically. It deals with velocity based problems, and also with pressure based problems. It deals with phase issues, frequency issues, and time issues all at once. But there's no way you could know that just by looking at the face. The only way you'd know it is if I told you how I designed each part of it, and why....

I never read the latest on that thread but if it is relevant to this then I will. I also never claimed to be able to tell you what something is by looking but we both do agree that Studio B as above is probably thick absorptive walls with slats.

Soundman2020 wrote:

As you can see a slat wall like this can break up parallel walls thus stopping standing waves

Well, that's just pure garbage! Absolute trash. It's one of the most persistent myths that continue to propagate around the internet.

Well then get it off your site! You can't blame someone for asking

Soundman2020 wrote:

and since I will have parallel walls, might be good to use these devices

This room has parallel walls, yet it does not have those devices:

abbey-road-JJ_02.jpg

Yeah but it is huge Not the same as a small room when the walls are so close to each other and so close to a condenser microphone.

Soundman2020 wrote:
I have read through that entire thread, and did not come across the part where you say " a lot of people are ... [disagreeing]" with me! All I see is some people who have a good background in acoustics (Jason, Dan Dan, Jens) actually AGREEING with what I have been saying!

I don't think you read the whole thing. Over and over in that thread the testing, the results and the conversation circles around a sealed helmholtz device that is full of fiber. Something which you claim can not exist. You said that if the box is fill of fiber then it is not a helmholtz resonator.

from post 39

A "slatted panel full of fiber" is still a "slat resonator" (helmholtz array). The amount and type of wool will affect the performance in different ways but as you see in the graphs from the other thread; you still get a helmholtz effect even if filled with fiber (assuming not to dense, or to high GFR).

Also, The acoustic modelling calculator doesn't agree with what you said as I already posted above. You said there is no resonance when the panel is full of fiber yet here it appears pretty much the same.

thanks for your response!

I'm no expert, but I'll try and help where I can here. I apologize if I give any wrong info.

That is what I am trying to do and that is why I posted this because on the one hand you are saying that a Helmholtz Resonator is tuned, but then on this same site it is said to be either tuned or broadband (when angled).

You still don't seem to understand Helmholtz 100%. Hopefully this horrible explanation will help you understand it in such a way that all the dots connect for you. Okay, so just know that even though a Helmholtz device works as a single unit (like a single bottle does, simply because it is one sealed cavity), it still works as individual slices or slugs. So a rudimentary way of looking at it would be like this. So your wall device has let's say 2 slots in it (between strips of wood on it's face). The device is angled on the wall. Slot #1 is 12" away from the wall. The sound goes through that slot straight towards the wall and comes straight back (quieter at the frequency that the Helmholtz calculator said it would work at, at that 12" depth). The second slot is at 10". The sound goes straight in and straight out, quieter, at it's calculated frequency. So, again, even though it's one sealed cavity, the device works in slices.
So, when anyone is referring to a Helmholtz that is broadband, they are clearly stating that a single Helmholtz device has several different slat/slot widths/depths or varying distances from the wall. And the same goes for them using the term "de-tuned". "Tuned" in general means that it is band limited and/or working on a specific frequency (like if a Helmholtz device had only the same slat, slot, and depth components across its face. Basically, they're just widening the Q of the device as naturally, a Helmholtz device has a very narrow and sharp Q.

You said in your earlier post that filling a slatted trap with fiber also makes it more broadband so naturally I was interested in the difference between the two.

Adding a little bit of insulation to the device (it works best at it's mouth) does widen it's Q, ultimately making it more broadband. Think about this: Adding a tiny tiny bit of insulation widen's it's Q. Add more, it widen's it more. Add enough and eventually it's got an extremely wide Q factor. Now, it won't absorb ALL the frequencies because it does still have wood slats on the front of it. Now, what those are going to do is cause the device to deflect the higher frequencies (the width of the slats determines that frequency) back into the room, and anything below that frequency will pass into the insulation and get dampened.

Then why do these results look so similar?

I'm not sure about the parameters of this as it says the insulation is 14.29%. % of what?

Also in the other thread on GS I posted to it has been said several times that a Helmholtz resonator is still a Helmholtz resonator when filled completely with fiber

When the frame of the device is sealed to it's backing (a wall) and the only way for air to get in and out is through the slots, then that classifies it as being Helmholtz.. just like a glass pop bottle. No insulation, or 100% insulation, it's core design is still Helmholtz. Take away the Green Glue sealant and now there's another escape route for the air and it's no longer Helmholtz. It won't resonate anymore.

So the slat wall that is ANGLED is broadband in nature because each slat is a different height from the back of the panel and the non-angled broadband absorber with slats on the front is broadband because it is fill of fiber. Did I understand you?

I believe you're understanding that correctly.

Exactly! Isn't this also the idea with an angled slat resonator?
From this website:
If the gaps vary say 5mm, 10mm, 15mm,20mm and the wall is angled as shown below, a broad band low mid absorber is created that still keeps the the high frequencies alive.

The slats will return high end, yes, but in a resonator design, the slats also play their role in tuning the device.

To control low mids while diffusing high frequnecies. Isn't that desireable in most rooms or do we need to measure the room first and then decide to use it? I record a lot of guitar/cello/hand percussion and the low mids are where most of the energy is so I thought this device a good option for these instruments

"Control" in what way? Measuring is the most effective way to figure out what your room needs. Stuart is basically saying this stuff to make you think about why you're doing what you're doing. He's trying to teach you to only do something because the math or a measurement indicates that you need to do it, not just because your friend told you that you should or because you saw some fancy studio with stuff on their wall and you think you should do it too. His Abbey picture is proof that if your room sounds good, you don't need this and that device. That is all. Basically, get your room isolated and ventilated. Take acoustic measurements, analyze the results and deal with one problem at a time.

