given room size and a rough estimate of RT60 ( or maybe to be more clear should I write : the time required for reflections of a direct sound to decay 60 dB ???
Joking !!!!! ) you can have an ESTIMATE of Schroeder freq
... and how will you know what the RT60 time will be if you don't yet know what the treatment will be?
RT60 depends also on room treatment, not just room size.
The problem you are giving yourself by trying to calculate what to treat based on the Schroeder frequency, is that by treating the room, you change the Schroeder frequency! So you change the parameter that you are using to calculate the parameter...
the time required for reflections of a direct sound to decay 60 dB
Right! RT60 is not even valid, technically, for small rooms. Small rooms do not have a statistically valid reverberant field at low frequencies, so you can't even talk about "RT60" for such rooms...
you can have an ESTIMATE of Schroeder freq
Try plugging in a few different treatment options into Andy's calculator: You can easily get results anywhere from about 100 Hz to over 200 Hz! That's a massively huge range! Wavelengths varying between 3.5m and 1.7m... Which one would you choose?
Keeping outer shell rectangular ( sound-proofed shell ) and then create slanted slat resonators with thick broad band absorption behind. This should create a variable depth structure that should lower the Q
... in which case you cannot treat modal issues! As I mentioned, modes are very tight, very high Q. If you want to damp a mode, then you need a device that has very tight,high Q. Trying to do it with a low-Q device is not going to be very effective: you'd need a huge surface area, and a huge volume to do that.
John uses slot walls such as you show in your diagram, but NOT to treat modes. John uses them as general mid-range devices, either broadband if the open area percentage is large enough, or individually tuned if it is low enough. I use them the same way. We also use them for diffusion, reflection, and absorption, as well as tuned devices, all at the same time. That's one of the great things about slat devices: you can design them to do many things at once, if you know how to do that... But we don't try to use them as tuned devices to treat modal problems, because that is almost impossible.
And you are missing the point: I asked if you have done the math to find out what dimensions you would need to treat the very low frequencies where modal issues occur: Since you still want to try treating modal issues with slot walls, I'd say that the answer is "No"!
When you do the math, you'll see what the issues are... and find out why people generally don't try to treat modes with slot walls...
There's another problem with your diagram: You show your slot walls as using the outer leaf (outer shell) of the room as the back of the resonant cavity, but that would be impossible. You would have no isolation at all if you did that. You have to use the INNER-leaf of your room as the back of the cavity, not the outer leaf.
Keeping outer shell rectangular ( sound-proofed shell )....
You've been reading too much Philip Newell....
It would be nice if you really could build a wall that performs exactly as NER theory says it should but in reality it's not that simple... Take a look at the Wyle report to understand why... Also, if you follow Newell, then your outer leaf CANNOT be rectangular! According to him, it must be asymmetric. On the other hand, it is indeed possible to have good isolation with a rectangular outer leaf, just as you show it (despite what Newell says...
) So you are fine there ... but you also need to show the inner-leaf.
how will be make sure that the tuning of your devices exactly matches the modal problem?
Well, I suppose I will measure my room before !!!!!
You are missing the point here too! I'll repeat the question. "How will you be sure that the TUNING OF YOUR DEVICES exactly matches the modal problem"? In other words, how will you check that your device really is tuned to the exact, precise frequency of the mode? Text book theory is one thing, but real-world devices are never as precise or accurate as the equations say. The equations assume that you have perfect materials, that is perfectly rigid, solid, uniform, with no variations, and that you cut it perfectly straight, nail it up perfectly parallel, not varying at all, and that the thickness and depth are perfect, and that the insulation is perfect, and that the air temperature and pressure are identical at all points, and exactly equal to standard temperature and pressure.... If any of those varies even slightly, then the real device will not be tuned to the predicted frequency.
For example, let's say you have a mode at 38.3 Hz, so you design a slot wall that is supposed to be tuned to 38.3 Hz. How will you check that it really is tuned to 38.3 Hz? What happens if it actually turned out to be tuned to 34 Hz? It is no use to you. None at all! Worthless. A 34 Hz Helmholtz resonator is no use for treating 38.3Hz! It is more than ten percent off!!! How will you find out what the device is
actually tuned to, and how will you
adjust your device to change the tuning so it is correct? You say you want to make your device broadband, by varying the depth, but then you would have very low Q like that, and it would not treat the mode effectively. Modes are very powerful: they store a lot of energy, and you need a device that can remove
all of that energy from the room, not just a small part of it. Let's say you de-tune your device to low Q so that it has a coefficient of absorption of 0.25 at 36.1 Hz. That will have practically no effect at all on the mode. It might reduce it by maybe 0.1 dB in intensity, and perhaps shorten the decay time by a few milliseconds... But you need it to reduce the intensity a hundred times more than that, and cut several hundred milliseconds off the decay times...
