John Sayers' Recording Studio Design Forum Archive

Hello again!

Is there a general consensus when it comes to bookshelves and DVD / CD shelves as diffusors?

I'm starting to jot down some ideas for the layout of things, inside the control room, and I've always had a lot of books, DVD's and CD's around in my control rooms.
I don't expect the shelves (and the content) to do much in the low end or low mid absorbtion dept. but i can imagine there would be some effectiveness in the mid to high freqency diffusion?

I wonder if I could do a bit of a 'mix and match' when it comes to diffusers, absorbers and bookshelves?
Not to replace what would be needed for a proper RFZ, per se, but have them co-exist, for aesthetic reasons, at least.
I can see (in a lot of pictures and designs) that there is quite a bit of real estate towards the front of the control rooms, where 2-3 decent sized absorbers are hung on the walls on either side of the listening position, and then the bare wall in between.

This, to me, would be an aesthetically interesting place to build some book/cd shelves.
(as opposed to the "four squares, evenly spaced" look.

Any thoughts on something as potentially broad and un-answerable as the "acoustical properties" of a book/cd shelf?

Is there a general consensus when it comes to bookshelves and DVD / CD shelves as diffusors?

Yes: The general consensus is that they don't work! You'll only see those "recommendations" from places that don't understand how diffusion actually works (or do understand, but don't care...).

First: real diffuses are tuned. They consist of a series of "wells" that lie below the acoustic "face" of the device. The effective low frequency limit of the diffuser is determined by the maximum depth of the wells, and the highest frequency is determined by the well width. The well depth should be 1½ times the wavelength for the desired lowest frequency. The well width should be half of the wavelength for the desired highest frequency. Do the math for a typical book size, or CD case size, and see how well the "diffusion" that you'd get from such a device, lines up with the needs of a typical room...

Second: real diffusers follow carefully arranged mathematical sequences to optimize the diffusion, and ensure that there are no "lobes" in intensity, frequency, or phase. A random arrangement of CD's on a rack will NOT produce a useful diffusion pattern. A lot of research and testing has gone into the mathematical design of diffusers. You might find this book interesting, if you'd like to really understand the subject: https://www.amazon.com/Acoustic-Absorbe ... 0415471745 It's THE definitive book.

I'm starting to jot down some ideas for the layout of things, inside the control room, and I've always had a lot of books, DVD's and CD's around in my control rooms.

How long is your control room going to be? If it is not long enough, then you cannot successfully use tuned diffusion at all! That same book above will explain why it is a really bad idea to have a diffuser within ten feet (3m) of your head. But here's a more intuitive diagram of why this is a bad idea:
That shows the typical lobing pattern created by all tuned diffusers. As you can see, close up to the diffuser there's a huge amount of "chaos" in the diffusion: moving your head a little in that area will give you a very different frequency, timing, and intensity response, so much so that your brain wont be able to accurately determine directionality and frequency response. It's only when you get far away form the device (right at the top of that image above), that the sound field truly becomes "diffuse". According to the theory, that distance is AT LEAST ten feet, for all diffusers, and could be even more, depending on the tuning. If the lowest scattering frequency is very low, then you also need to consider three times the longest wavelength as the critical distance. So it could be more than ten feet, but never less.

However, you do often see small control rooms with large and very expensive tuned diffusers on the rear wall, beautifully made from exotic woods, but just a few feet behind the mix engineer's head.... or even worse, where the client couch is just a few inches in front of a diffuser! Clearly, some people have a lot more money than they do actual understanding of acoustics...

So, your room would have to be long enough to be able to use diffusion. That implies at least ten feet, plus the distance from the front wall to the mix position, plus the depth of the bass trap behind the diffuser... That implies a MINIMUM room length of about 19 feet. Is yours big enough?

I don't expect the shelves (and the content) to do much in the low end or low mid absorbtion dept. but i can imagine there would be some effectiveness in the mid to high freqency diffusion?