I don't think you read the whole thing. Over and over in that thread the testing, the results and the conversation circles around a sealed helmholtz device that is full of fiber. Something which you claim can not exist. You said that if the box is fill of fiber then it is not a helmholtz resonator.

The structural design is Helmholtz, but at some point, you add enough insulation, it negates the function of Helmholtz an therefore doesn't function as a Helmholtz anymore. So I suppose, then you can say that it is not a Helmholtz haha

Also, The acoustic modelling calculator doesn't agree with what you said as I already posted above. You said there is no resonance when the panel is full of fiber yet here it appears pretty much the same.

Again, I don't understand the parameters fully.

Hopefully I didn't confuse you more. It's hard to explain and heck, maybe I don't even understand it properly. I think I do. I hope you do now too.

Greg

Then why do these results look so similar? Also in the other thread on GS I posted to it has been said several times that a Helmholtz resonator is still a Helmholtz resonator when filled completely with fiber

What software is that?

Gregwor wrote:

Then why do these results look so similar? Also in the other thread on GS I posted to it has been said several times that a Helmholtz resonator is still a Helmholtz resonator when filled completely with fiber

What software is that?

http://www.acousticmodelling.com/multi.php

The parameters can be seen at the bottom of the image.

Gregwor wrote:I'm not sure about the parameters of this as it says the insulation is 14.29%. % of what?

the opening percentage.

Gregwor wrote:When the frame of the device is sealed to it's backing (a wall) and the only way for air to get in and out is through the slots, then that classifies it as being Helmholtz.. just like a glass pop bottle. No insulation, or 100% insulation, it's core design is still Helmholtz. Take away the Green Glue sealant and now there's another escape route for the air and it's no longer Helmholtz. It won't resonate anymore.

Ok but that is not what Stuart said to me.

Gregwor wrote:The slats will return high end, yes, but in a resonator design, the slats also play their role in tuning the device.

Ok got it. But with an absorber panel the slats also play their role in "tuning" to some extent because they are what define how the absorber will perform.

Gregwor wrote:Hopefully I didn't confuse you more. It's hard to explain and heck, maybe I don't even understand it properly. I think I do. I hope you do now too.

Nah, your post is great. I think you understand it so look forward to your responses too

on the one hand you are saying that a Helmholtz Resonator is tuned, but then on this same site it is said to be either tuned or broadband (when angled).

I don't see where there's a conflict here! It's very simple: it can be tuned tightly (high Q), which is the "traditional" and conventional way of doing it, or it can be tuned more broadly (low-Q), which is accomplished in any of several ways, one of which is to vary the depth of the cavity. There's no conflict in saying that Helmholtz devices can be high-Q (tightly tuned) or low-Q (loosely tuned), or even tuned to several frequencies at once (both broad-band and also high-Q). It's all a matter of what you need the device to do, and how you design it.

The big problem with high-Q devices (of all types, not just Helmholtz-based) is that is very difficult to tune them precisely. And since the very purpose of tightly tuned devices is to hit the exact frequency associated with a specific room problem), it's not such a good idea to go with very high Q devices, unless you are certain that it is tuned perfectly, or have some means of adjsuting the tuning after it is finished. Hence, in this room, I tuned the rear corner devices broadly in two different ways: by varying the slot widths AND cavity depths slightly, and also by adding some additional absorption on the rear face of the cavity and the floor of the cavirty, which deliberately de-tunes them (lowers the Q). That way, he did not need to build them with extreme precision and test / moidfy / test / modify / test / modify until they worked. Rather, they worked straight away, since they cover several frequencies (differing slot widths, differing cavity depths), AND ALSO have had their Q lowered a bit, due to the interior damping.

So, that device that you see there (and see the results of, in the graphs) is indeed a Helmholtz resonator that is both tuned and also broad-band: it covers a range of frequencies, that I specifically choose as they were problematic in the room. I tuned it to that range. So it is "tuned" and "broadband". The two terms are not mutually exclusive. You can tune an absorption device tightly, or broad-band, in much the same way that you can tune a diffuser to be tight or broad-band.

You said in your earlier post that filling a slatted trap with fiber also makes it more broadband so naturally I was interested in the difference between the two

You seem to be confusing two different types of acoustic device here. As I mentioned before, two devices can APPEAR identical at first glance, but actually have very different functions, and be based on very different acoustic principles. A true "slat wall" or "slot wall" is one thing, but a slatted device filled with porous absorption is another, very different thing. They might look the same from the front, but work very differently internally. Once again, take a look at the rear corner treatment in the room:
Here's another view:
See those three identical devices in the rear corner? One at the top, one in the middle, and one at the bottom? They all look the same, so they must all act the same, right? Except that they don't. The top and bottom ones are the same, and act the same, but the one in the middle is very different. The top and bottom ones really are Helmholtz devices, but the middle one is not. So they act very differently. Here is the finished system, after he built it:
It works as designed: the top and bottom modules do their Helmholtz thing, and deal with the frequency range they are designed to deal with, while the one in the middle has no Helmholtz effect at all, and does other stuff, that it was designed to do.

why do these results look so similar

Two reasons: one, the absorption is very light for the application: only 7000 MKS rayls. two, the open percentage is rather high.

it has been said several times that a Helmholtz resonator is still a Helmholtz resonator when filled completely with fiber

And a room mode is still a room mode, even when it is completely damped! I don't see your point. Yes, a Helmholtz resonator is still a Helmholtz resonator when filled completely with fiber, but it is no longer effective as a Helmholtz resonator if it is over-damped! A car that has been dropped off a hundred foot cliff is still a car, but it's not going to act like a car any more!

I am trying to question you to be able to learn from you Stuart not to annoy you.