This is not what I meant !!! I don’t want to resolve ALL modal issues with side walls…
Then why build them as modal devices? That doesn't make a lot of sense.... If a device only deals with one specific problem, but it takes up half of an entire wall, then what wall area will you use to treat the dozens of other problems that you will have?
Idea is to put a membrane absorber on the ceiling…
So you plan to make your ceiling reflective at mid and high frequencies?
How will you deal with the flutter echo caused by that plan? And which of the TEN modal frequencies that you will have in your room below 100 Hz, would you tune that to? Yes, there are ten modal issues that involve the ceiling in your room, and those are only the ones below 100 Hz. If your Schroeder frequency happened to be 150 Hz, then there are 32 modes that involve the ceiling below 150 Hz. Which ONE of those would you tune your ceiling for? What about the other 31 modes?
Also, I'd guess that you have never actually tried to build a tune a membrane trap... That is notoriously hard to do. Not quite as hard as a Helmholtz resonator, but still nowhere near as easy as the text books make it sound. Text book theory is one thing, but real-life acoustic devices are something else...
Have you done the math on having such a device on your ceiling? How deep does it need to be to hit your first vertical axial mode, at 57.4Hz? Are you willing to give up that much ceiling height? Are you aware that membrane traps continue to vibrate for a couple of cycles AFTER the triggering signal has stopped, and can feed some of that energy back into the room again, thus extending modal ringing even longer? You have several modes that involve the ceiling at low frequencies (at least 8 that will potentially be a problem): Which ones will you treat, and which ones will you NOT treat? You would have to divide up the total ceiling area such that you are giving a small area to each problem, but the more areas you have, the smaller each one is, so it wont be very effective as it only covers part of the area....
Do you even have enough volume available in the room, to build the huge resonators that you need to deal with low frequencies? Do you have enough surface area at the key pressure points?
Yes, I think so !!
"Thinking" it might be so is not the same as doing the math to check if it really is so! So let's do some quick calculations: You need about 1% of the room volume inside a resonator to effectively deal with each problem. I can see that you will have about nine or ten really problematic modes in your room. The total volume of your room is 94m3. So you will need nearly TEN CUBIC METERS of air cavity inside your slot walls to deal with those.
Your design has one resonant device on each side wall, so you need 5m3 inside each. 5m3 on the left, and 5m3 on the right side of the room. Your room height is 3m, but let's call it 2.5m to make the math easier, and also to allow for the realities of construction. Therefore, you will need to make each of your slot walls
two meters long and one meter deep. But you also want to angle them, so let's say the shallowest part will need to stick out roughly half a meter into the room, and the deepest part will need to stick out nearly 2 meters... the room is only 4.5 m wide, so at the point where each side wall sticks out 2m, the actual room will be just 50cm wide..... Can you fit your desk and console and speakers and you into a room that is only 50cm wide?
Ummmm. I think I'd say that you do NOT have the volume to do what you are proposing to do.... The math tells the truth. You MIGHT be able to afford enough space to treat the three most important modal issues... but two of them cannot be treated by the side walls anyway! In the very best case, you could treat one single mode with your plan, but you would use half of the entire side wall area to do it, and even then it would not be effective.
If you have carefully tuned slots that will deal with a certain mode, and those slots cover an area equivalent to, say, 5% of the wall, that means that 95% of the wall is NOT dealing with that frequency... yet 100% of the wall is seeing it...