Diffusers don't absorb. Well, they do actually, but only a little, and that's not their purpose. What they do is, they diffuse! (duh!). So what is diffusion? How is it different from absorption? Simple: with absorption, the device physical removes some of the acoustic energy from the room. The sound wave energy is converted into low-grade heat energy by the internal structure of the absorber. So after the wave hits an absorber, it leaves with a lot less energy. And each time it hits, the energy goes down again. That's why small rooms often sound "dead": because there is too much exposed absorption that is sucking the life our of the room.

A diffuser does NOT remove energy from the wave: rather, it scatters and re-directs that energy in multiple different directions, all with the same intensity, frequency, and timing changes. So the TOTAL energy remains the same, but instead of traveling in one single direction like it did when it arrive, it is sent off in many different directions, each one with a much lower energy... but if you add up all those "lower energy" reflections, you still get to the same total energy as the original wave had (with only slight losses). A diffuser spreads the original wave around smoothly, evenly, and consistently... but not up close. Up close, the effect is NOT smooth, even or consistent! You have to be far enough away (ten feet....) before you are in the truly diffuse field. Your brain DOES need that diffuse field to make sense of the room, and feel comfortable in it.... which is why early room design philosophies, such as LEDE, didn't work very well. They were found to be "fatiguing" and "uncomfortable" for long sessions, due to the lack of sufficient diffuse field at the mix position.

So, that's my long lead-in rant to answer your comment: Yes, your randomly placed CDs and books will certainly have "some effectiveness in the mid to high freqency", but the effect would be lousy: it would not be even, not smooth, not consist... at ANY distance. And it would be tuned to the wrong frequency range for your room anyway.

I wonder if I could do a bit of a 'mix and match' when it comes to diffusers, absorbers and bookshelves?

You will undoubtedly need a LOT of absorption! All small rooms do. The smaller it is, the more it needs in the low end. That's a given. It goes without saying. If the room is large enough, then you MIGHT be able to use tuned diffusion, such a Schroeder's, QRD's, PRD's, binary arrays, etc. Otherwise, you'd be restricted to reflection. See this thread : http://www.johnlsayers.com/phpBB2/viewt ... =2&t=21368 to see how the procedure goes for tuning small control rooms that are too small for numeric-series diffusion. Note: no bookshelves or CD racks!

Not to replace what would be needed for a proper RFZ, per se, but have them co-exist, for aesthetic reasons, at least.

For a true RFZ design, you DO need diffusion! But if the room is not big enough, then it cannot be achieved with numeric-based diffusion. It pretty much has to be purely reflective, or poly-cylindrical based. See the room below...

I can see (in a lot of pictures and designs) that there is quite a bit of real estate towards the front of the control rooms, where 2-3 decent sized absorbers are hung on the walls on either side of the listening position, and then the bare wall in between.

Are you SURE it is bare wall back there? If so it is not a true RFZ room... What you often see in photos of finished rooms is something that LOOKS like a wall, but really isn't a wall at all.

For example, take a look at this photo (from the most recent post on his thread):
See that nice solid back wall there, between the two slotted poly diffusers on each side? That's not a wall! Read the thread, and you'll see that there's several FEET of treatment behind it. And that treatment is doing a lot of the "magic" that I needed to tune the room, as you can see from the graphs.... It's a complicated room to tune, but by careful use of the right things in the right places, it is coming on nicely... (and no bookshelves or CD racks! : )

This, to me, would be an aesthetically interesting place to build some book/cd shelves.

... and each time you take out a book, or re-arrange your CD's, or buy some new ones, or lend some books, the tuning of your room would change... Probably not a good idea, if you hope to meet the ITU BS.1116-3 spec!

(as opposed to the "four squares, evenly spaced" look.

I don't think you could accuse that room of having a "four squares, evenly spaced" look.! To me, it looks rather organic, nicely curved, with no even spacing anywhere... it's pseudo-exponential spacing on the slats for the slotted poly, for example.... which is why we are now getting that nice even diffusion that you can clearly see in the last set of graphs I posted there, comparing the "before" and "after" impulse response images...