And that's fine! Excellent! It's what the forum is all about! But when you post a that questions if I even understand how acoustics works, suggesting that my understanding of Helmholtz resonators is flawed, wrong, incorrect, and inviting the whole world to take a look at another thread to learn the "truth" about them, them please do expect a "condescending" and "presumptuous" response. I don't think such a response is unreasonable, give that I already linked you a thread where I designed and implemented a rather complex set of Helmholtz devices, which would not have produce the results that you can clearly see in the graphs if my understanding of Helmholtz resonators is wrong.

Here's another thread that you might want to look at: http://www.johnlsayers.com/phpBB2/viewt ... =2&t=20471 Many of the devices in that room are Helmholtz based: There are slot walls at the front of the room, perf panel devices in the devices on the side walls, a very large _ low tuned slotted device on the rear wall, and several others. The results you see in that room would not have been achieved if Helmholtz resonators do not work the way I understand them to work. So your post questioning my understanding, and pointing to another thread where "lots of people" apparently "disagree" with what I said (when in reality they do not disagree at all), is rather insulting.

So the slat wall that is ANGLED is broadband in nature because each slat is a different height from the back of the panel and the non-angled broadband absorber with slats on the front is broadband because it is fill of fiber. Did I understand you?

Yes and no! Sorry to be cryptic, but there's not enough information in your two scenarios to be able to give you a better answer. Yes, angling the slats does create different cavity depths, and that is one way of getting a Helmholtz resonator to act over a broader frequency range, but that might or might not lower the Q, depending on how it is built internally. Yes, stuffing the cavity full of insulation will lower the Q (assuming that it isn't very light insulation [low GFR], of course), and that alone, in and of itself, will broaden the bandwidth, which is implied by "low Q". But you aren't doing a straight comparison of apples to apples.

Exactly! Isn't this also the idea with an angled slat resonator?

No, it isn't. You are still assuming that all slat walls are Helmholtz devices, when in reality they are not. They can be, if you build them that way, but if you DON'T build them specifically to be Helmholtz, then they will not exhibit any Helmholtz effect. I can build you a car made of cardboard, and paint it to look exactly like a real car, but you won't be able to drive it anywhere! Just because ti looks like a car from the front, does not mean that it really is a car. Just because something looks like a tuned Helmholtz slot wall does mean that it really is a tuned Helmholtz slot wall.

If the gaps vary say 5mm, 10mm, 15mm,20mm and the wall is angled as shown below, a broad band low mid absorber is created that still keeps the the high frequencies alive

Exactly. In other words, it is TUNED! I don't see what's so hard to understand here. If it does NOT absorb the highs, and does NOT absorb the high-mids, and does NOT absorb the lows, instead absorbing ONLY the low-mids, then by definition it is tuned to reject the lows, high-mids and highs!

I agree, I never said it was a slat resonator.

Yet you posted the image of a slat resonator...

To control low mids while diffusing high frequnecies. Isn't that desireable in most rooms or do we need to measure the room first and then decide to use it?

Depends on the purpose of the room, and it's size, among other things. A small control room will need major absorption in the deep lows, some on the low mids, and little to none in the highs. The same room will need even, smooth diffusion across the entire spectrum, or at least across the region from the Schroeder frequency upwards. However, a large live room for tracking grand piano or drums, might NOT need such diffusion, and might be fine with general mid-to-high scattering, and generally reflective, not diffusive. While the same size room for tracking a string ensemble might do better with a lot more high-end absorption, and the same room for tracking a full rock band, or jazz band, or Gregorain chant choir, or Taiko drums, or __________ [fill in the blank] .... might need yet different treatment. In general, control rooms need tightly controlled acoustics in order to make them totally neutral and "invisible", while live rooms need to be more... well "live"! They have the character hat is needed for each session: Hence, most live rooms have variable treatment, either in the form of gobos that can be wheeled around, or panels that can be added/removed, or truly variable devices that can be adjusted in some way, in place, to produce the required acoustic effect.

I never read the latest on that thread but if it is relevant to this then I will.

It is very relevant. It deals with the process of tuning a control room, and uses tuned slot resonators to produce low frequency broad-band absorption while also diffusing lows, mids and highs in different ways.

do agree that Studio B as above is probably thick absorptive walls with slats.

... but with no way of knowing if they are Helmholtz resonators or not. I suspect that most are not, while some might be, but that's just conjecture. You'd have to ask the designed, or look at the plans, or open up the walls to see inside, before you could arrive at a conclusive definition.

Not the same as a small room when the walls are so close to each other and so close to a condenser microphone.

Your mention was about parallel walls, not about room volume. Yes, volume is a major issue, but that's the entire point of my posting that image; Just because a room has parallel walls does not mean that it is terrible acoustically. Even small rooms can have parallel walls and still be good acoustically. There's no relationship between the simple fact of having parallel walls, and whether or not the room will have good acoustics. You can have rooms with non-parallel walls that are terrible, and you can have rooms with parallel walls that are great (the same size in both cases). Parallel or non-parallel CAN be an issue, but it does not HAVE to be an issue.

I don't think you read the whole thing.

I did, actually, and it's not the first time I've read it. You missed once key point that Jens raised, repeatedly: "(assuming not to dense, or to high GFR)." It is extremely simple to kill a Helmholtz resonator by completely filling the cavity with insulation that is too dense FOR THE DEVICE! Notice that: TOO DENSE FOR THE DEVICE! Here's a very simple, clear example:
That's a simple, basic perf panel device, tuned to about 55 Hz. Two curves show two different cases of the exact same device. In one if them, it is filled completely with plain old ordinary OC-703, which is VERY commonly used in acoustics. Nobody would consider that 703 is overly dense, or has a GFR that is too high: There are various estimates of the real GFR of 703, but the one I use is 15,000 MKS rayls, so that's what I used here. That's the BLUE curve!!!! As you can see, that device filled with plain olf 703 is DEAD! It does not work. Exactly as I said. Sure, a pedantic anal dolt could say "But look at the graph! It is still resonating! You can see the curve! It's still a Helmholtz resonator!" And I'd call "total BS" on that. No acoustician would bother considering a device that only produces a coefficient of absorption of less than 0.2, best case! Yeah, it "resonates", but yeah, its DEAD! No use. It "resonates" in the same sense that the car I dropped of the cliff can still be said to "run" if I drag it to the top of a very steep hill, and give it a push.... It still "runs" down the hill, so the same pedantic anal dolt could claim that it is still a valid car, since it looks sort of vaguely like a car and it moves like one down the hill, accelerating even... but no sane person would consider that it really is a viable car! Just like nobody would consider that the device in the blue curve is a viable Helmholtz resonator.