This is not what I meant !!! I don’t want to resolve ALL modal issues with side walls…
That's not what I'm talking about. You missed the point. I'm talking about one single mode that you
could treat, and the surface area
available to treat it. Let's talk about your 0,1,0 mode, which is at 38.3 Hz. It
does have pressure nodes on the side walls, so it would be possible to treat it on the side walls, in theory. Let's assume that you are able to design and build a slot wall that has a resonant frequency of exactly 38.3 Hz, very accurate, and high Q. Let's say you really did build those enormous modules, 2m wide and 2.5m high, so you have 5m2 of surface area on your slot wall. However, only about 3% of that area is the actual open area of the slots! The rest is just reflective slats.... so only 3% of that 2.5m2 area is actively absorbing that frequency. That's just 0.15m2. The other 97% of the surface area is not absorbing that frequency! Even worse, the mode is affecting the
entire wall, not just the part where the resonator is, and the wall has an area of 7x3m = 21m2. So you have a wave that is affecting 21m2 of surface area, and you are hoping to effectively eliminate the problem with just 0.15m2? That's 0.7% of the entire surface. So the wave hits 100% or the wall, and you plan to deal with it using just 0.7% of the wall.... 99.3% of the wave does not even see your treatment...
And that's just ONE mode we are talking about. What if you need to deal with five or six modes on your side walls? Each tuned device would be able to have only about 0.1% of the total wall area.... Even if you could tune all of those perfectly to the exact mode, and design each one perfectly so that it has a coefficient of absorption of 1.0 at that frequency, with very high Q, you would still only be able to deal with 0.1% of the wave....
Ummmm... so I think I'd say that No, you do NOT have enough area available.
The more you work through the math and the reality, the more it looks like your plan for dealing with modes using slot walls, is not viable. (That's probably why you don't see many real-world studios actually built that way....
)
SBIR is quite predictable… so I can manage to control it !!
I'm glad you think it is so easy! I'm guessing that you have not actually built many rooms, and have not actually dealt with SBIR successfully in them... SBIR is a HUGE problem. Yes, it is somewhat predictable, and yes it is dead easy to measure, but dealing with it is another matter entirely... I often spend days coming up with effective, successful methods for treating SBIR in real-world studios....
OK, here's a little test for you to think about: let's assume you have an SBIR null at 80 Hz at your mix position: You say it is easy to fix, so how exactly would you "control" that? What treatment would you build, and where would you put it? Please explain in detail what your plan would be. Membrane trap? Helmholtz resonator? QRD diffuser? Schroeder diffuser? Fractal diffuser? Slot wall?
there’s a lot of materials and infos about LEDE, NE… but I actually didn’t find many precise guidelines for RFZ….
LEDE is dead. Nobody in their right mind still builds true LEDE rooms, as originally defined. Modified forms of LEDE, such as RFZ and NE and CID ad others, are very much alive, and very much used. However, RFZ is actually patented or trademarked, or something. As far as I know, RPG holds the rights to that, but I may be wrong. Maybe it is Cox or D'Antonio. Not sure. If you wanted to copy their patented design for your own personal room, then that might be allowable (but don't take my word for it: I'm not a lawyer), but if I wanted to copy it for one of my customers, that would probably be a BIG problem, because I am a studio designer, and I design studios for a living! That would be commercial use if I did it, so I can't do that. However, I
can use the same principles and achieve comparable results, using different designs. One of the most basic principles for building an RFZ-style room, is that there must not be any reflections arriving at the mix position within 20ms of the direct sound and above -20 dB, plus all reflections that do arrive at the mix position must be diffuse, or at least 20 dB down. The spec also talks about a 20ms ITDG at -20dB, then comes a higher level diffuse (reverberant) field at maybe -10dB for a short time, but I have found that to not be so necessary, especially in small rooms where it is theoretically impossible to achieve anyway! I prefer to have a natural, slower, smooth decay starting immediately after the direct sound, starting -20 dB down. So while my rooms do indeed create a reflection free zone around the mi position, my rooms are not RFZ... even though they do the same thing! RFZ(tm) is not the same as rfz, the concept.
I'm pretty sure there is info on the actual RFZ specifications somewhere...
but I can't give it to you, as that would not be legal for me to do!
Well I suppose that I can tune the front ( or THE REAR wall ) on Axial length-related mode….
How would you tune the front wall, when two thirds of the entire front wall is taken up by the speaker infinite baffles?
You only have one third of the area available, and you will need to treat many modes with that small area....
How would you tune the rear wall to deal with both that specific modal issue, and all the other modal issues that involve it, and also deal with SBIR?
That's sort of like saying: "I want to build a bicycle that is also a car and a truck and an airplane and a boat, all at the same time"....
Higer and mid-high frequencies can be deflected… so no need to be absorbed
Are you SURE about that?
For example, how would you deflect a frequency of 440 Hz? That's a mid frequency, for sure! It is middle C on the piano. When that tone hits your wall, would that react like a ray, and bounce back tightly focused at exactly the opposite angle of reflection, from a single point? Or would it act more like a pressure wave, bouncing from a broad area where it was reflected, expanding out in all directions from there? ... tough question...