Any thoughts on something as potentially broad and un-answerable as the "acoustical properties" of a book/cd shelf?

The acoustic properties are known, broadly, but are unpredictable in reality, since every time you move a book, the entire pattern would change. If you really wanted to use this idea, you could take your bookshelf and books to a place like Riverbank Acoustic Labs and have it tested in their highly specialized, world-class testing labs. You could even test a whole series of different book sizes, position, locations, depths, patterns, etc. It might be a bit expensive though: it costs thousands per day to do that, and you'd need many days.

But for a more definitive answer: Take a look at photos of the high-end control rooms from around the world that you see right here on the forum (designed by John and others), or in magazines such as Sound on Sound, Mix, Tape Op, Pro Sound, and others.... How many of the world-class control rooms you see there use bookshelves and CD racks as their acoustic treatment? I think that should answer your question...

One other minor issue: diffusers have to be sealed in order to work effectively... In other words, you'd have to glue all your CDs together, and all your books too, so that there are no air spaces between them, above them, or below them... That might not be much of an option, if you ever wanted to read those books again, or play a CD!

The ONLY place you might perhaps maybe could be try something like that is in a bedroom studio with no budget at all, and where there's no real possibility of getting decent acoustic response out of it. They might provide a tiny bit of benefit, but for the type of pro-level studio you are talking about, that's out of the question.



- Stuart -

Hello again Stuart

and thanks yet another thoughtful reply.
really invaluable, all this information!!

yeah, of course this is all in the planning stages, still, and who knows what the "available real estate" will end up being.
i.e. what (if any) space I will end up with, that does not to be treated (on the walls).

that room you worked on really looks like a "rabbit out of a hat" situation!
well done!!
what an extraordinary amount of work = and some pretty remarkable results!!

i suppose maybe i was a bit unclear in my wording, re: the bookshelves.
it was meant to be more along the lines of: "If i find myself with some available blank walls btw absorbers in the RFZ (if that's even a possibility) what would it do to the sound if i used that space for some bookshelves"

in other words, and like i said in the other post, i'm not nescessarily trying to replace proper diffusers with bookshelves (and certainly not aiming to replace proper absorbers) but rather: wouldn't it be a viable option (and nice aesthetically and for my clients) to have a couple of bookshelves in the control room, as opposed to a blank (untreated wall). Hence the "What are the acoustical properties of a bookshelf" question.. even if it my have sounded a bit shortsighted.

and, like i said at the top, this is all theoretical at this point.. who knows what I'll end up needing as far as wall and corner treatments.. this will only become clear after i nail down the dimensions of the control room, and the obviously, manage to actually measure the real thing after the build - and then bring the graphs here to you guys for help !!

btw.. the current plan for the control room is 20ft W, 29ft L, 11.5ft H (with some flexibility on the Length, based on ratios, possible bass trapping needs etc - i.e. could be longer if i feel i need 4 feet of bass traps at the back end, and thus wanted more space...or could be shorter if i feel like it does't look good on paper according to Amroc / Bonello - or you )


thanks again!!

You might find this book interesting, if you'd like to really understand the subject: https://www.amazon.com/Acoustic-Absorbe ... 0415471745 It's THE definitive book.

And thanks for this, Stuart!
I'll have a look and see if it's available at the library!

take care

In talking to HVAC people ("specialists") I'm getting conflicting reports on where to put supplies and where to put returns (believe it or not)

What are your thoughts on the control room airflow designs?
Would the concensus be to have more cold air enter where most of the heat is generated (in my case, the front end of the control room, where there is more gear and a large window - with sunlight) and have the air then pulled out from the rear of the room?

Or, have the cold air distributed more evenly, and have the returns over the heat generators, so suck out the hot air?
Obviously having more people in the room would change the 'heat generation' somewhat, but at least the gear is a constant, and there is always going to be more heat generated in the front..

so?

Is there ANY way to get sprinklers into the control room and tracking room - through two leaves of our beautifully designed, and acoustically sound, walls - without all our hard work going to heck?