On the other hand, if you use REALLY light, low density, low GFR insulation in there, stuffing the device 100%, then look at the wonderful curve you get! Nearly 0.9, and tightly tuned! Amazing! Except that I'm not aware of any insulation that has a GFR of only 1,000 rayls.... Air is about 400 rayls, so the insulation you'd need for that device is non-existant, as it it would need to be barely twice the impedance of empty air....

So let's add a third option to that device: replace the insulation with a 2mm thick layer of fabric, with typical GFR:
That's the red curve: 2mm thick, 5,000 rayls. And suddenly we have a viable device! It is tuned tightly, coefficient over 0.8, absorbs well, and can actually be built in the real world (it would not be verify efficient for other reasons, but it would work).

So we get back to my original point, and the point that Jens was making too (but that you missed in both cases): A Helmholtz resonator that is filled with insulation that is too dense for the application, is dead. It won't work.

Also, The acoustic modelling calculator doesn't agree with what you said as I already posted above

Yes it does, when you put in realistic values for real-world applications.

Sure, you can cherry pick certain cases where filling or not filling the cavity with light, medium, or dense insulation only makes a minor difference, but in real world examples, such as the ones I'm giving you here, a Helmholtz device that is filled with overly dense insulation, is dead.

Instead of continuing to argue this "dead" point, here's a very simple real-world experiment you can do yourself: go find an empty glass beer bottle or coke bottle that resonates nicely when you blow across the top. Choose the best one you can find, the one that you REALLY like, since it resonates the loudest. Go crazy: test a hundred bottles, to find one that is REALLY good: powerful, loud, strong sound. Now stuff it full of cotton wool. the entire bottle: fill it with cotton wool, which is a porous absorber. Now blow across the top again... Case closed.

I'm not sure if you came here to argue, or to learn, but I'm done arguing. The principles of Helmholtz resonance work. The equations are correct. The acoustic modellers are correct when used realistically. I've have designed and used these devices in real world situations, and shown you links that demonstrate this. I'm not sure what more I can do, but one thing is for sure: I'm not going to continue arguing with you, when you claim that they don't work, or try to point other threads to justify your claims. They do work, the math is clear, and has been clear ever since Hermann von Helmholtz and his colleges described it over 150 years ago. I think that if they were mistaken in some way, and Helmholtz resonance does not actually work they way the theory says it does and they way the equations document it, then we would have found out by now. So if you are here to learn, then ask all you want, and the stuff I know I'll be happy to explain. Ask away! But if you are here to argue with us and claim that these things don't work, and that other people are contradicting what I said, when in reality they aren't, then I don't see a lot of point in carrying on.

Stuart is basically saying this stuff to make you think about why you're doing what you're doing. He's trying to teach you to only do something because the math or a measurement indicates that you need to do it, not just because your friend told you that you should or because you saw some fancy studio with stuff on their wall and you think you should do it too. His Abbey picture is proof that if your room sounds good, you don't need this and that device. That is all.

Thank you Greg! Exactly. You hit the nail on the head.

The structural design is Helmholtz, but at some point, you add enough insulation, it negates the function of Helmholtz an therefore doesn't function as a Helmholtz anymore. So I suppose, then you can say that it is not a Helmholtz

Exactly. Car. 100 foot cliff. Drop. "But it is still a CAR!". Nope. Not any more!

Ok got it. But with an absorber panel the slats also play their role in "tuning" to some extent because they are what define how the absorber will perform.

Point missed again: The reflective properties of the slats have zero bearing on the tuning of the device as a Helmholtz resonator. You are confusing the issues again. Some acoustic devices do several things at once. A slot wall will certainly reflect some frequencies simply due to the slats and their dimensions, as Greg pointed out, but that's irrelevant and has no relationship to the Helmholtz absorption, which is an entirely DIFFERENT effect produced by the exact same device. In fact, I could design you a slot wall that bot reflects and absorbs the same frequencies, if you really wanted me too! I could carefully tailor the slats dimensions to reflect a certain frequency range, then also tailor the other parameters to absorb that exact same frequency range. The device would be rather worthless and useless, of course, but it would be possible to do that, since the reflection is totally independent of the Helmholtz absorption. There's no relationship between the two.


- Stuart -

Soundman2020 wrote:

on the one hand you are saying that a Helmholtz Resonator is tuned, but then on this same site it is said to be either tuned or broadband (when angled).

I don't see where there's a conflict here! It's very simple: it can be tuned tightly (high Q), which is the "traditional" and conventional way of doing it, or it can be tuned more broadly (low-Q), which is accomplished in any of several ways, one of which is to vary the depth of the cavity. There's no conflict in saying that Helmholtz devices can be high-Q (tightly tuned) or low-Q (loosely tuned), or even tuned to several frequencies at once (both broad-band and also high-Q). It's all a matter of what you need the device to do, and how you design it.