There is no hard dividing line between ray-like behavior and wave-like behavior. Very high frequencies are mostly ray-like, very low frequencies are mostly pressure-wave-like, but there's a gradual progression across the scale, not a single frequency where it flips. And even then, lows are slight ray-like, and highs are slightly wave-like...
Under a certain frequency deflecting become a non sense … so why don’t we try to absorb them
Are you
sure that deflecting a 50Hz wave is "nonsense"? If so, then what does your back wall do? Are you saying that your back wall will NOT reflect a 50 Hz wave and return it to the room again? And if your back wall can do it, then how come your side walls cant?
so why don’t we try to absorb them
How would you absorb a 50 Hz wave? The wavelength is nearly 7 meters... How thick do you think you'd need to make your absorption to fully absorb a wave that is nearly seven meters long? The quarter wave is 172 cm. ... Assuming you wanted to make your absorption a quarter wave thick, can you spare the space for 1.72 meters of thickness on each side wall to fully absorb that wave? Even if you could, where would you put that, since your entire side wall area at the front of the room is already used up by your slot walls?
We can create a sidewall that ... - Reflect mid/high ... - Absorb lower frequency
No we can't. Sorry. You are thinking about
part of the basic theory, but only a small part, and you are not taking into account reality. It is not feasible to completely absorb a low frequency wave. High frequencies can be reflected fairly accurately in other directions, mids cannot. They CAN be reflected, yes, and so can lows, but NOT in a tightly controlled manner to precise directions. (At least, not with any device that would fit inside a typical studio.) Waves do not behave like a stream of small rubber balls being fired at the surface: that is only true for very high frequencies. Low frequencies behave more like a balloon being inflated and deflated, or maybe like thousands of streams of rubber balls being fired over a wide arc of directions, in 3D. The lower the frequency, the wider the arc.
EDITED TO ADD: To avoid confusion, I should clarify that it is certainly possible to use absorption to
damp low frequencies, just not to actually
absorb them, which is not the same thing... On reading this over again, I noticed that this could be confusing, so I added the comment.
Why you say “ You can absorb some frequencies to a certain extent, yes, but not the low-mids or lows. “ ???
I don't know what is hard to understand about that: It simply is not possible to absorb low frequencies completely with any reasonable absorption material! How would you absorb a wave that is 15m long, in a room that is only 7 m long? I'm not sure what you are questioning here. It is simple to absorb high frequencies, because the wavelengths are very short: just a few cm. Even a carpet or blanket can absorb most of the highs, reasonably well. High-mids are more problematic, but a small thickness of porous absorber can do the job. The middle of the mid-range is tougher, but once again thick, dense porous absorbers with good coefficients in the mid range can do the job, to a certain extent. But the low end of the mid range, and all of the lows are another thing entirely... We are talking about waves that are over a meter in length, and there's no way of absorbing those completely with any reasonable thickness of porous absorption, even if it is low density and has great coefficients.
If you do not believe me, then here's an experiment you can do yourself: take one of your speakers, lay it down on your bed on its back, so it is facing upwards, towards the ceiling, and put a very big, thick pillow or cushion over it, covering the drivers and ports completely. Now play music through your speaker, at typical listening level (85 dBC before you put the pillow on it). Can you hear it? Of course you can! You don't hear the highs at all, you hear some of the high-mids vaguely, most of the low-mids, and you certainly do hear all of the lows, unaffected. You will hear the kick, toms, snare and bass without any problem. Muffled, yes, because you are removing the high frequency component of those sounds, but the low frequencies are still there, loud and strong, coming through your pillow as though it was not even there.
If what you say really was true, then you would hear nothing at all when you played music like that. It would be silent. But since you can still hear the lows and low-mids clearly, there's your proof that it is impossible to stop them with any reasonable thickness of porous absorber. If you STILL don't believe me, then put another pillow on top of that one, and a few blankets as well! But don't run the speaker for long like that, as it will overheat and burn out....
I can REFLECT and DEFLECT higher frequencies
I'm not sure what you think the difference is between "reflect" and "deflect", acoustically. Maybe you can explain that? Deflection is normally accomplished by reflection. I can't think of any other way of deflecting a sound wave, except by reflecting it: I suppose you cold diffract it, or even refract it, but that would be complicated....
for lower frequencies I can REFLECT them but will they be DEFLECTED ??