I'm trying to remember, in the studios I've worked in, what the sprinkler situation was.. never really paid much attention to that side of things...

I wonder if it would have to be a separate system - for each room, coming in through the roof?? The building has concrete ceilings (roof) so that might be an option?

In talking to HVAC people ("specialists") I'm getting conflicting reports on where to put supplies and where to put returns (believe it or not)

What are your thoughts on the control room airflow designs?
Would the concensus be to have more cold air enter where most of the heat is generated (in my case, the front end of the control room, where there is more gear and a large window - with sunlight) and have the air then pulled out from the rear of the room?

This is actually fairly simple, but so many people get it wrong. Especially even HVAC installers who do ordinary houses / shops / offices / etc, but have no idea that studios are sealed systems!

Where it the WARMEST part of the room? That's where you want to suck out the WARM air! Simple. That's usually at the front of a control room, but could be in any pf several places for a live room.

So where does the inlet come in then? As far away as possible from the outlet! In order to move as much of the room air volume as possible through the system. So if your return registers are above the speaker soffits at the front of the room, then the supply registers should be at the rear of the room, and oriented to blow the air AWAY from the return vents: in other words, towards the side walls, or the rear wall, but not towards the front wall. Once again, to get the most even air flow throughout the room.

But why do we want cold air coming from the ceiling, and also suck hot air out at the ceiling? Some people seem to think that registers need to be placed down low, or mid-wall, or in other strange locations. but once again the principle is simple: Warm air rises, cool air sinks. So if you pump in cool air at ceiling level, it will descend to floor level of its own accord, simply because it is COOLER than the room air. And as it flows through the room and gets warmed up by the gear and people, it will rise all by itself as well... to the highest point, at the OTHER end of the room, where it can be sucked out by the return registers..

Obviously having more people in the room would change the 'heat generation' somewhat, but at least the gear is a constant, and there is always going to be more heat generated in the front..

Right... which is why you need sensors and a controller to take care of running the system at the correct fan speed, to get the correct air flow rate for that room, with the correct dehumidification, and the correct cooling, as well the correct amount of stale air exhaust (percentage of recirculated air), plus the same amount of make-up fresh air, to ensure that the CO2 levels don't rise too high, and the humidity is held near 40%, and the temperature is correct... throughout the entire room...

Is there ANY way to get sprinklers into the control room and tracking room - through two leaves of our beautifully designed, and acoustically sound, walls - without all our hard work going to heck?

Yes. With flexible couplers at the leaf surfaces.

Darth Fader wrote: I'll have a look and see if it's available at the library!

Be warned: It's heavy going... not for the faint of heart. Lots of fun math and equations and stuff, along with the text...

- Stuart -

This is actually fairly simple, but so many people get it wrong. Especially even HVAC installers who do ordinary houses / shops / offices / etc, but have no idea that studios are sealed systems!

Where it the WARMEST part of the room? That's where you want to suck out the WARM air! Simple. That's usually at the front of a control room, but could be in any pf several places for a live room.

So where does the inlet come in then? As far away as possible from the outlet! In order to move as much of the room air volume as possible through the system. So if your return registers are above the speaker soffits at the front of the room, then the supply registers should be at the rear of the room, and oriented to blow the air AWAY from the return vents: in other words, towards the side walls, or the rear wall, but not towards the front wall. Once again, to get the most even air flow throughout the room.

But why do we want cold air coming from the ceiling, and also suck hot air out at the ceiling? Some people seem to think that registers need to be placed down low, or mid-wall, or in other strange locations. but once again the principle is simple: Warm air rises, cool air sinks. So if you pump in cool air at ceiling level, it will descend to floor level of its own accord, simply because it is COOLER than the room air. And as it flows through the room and gets warmed up by the gear and people, it will rise all by itself as well... to the highest point, at the OTHER end of the room, where it can be sucked out by the return registers..