The big problem with high-Q devices (of all types, not just Helmholtz-based) is that is very difficult to tune them precisely. And since the very purpose of tightly tuned devices is to hit the exact frequency associated with a specific room problem), it's not such a good idea to go with very high Q devices, unless you are certain that it is tuned perfectly, or have some means of adjsuting the tuning after it is finished. Hence, in this room, I tuned the rear corner devices broadly in two different ways: by varying the slot widths AND cavity depths slightly, and also by adding some additional absorption on the rear face of the cavity and the floor of the cavirty, which deliberately de-tunes them (lowers the Q). That way, he did not need to build them with extreme precision and test / moidfy / test / modify / test / modify until they worked. Rather, they worked straight away, since they cover several frequencies (differing slot widths, differing cavity depths), AND ALSO have had their Q lowered a bit, due to the interior damping.

So, that device that you see there (and see the results of, in the graphs) is indeed a Helmholtz resonator that is both tuned and also broad-band: it covers a range of frequencies, that I specifically choose as they were problematic in the room. I tuned it to that range. So it is "tuned" and "broadband". The two terms are not mutually exclusive. You can tune an absorption device tightly, or broad-band, in much the same way that you can tune a diffuser to be tight or broad-band.

You said in your earlier post that filling a slatted trap with fiber also makes it more broadband so naturally I was interested in the difference between the two

You seem to be confusing two different types of acoustic device here. As I mentioned before, two devices can APPEAR identical at first glance, but actually have very different functions, and be based on very different acoustic principles. A true "slat wall" or "slot wall" is one thing, but a slatted device filled with porous absorption is another, very different thing. They might look the same from the front, but work very differently internally. Once again, take a look at the rear corner treatment in the room:

Frank---S2870004.jpg

Here's another view:

Frank---S2870022.jpg

See those three identical devices in the rear corner? One at the top, one in the middle, and one at the bottom? They all look the same, so they must all act the same, right? Except that they don't. The top and bottom ones are the same, and act the same, but the one in the middle is very different. The top and bottom ones really are Helmholtz devices, but the middle one is not. So they act very differently. Here is the finished system, after he built it:

20180125_151839.jpg

It works as designed: the top and bottom modules do their Helmholtz thing, and deal with the frequency range they are designed to deal with, while the one in the middle has no Helmholtz effect at all, and does other stuff, that it was designed to do.

why do these results look so similar

Two reasons: one, the absorption is very light for the application: only 7000 MKS rayls. two, the open percentage is rather high.

it has been said several times that a Helmholtz resonator is still a Helmholtz resonator when filled completely with fiber

And a room mode is still a room mode, even when it is completely damped! I don't see your point. Yes, a Helmholtz resonator is still a Helmholtz resonator when filled completely with fiber, but it is no longer effective as a Helmholtz resonator if it is over-damped! A car that has been dropped off a hundred foot cliff is still a car, but it's not going to act like a car any more!

I am trying to question you to be able to learn from you Stuart not to annoy you.

And that's fine! Excellent! It's what the forum is all about! But when you post a that questions if I even understand how acoustics works, suggesting that my understanding of Helmholtz resonators is flawed, wrong, incorrect, and inviting the whole world to take a look at another thread to learn the "truth" about them, them please do expect a "condescending" and "presumptuous" response. I don't think such a response is unreasonable, give that I already linked you a thread where I designed and implemented a rather complex set of Helmholtz devices, which would not have produce the results that you can clearly see in the graphs if my understanding of Helmholtz resonators is wrong.

Here's another thread that you might want to look at: http://www.johnlsayers.com/phpBB2/viewt ... =2&t=20471 Many of the devices in that room are Helmholtz based: There are slot walls at the front of the room, perf panel devices in the devices on the side walls, a very large _ low tuned slotted device on the rear wall, and several others. The results you see in that room would not have been achieved if Helmholtz resonators do not work the way I understand them to work. So your post questioning my understanding, and pointing to another thread where "lots of people" apparently "disagree" with what I said (when in reality they do not disagree at all), is rather insulting.

So the slat wall that is ANGLED is broadband in nature because each slat is a different height from the back of the panel and the non-angled broadband absorber with slats on the front is broadband because it is fill of fiber. Did I understand you?

Yes and no! Sorry to be cryptic, but there's not enough information in your two scenarios to be able to give you a better answer. Yes, angling the slats does create different cavity depths, and that is one way of getting a Helmholtz resonator to act over a broader frequency range, but that might or might not lower the Q, depending on how it is built internally. Yes, stuffing the cavity full of insulation will lower the Q (assuming that it isn't very light insulation [low GFR], of course), and that alone, in and of itself, will broaden the bandwidth, which is implied by "low Q". But you aren't doing a straight comparison of apples to apples.

Exactly! Isn't this also the idea with an angled slat resonator?

No, it isn't. You are still assuming that all slat walls are Helmholtz devices, when in reality they are not. They can be, if you build them that way, but if you DON'T build them specifically to be Helmholtz, then they will not exhibit any Helmholtz effect. I can build you a car made of cardboard, and paint it to look exactly like a real car, but you won't be able to drive it anywhere! Just because ti looks like a car from the front, does not mean that it really is a car. Just because something looks like a tuned Helmholtz slot wall does mean that it really is a tuned Helmholtz slot wall.

If the gaps vary say 5mm, 10mm, 15mm,20mm and the wall is angled as shown below, a broad band low mid absorber is created that still keeps the the high frequencies alive

Exactly. In other words, it is TUNED! I don't see what's so hard to understand here. If it does NOT absorb the highs, and does NOT absorb the high-mids, and does NOT absorb the lows, instead absorbing ONLY the low-mids, then by definition it is tuned to reject the lows, high-mids and highs!

I agree, I never said it was a slat resonator.

Yet you posted the image of a slat resonator...

To control low mids while diffusing high frequnecies. Isn't that desireable in most rooms or do we need to measure the room first and then decide to use it?