See above: If you
reflect them, then they have been
deflected! If you reflected a wave, and it did not deflect, then where did it go?
Since frequencies are lower, angled walls appears less angled in relation to wavelength.
Really? Why? Why would the angle appear smaller to a lower frequency? Please explain the physics of that. Why would longer wavelengths "see" a different angle to the reflecting surface as compared to shorter wavelengths? How much does the apparent angle change? For example, lets say I have a wall at an angle of 30° to the wavefront: What angle do the highs see, and what angle do the lows see? Is it maybe 29.9° for the highs and only 17° for the lows? Or maybe 21° for the lows? or only 5°? OR maybe it's the other way, and the lows see 36°? Or 40°? How do I calculate that? Do you have an equation that predicts what the apparent angle will be for each frequency? Does it vary with intensity? Will a very loud 50Hz wave "see" a different angle than a very quiet 50 Hz wave? I have never heard of this theory before, so I'd really like to learn more about it...
If it were true that lower frequencies are reflected at different angles than higher frequencies, from the same surface, then when you looked at the reflection of a rainbow in a mirror, the red part of the rainbow would be reflected at a larger angle than the blue part, so the reflection would be all smeared out, and the reflection of the rainbow would make it look much bigger than it is... But when I look at reflections in my mirrors, all the colors seem to be reflected at the exact same angle....
Yes , I’m aware of that and I’m considering soffit mounting monitors but I see many many rooms that SEEMS to be RFZ with non-seffitted monitors… where’s the trick there ?
There's no trick: They are not actually RFZ! You are correct when you say that they only "seem to be RFZ". But they are not true RFZ, and there will be some reflections at the mix position. If you don't put your speakers in soffits, then your mix position will not be fully free from reflections.
Also, there's the issue of semantics: Did you look at the room in the link I gave you in my previous post? Take a careful look at that room: that one does, indeed, create an almost perfect reflection free zone around the mix position, but it is NOT an RFZ room!
Careful with the semantics here: When I designed that room, I designed such that it creates a complete reflection free zone around the listening area, and provides a low level diffuse field after a suitable delay. But it is NOT done the same way that the RFZ patent or trademark indicates. The rights to the name "RFZ" are legally owned by an individual or corporation, so I cannot say that a room I designed is true RFZ. But I CAN say that it accomplishes the objective of creating a zone around the mix position that is free from early reflections for 20ms and at -20 dB, which is the most important part of the RFZ spec! There are other ways of achieving that, which do no infringe on the patent or trademark, or whatever it is. If you look at the detailed acoustic analysis graphs for that room that I linked you to, you will see that it does meet those criteria, and it also meets or beats all the other criteria from various organizations, for a high quality critical listening room. But I can't call it a true "RFZ" room, even though I can call it an rfz room....
But getting back to your point: One thing I will tell you: if a room does NOT have the speakers in soffits, then it is very, very, very, very, very hard to create a reflection free zone around the listening position... I won't say that it is totally impossible, but it is very complex, and impractical for most studios. For example, it can be done with a true CID design, but that involves complicate 3D geometric calculations and angling multiple surfaces at precise compound angles all over the front half of the room, ... as far as I know there have only ever been a handful of rooms built like that. Impractical. It is also possible to create a reflection free zone like that with huge amounts of very deep diffusion, but once again, it is impractical, and once again there's only a couple of rooms that have actually been built like that. It is far, far simpler to just mount the speakers in soffits, and angle the side walls accordingly. And you can't do it with absorption alone....
And just so that you don't' get the wrong idea: in my designs (and John's too, as far as I know), the soffits themselves (and also the angled transition walls), are not completely reflective: the CENTRAL part is fully reflective, down to low frequencies (it is very hard, thick, solid, dense, massive), but the top and bottom parts (up near the ceiling and down near the floor) are totally abortive. They normally feed into very large bass traps that are hidden under, over, and even behind the soffits.
In short, real-life acoustics is a lot more complex than just a simple set of equations. Each equation can predict certain things, but to get the full understanding of how the room will perform you need to grasp how all of those equations interact in theory, and what the mean for real sound fields, in the real world, as well as taking into account how actual materials and actual construction techniques can affect the perfect predictions of the equations.
If designing a room were simply a matter of following a couple of the most important equations, John and I would be out of a job! It's a lot more complex than that...
- Stuart -
is :