Great stuff, Stuart. Thank you!
(looking forward to some "spirited" conversations with my HVAC guys... )

and re: the Sprinkler systems. Amazing! I had no idea this even existed. Thank you!
I can see how it would be advantageous to allow for a little extra room here and there, btw the leaves, for things like this - as well as HVAC silencers, etc.
For example if i had a little extra wiggle room between the two leaves separating the control room from the hallway (the outside world) and the same for the tracking room - i would try to design the sprinkler systems so they enter from a 'convenient' point in the design (if there's even any flexibility in doing that, within the building code) - as opposed to having to cross through and between multiple recording spaces.

onward!!!

PS - yeah, that book, I'm sure, is on the dense side. hence the library option ...

Hello again

doing some math on the HVAC system, wondering if you guys could have a quick look over my numbers, to see if I'm on the right track?

(this is for the proposed control room)

Size: 20x30x11.5
Volume: 6900 cubic ft
x6 changes per hr: 41400 cubic ft
divide by 60 mins: 690 (cfm?)

according to my online duct calculator, given the target would be 300 fpm at registers, the ductwork inside the control room would need to be somewhere close to 20.5 in diameter, for a round duct, and 10x30 for rectangular ducts.

this sounds pretty substantial for a 20x30 room.
is this correct?
(not sure i've ever even seen a 20" round duct, in person )

or... would this be split before entering the control room?
and that then bringing the total number of silencers to 8, for a single room??

ps (edit) - actually, just got out my measuring tape and looked around the room a bit... 20" in diameter, for a single duct, doesn't actually look crazy.. it just sounds kinda outrageous on paper.. so maybe a single 20" supply wouldn't be quite as Hindenburg looking as i was afraid it would be

or, maybe a single 10x30 duct down the middle - or halfway - would look pretty ok as well.

Back to the Modes and room tuning theory - for a quick sec.

I've done a lot of reading here, and as before, TONS of great info!!

I feel like I'm getting a fair grasp of the whole MODE thing.. room sizes, ratios, absorbers vs diffusers etc etc.
It's slow going, but I'm happy to do the research.

- It's my understanding that every room has modes, and that's all good.
- it's my understanding that in designing a room the more evenly distributed those modes are, the better (if we focus on the 200hz and below, for now)
- It's my understanding that bass traps' primary goal is to convert low hz energy into heat - lessening the "ringing"/ RT / sustain of said modes, and thus making the listening room more accurate.

where i get lost is: when there is a 'gap' in the modes (invariably, in every design), for example room "x" has a dip at 123hz, that's VERY audible, and troublesome, how is that treated? Or, more accurately, how does bass trapping adjust for that dip?
or does it?

with my engineering hat on, i see that frequency plot of the room as an eq curve, and if "broadband low freq absorbtion" is applied - i.e. bass traps that affect 200hz and below, somewhat evenly, my engineering mind would say: if the "peaks" are being brought down by x dB, wouldn't that dip (at 123hz) come down by the same amount??

short version: how do the bass traps "know" to not bring that 123hz dip even further down??

sorry if this sounds obtuse
i'm really giving it my best here.. reading, sifting, doing the research the best i can

hope you all had a great weekend!
cheers from L.A.

- It's my understanding that every room has modes, and that's all good.
- it's my understanding that in designing a room the more evenly distributed those modes are, the better (if we focus on the 200hz and below, for now)
- It's my understanding that bass traps' primary goal is to convert low hz energy into heat - lessening the "ringing"/ RT / sustain of said modes, and thus making the listening room more accurate.

More or less correct, yes. Technically, the bass trap is an acoustic damper, and yes, one of the jobs it does is to convert sound energy into heat energy.

where i get lost is: when there is a 'gap' in the modes (invariably, in every design), for example room "x" has a dip at 123hz, that's VERY audible, and troublesome, how is that treated? Or, more accurately, how does bass trapping adjust for that dip? or does it?