Depends on the purpose of the room, and it's size, among other things. A small control room will need major absorption in the deep lows, some on the low mids, and little to none in the highs. The same room will need even, smooth diffusion across the entire spectrum, or at least across the region from the Schroeder frequency upwards. However, a large live room for tracking grand piano or drums, might NOT need such diffusion, and might be fine with general mid-to-high scattering, and generally reflective, not diffusive. While the same size room for tracking a string ensemble might do better with a lot more high-end absorption, and the same room for tracking a full rock band, or jazz band, or Gregorain chant choir, or Taiko drums, or __________ [fill in the blank] .... might need yet different treatment. In general, control rooms need tightly controlled acoustics in order to make them totally neutral and "invisible", while live rooms need to be more... well "live"! They have the character hat is needed for each session: Hence, most live rooms have variable treatment, either in the form of gobos that can be wheeled around, or panels that can be added/removed, or truly variable devices that can be adjusted in some way, in place, to produce the required acoustic effect.

I never read the latest on that thread but if it is relevant to this then I will.

It is very relevant. It deals with the process of tuning a control room, and uses tuned slot resonators to produce low frequency broad-band absorption while also diffusing lows, mids and highs in different ways.

do agree that Studio B as above is probably thick absorptive walls with slats.

... but with no way of knowing if they are Helmholtz resonators or not. I suspect that most are not, while some might be, but that's just conjecture. You'd have to ask the designed, or look at the plans, or open up the walls to see inside, before you could arrive at a conclusive definition.

Not the same as a small room when the walls are so close to each other and so close to a condenser microphone.

Your mention was about parallel walls, not about room volume. Yes, volume is a major issue, but that's the entire point of my posting that image; Just because a room has parallel walls does not mean that it is terrible acoustically. Even small rooms can have parallel walls and still be good acoustically. There's no relationship between the simple fact of having parallel walls, and whether or not the room will have good acoustics. You can have rooms with non-parallel walls that are terrible, and you can have rooms with parallel walls that are great (the same size in both cases). Parallel or non-parallel CAN be an issue, but it does not HAVE to be an issue.

I don't think you read the whole thing.

I did, actually, and it's not the first time I've read it. You missed once key point that Jens raised, repeatedly: "(assuming not to dense, or to high GFR)." It is extremely simple to kill a Helmholtz resonator by completely filling the cavity with insulation that is too dense FOR THE DEVICE! Notice that: TOO DENSE FOR THE DEVICE! Here's a very simple, clear example:

dead-helholtz-resonator.jpg

That's a simple, basic perf panel device, tuned to about 55 Hz. Two curves show two different cases of the exact same device. In one if them, it is filled completely with plain old ordinary OC-703, which is VERY commonly used in acoustics. Nobody would consider that 703 is overly dense, or has a GFR that is too high: There are various estimates of the real GFR of 703, but the one I use is 15,000 MKS rayls, so that's what I used here. That's the BLUE curve!!!! As you can see, that device filled with plain olf 703 is DEAD! It does not work. Exactly as I said. Sure, a pedantic anal dolt could say "But look at the graph! It is still resonating! You can see the curve! It's still a Helmholtz resonator!" And I'd call "total BS" on that. No acoustician would bother considering a device that only produces a coefficient of absorption of less than 0.2, best case! Yeah, it "resonates", but yeah, its DEAD! No use. It "resonates" in the same sense that the car I dropped of the cliff can still be said to "run" if I drag it to the top of a very steep hill, and give it a push.... It still "runs" down the hill, so the same pedantic anal dolt could claim that it is still a valid car, since it looks sort of vaguely like a car and it moves like one down the hill, accelerating even... but no sane person would consider that it really is a viable car! Just like nobody would consider that the device in the blue curve is a viable Helmholtz resonator.

On the other hand, if you use REALLY light, low density, low GFR insulation in there, stuffing the device 100%, then look at the wonderful curve you get! Nearly 0.9, and tightly tuned! Amazing! Except that I'm not aware of any insulation that has a GFR of only 1,000 rayls.... Air is about 400 rayls, so the insulation you'd need for that device is non-existant, as it it would need to be barely twice the impedance of empty air....

So let's add a third option to that device: replace the insulation with a 2mm thick layer of fabric, with typical GFR:

un-dead-helmholtz-resonator.jpg

That's the red curve: 2mm thick, 5,000 rayls. And suddenly we have a viable device! It is tuned tightly, coefficient over 0.8, absorbs well, and can actually be built in the real world (it would not be verify efficient for other reasons, but it would work).

So we get back to my original point, and the point that Jens was making too (but that you missed in both cases): A Helmholtz resonator that is filled with insulation that is too dense for the application, is dead. It won't work.

Also, The acoustic modelling calculator doesn't agree with what you said as I already posted above

Yes it does, when you put in realistic values for real-world applications.

Sure, you can cherry pick certain cases where filling or not filling the cavity with light, medium, or dense insulation only makes a minor difference, but in real world examples, such as the ones I'm giving you here, a Helmholtz device that is filled with overly dense insulation, is dead.

Instead of continuing to argue this "dead" point, here's a very simple real-world experiment you can do yourself: go find an empty glass beer bottle or coke bottle that resonates nicely when you blow across the top. Choose the best one you can find, the one that you REALLY like, since it resonates the loudest. Go crazy: test a hundred bottles, to find one that is REALLY good: powerful, loud, strong sound. Now stuff it full of cotton wool. the entire bottle: fill it with cotton wool, which is a porous absorber. Now blow across the top again... Case closed.

I'm not sure if you came here to argue, or to learn, but I'm done arguing. The principles of Helmholtz resonance work. The equations are correct. The acoustic modellers are correct when used realistically. I've have designed and used these devices in real world situations, and shown you links that demonstrate this. I'm not sure what more I can do, but one thing is for sure: I'm not going to continue arguing with you, when you claim that they don't work, or try to point other threads to justify your claims. They do work, the math is clear, and has been clear ever since Hermann von Helmholtz and his colleges described it over 150 years ago. I think that if they were mistaken in some way, and Helmholtz resonance does not actually work they way the theory says it does and they way the equations document it, then we would have found out by now. So if you are here to learn, then ask all you want, and the stuff I know I'll be happy to explain. Ask away! But if you are here to argue with us and claim that these things don't work, and that other people are contradicting what I said, when in reality they aren't, then I don't see a lot of point in carrying on.

Stuart is basically saying this stuff to make you think about why you're doing what you're doing. He's trying to teach you to only do something because the math or a measurement indicates that you need to do it, not just because your friend told you that you should or because you saw some fancy studio with stuff on their wall and you think you should do it too. His Abbey picture is proof that if your room sounds good, you don't need this and that device. That is all.

Thank you Greg! Exactly. You hit the nail on the head.

The structural design is Helmholtz, but at some point, you add enough insulation, it negates the function of Helmholtz an therefore doesn't function as a Helmholtz anymore. So I suppose, then you can say that it is not a Helmholtz

Exactly. Car. 100 foot cliff. Drop. "But it is still a CAR!". Nope. Not any more!

Ok got it. But with an absorber panel the slats also play their role in "tuning" to some extent because they are what define how the absorber will perform.

Point missed again: The reflective properties of the slats have zero bearing on the tuning of the device as a Helmholtz resonator. You are confusing the issues again. Some acoustic devices do several things at once. A slot wall will certainly reflect some frequencies simply due to the slats and their dimensions, as Greg pointed out, but that's irrelevant and has no relationship to the Helmholtz absorption, which is an entirely DIFFERENT effect produced by the exact same device. In fact, I could design you a slot wall that bot reflects and absorbs the same frequencies, if you really wanted me too! I could carefully tailor the slats dimensions to reflect a certain frequency range, then also tailor the other parameters to absorb that exact same frequency range. The device would be rather worthless and useless, of course, but it would be possible to do that, since the reflection is totally independent of the Helmholtz absorption. There's no relationship between the two.


- Stuart -

It seems clear to me that you are trying to protect your reputation which is perfectly fine because you have earned it but you did say to me that a helmholtz resonator does not function as a helmholtz resonator when it is full of fiber and when I provided two sources (what Jens said and the calculator) that contradict that, you say that I don't understand what Jens is saying or that the insulation is too low GFR (does insulation have high GFR where you are from, over here it does not ). I will let other people who are interested in the topic decide for themselves. At some point in this field when you go really deep into stuff you can see that it isn't black and white and I think you sometimes need to go with what you believe to be right. Especially when senior people say things that don't always match up (this certainly isn't the first time it has happened ). That is what I have learned from all of this.

Thanks Stuart

does insulation have high GFR where you are from, over here it does not

Really???? I would be FASCINATED to hear your theory on how porous insulation does not have high Gas Flow Resistivity! Now, maybe I'm wrong, and the stuff you buy in Ireland is totally impermeable to air, and therefore has no GFR at all... in which case it would be absolutely useless as an acoustic absorber. You might want to check what the term GFR actually implies.... Another word for it is "acoustic impedance". That's the principle on which porous absorption works: since it does, in fact, have acoustic impedance, it "impedes" the passage of sound through it. And it also creates an acoustic impedance mismatch at the boundary surface, which can be another factor in how porous absorbers work. I'd really like to see how acoustic absorbers are made in Ireland, where apparently insulation is manufactured in some magical manner that prevents it from impeding the passage of sound. Do you know if they use unicorn hair in making those devices? Or maybe pixie dust?

Sorry to be so sarcastic, but the very fact that you claim such a thing ("insulation does not have high GFR"), indicates that you still don't really understand the principles that we are talking about here.

Click on some of the other buttons on the calculator that you used. You might be especially interested in the "imaginary acoustic impedance" button, which shows the graph of the imaginary component of the acoustic impedance. Maybe you can explain to the "other people who are interested in the topic " what that means, and why it is important. Explaining the "real acoustic impedance" would be a welcome second.

But basically, real facts contradict your claim. Plain old Rockwool RW5 is widely available in Ireland, listed on many building supplier websites. Build4Less has it, Topline has it, and many others do to. Simple Googles search found those real fast. Rockwool RW5 does, in fact, have rather high GFR: Rockwool themselves lists it as "around 60,000 mks rayls". That's TWELVE TIMES HIGHER than the value I used in the graphs from yesterday. Isover is also widely available in Ireland: their U Tech Ultimate range has products with GFR up to 70,000 rayls... or FOURTEEN times higher than what I used (and about seven times higher than the typical value for OC-703). Pretty darn high by any standards!

So your claim is false. Insulation commonly sold in Ireland does, in fact, have high GFR. Much higher than would be useful in Helmholtz resonators, for example. Try repeating that same acoustic prediction using RW5 and U Tech.... OK I'll save you the trouble:
See the blue and orange curves, right down at the bottom? Blue is U Tech, orange is RW5. And yes, that "Helmholtz resonator" filled with those is dead. Totally dead. As in: "not even a tiny bit of life in it". Coefficient of absorption is around 0.04! And the peak absorption is at ... well... umm... errr ... it isn't "at" anywhere, because it has no absorption at all, let alone "peak" absorption! The minuscule amount of absorption that some extreme pedants might be construe it as having, covers well over half of the entire musical spectrum!

It seems clear to me that you are trying to protect your reputation

I'm not trying to protect my reputation: When I'm wrong, then I'm wrong, and I don't have a problem admitting it. I can make mistakes, just like anyone else. But I'm not wrong here: As I stated, a Helmholtz resonators that is overfilled with insulation is dead, and useless. Yes, you can choose some special cases where filling it does NOT kill it entirely, but for the typical insulation that most studio builders commonly use (not even the high GFR ones that "don't exist" in Ireland!!!), it is DEAD. Here's the same case as yesterday and above, but using OC-703 insulation, which is NOT high density, and IS commonly used in studios:
Orange curve: GFR of 15,000 rayls. Certainly not high, but the device is still dead.

So, once more, I'll repeat what I said, and what Jens also said (indirectly): a Helmholtz resonator that is filled with insulation where the GFR is too high for the application, is dead. What Jens didn't say directly (but implied), is the same thing you can see in the above graphs: it's fairy easy to use insulation where the GFR is too high, in a typical home-built Helmholtz resonator, just by filling it with commonly used, medium density insulation. If you read Gearslutz regularly, you'll know that DanDan himself once built a Helmholtz device that didn't work, for the same reason.

I'm really not sure why you continue to attack this truth so persistently when I've already demonstrated it several times. I don't get it. There's no point in it for you that I can see: you just continue to dig the hole deeper! You've already shown that you haven't yet grasped the basic principles behind Helmholtz resonators, yet you choose to argue with someone who has used them successfully in studios that he has designed.... I can't understand why you want to carry on down that road.

I've pointed you at threads that show that theses devices DO work, when designed and built right, but it seems you never even bothered reading through them (by your own admission). I've given you the math for figuring it out. I've even give you a simple home experiment that you can carry out yourself, which you have apparently not done either! In addition you have the calculator for figuring it all out, but you claim that the calculator itself is "wrong" . Yet, I have shown you that the calculator is NOT wrong, it DOES produce correct answers (provided that you plug in realistic values, of course: it does not do any data checking, to ensure that you gave it actual values that make sense in the real world), yet you still insist that I am wrong. I simply don't understand why.

At some point in this field when you go really deep into stuff you can see that it isn't black and white and I think you sometimes need to go with what you believe to be right.

Science works DESPITE what you believe, not BECAUSE of what you believe! You can believe that the sky is actually red and that pigs really do fly, but that does not make it so. The science that I have shown you clearly demonstrates that the point I am making is correct. If you choose to NOT believe that, then that's your problem, not mine! I can't help anyone who denies that the science of acoustics is correct, and can correctly predict the performance of acoustic treatment. There's simply no point in even trying to help someone who prefers belief over fact. The fact is that the equations work, the predictions are valid, and they do pan out in the real world when you actually build and test devices. This room is one case. Studio Three is another. There are MANY more, all over this forum, and other forums too. The Helmholtz equations have been known for well over a century, have been tested countless, numerous times, and even updated with slight correction factors to be even more accurate, but the basic underlying principle is not a belief: it's about as proven as you could want! It's right up there with Sabine's equations.

Especially when senior people say things that don't always match up (this certainly isn't the first time it has happened

The reason that acousticians often appear to be disagreeing is often because the people trying to follow the difference of opinion simply don't understand the basics well enough to comprehend what is really being discussed! You might well read one person say that he always designs control rooms to be rectangular, since that's what works best, while someone else says that control rooms should never be rectangular, and should always be trapezoidal, because that's what works best. There's actually no conflict there at all! They are both absolutely correct. An outsider who doesn't understand the underlying concepts might well conclude that they are in total disagreement, and that they cannot both be right... but they can! For example, if the first guy only designs very large control rooms, and the second guy only designs small control rooms, then they are both perfectly correct. Also if the first guy uses complex procedures to empirically determine the optimal layout for speakers, mix position and treatment in every rectangular room he does, while the second guy always does RFZ-style rooms, then once again they are both correct. But to the outsider who doesn't "get it", they are both liars: Or at least one of them is a liar.

Likewise, there can be differences of opinion on some aspects of the best way to treat a live rooms, simple because there is no "best!" way! Live rooms are personal, subjective, individual: they are supposed to sound the way that the owner wants them to sound, period. If the owner wants a "boomy" room, then nobody has any right to tell him that his room is "wrong" if it sounds boomy. Another guy might prefer a sizzlingly zingy room, and the "boomy" guy would have no right to say that THAT was wrong. If that's what the owner wants for his style of tracking, and his genre, and his personal preference, then that's what is "right" for the room. Control rooms are different: there are specifications and solid, logical reasons for those specs. There's only really one "prefect" goal for a control room, and everybody can agree on that (even though there are multiple ways of getting there), but there can be as many goals for a live room as there are studio owners! So nobody holds the "only correct" opinion on how a live room should sound. Right now I'm working with a customer in the north-eastern USA on his new studio, and he has very specific goals for his piano room, which houses a baby grand, but less so for the live rooms, which he wants highly variable. I came up with some unusual design elements for both of those rooms, and he's busy with his carpenters building them right now. But I didn't tell him how his rooms should sound! He's an experienced producer and sound engineer who has worked with some big-name artists, so he knows much better than I do what his rooms should sound like. He explained to me what he wanted, we took measurements in his rooms, I made some suggestions, did some modeling and predictions, and he liked the concepts, so he's building things. But it's his choice on how he wants them to sound, not mine. And nobody else would be correct in saying that the sound he is after is "wrong". Hopefully, after we are done in a few weeks, he'll allow me to post some of the details of what we did there, so others can get ideas. Maybe not: That's also up to him.

The point being that people who don't understand acoustics well, don't "get it" when acousticians appear to disagree, or appear to make absolutely contradictory statements. Very often, they are not really disagreeing or contradicting at all! Very often they are simply talking about entirely different cases, or they are talking about personal preferences. To "outsiders", that appears to be "not matching up", but in reality it isn't.

That is what I have learned from all of this.

If that's all that you have learned from this, then you have missed the point. I asked you if you came here to argue, or came here to learn: it seems that the answer is "neither", which I have a hard time figuring out...


- Stuart -