Depending on the CAUSE of that dip, and the location of the treatment, it might make a big difference, or it might make no difference at all. Look at the two frequency response graphs here: http://www.johnlsayers.com/phpBB2/viewt ... 8&start=33 , just before and after he put the ceiling insulation in (graphs # 5 and 6 in that post). You can clearly see that there was a large dip at 144 Hz, but after adding the insulation to the ceiling, it improved very nicely, RISING in level by about 8 dB. On the two graphs below that (the RT60 graphs), you can see that the decay times around 144 Hz dropped from over 400 ms to around 300 ms. That's a clear-cut case of a bass trap doing it's job, and damping a problem.

my engineering mind would say: if the "peaks" are being brought down by x dB, wouldn't that dip (at 123hz) come down by the same amount??

Yet that did not happen! Therefore, it must be ... MAGIC! The truth is out! We made that insulation from Unicorn tail hair!

OK, seriously, you need to put that engineering hat on a little tighter, and start thinking of the entire room being filled with waves that move in three dimensions. Some of those waves are "standing waves", where the wavelength happens to fit in exactly between a pair of walls, or four walls, or all six "walls" (where ceiling and floor are also considered to be "walls"). Where that happens, the wave appears to NOT move around the room, but rather to stand still with the peaks and nulls for that specific frequency always occurring at the same locations in the room. If you could see the entire wave, it would appear to be stationary in the room (even though it is NOT stationary: the energy does still move).

So, looking at your visible wave, you would see some parts of it having high intensity (peaks), while other parts have low intensity (nulls). With me so far? The problem when you look at a REW graph, is that you are looking at only one very tiny spot on that wave, not the entire room. The mic is sitting still at one precise point, so it only "sees" how the wave looks from it's perspective at that point. It CANNOT see the entire wave, because it is a very simple device: all it does, is to detect the sound pressure level at that one spot, and it "knows" nothing at all about the rest of the room.

OK, so if the mic happens to be positioned in the NULL of a certain wave, then it only sees the very low intensity at that one spot, but there are OTHER spots in the room where the rest of the wave is located, and those will be the PEAKS. But the mic doesn't know that: it only sees the null at the point where it is located.

In other words, what you see on the REW graph only tells you about what the waves are doing where the mic is, even though they are doing many other things at other locations in the room. So, that pressure null that you see is one "end" of the wave, and the other "end" is at some other point. And if you put a bass trap at the "other" point, where the peak is, you will be removing energy from that peak, thus removing energy from the wave, so the null won't go down: it will go UP! Removing energy from a wave implies that the extremes become reduced, and the wave tends towards a lower energy level all over. That includes the "null", which actually reflects something very different from what you think it does...

A little more: Because you have your engineering cap firmly in place, you already know that the total energy in that wave CANNOT just disappear and re-appear at a different spot: the total energy must remain constant at all points. And since the measurement mic only sees the "pressure" component of the wave, and that varies all over the place, the obvious conclusion is that there must be ANOTHER component to that wave, besides pressure. And there is: velocity! IF you consider the wave at it's most basic level, it is simply energy moving through air, and they way it does that is by causing the air molecules to bang into each other, thus transferring energy from one ,molecule to the next. so each time a molecule hits its neighbor and transfers it's energy, it bounces back to where it was before: It doesn't actually move; it merely vibrates in place. If you imagine how that particle moves, you'll see that at some points in its "vibration cycle" it is going very fast, while at the point where it hits its neighbor and bounces back, it actually has no speed at all. At some point, it stops completely, then starts going back the other way, picking up speed again. However, at the point where it is totally stopped, it is also applying maximum pressure to the neighboring particle, causing that one to start moving. So. two things are happening in the wave: at any given point, the TOTAL energy is a combination of the VELOCITY energy and the PRESSURE energy. At some points along the wavelength, the velocity will be high but the pressure will be low, and at other points the PRESSURE will be high while the VELOCITY is low. But the total energy always sums to the same amount.

So, that brings us back to the measurement mic: since it is a pressure-based mic (electret condenser capsule), it only knows about the PRESSURE component of the wave as it passes, and that's all it can register. It has no way of knowing what the velocity component is. If you wanted to know that, you could out a ribbon mic at the same point, since ribbon mics are velocity based, not pressure based. If you did that, you'd see that where you have a PRESSURE null in the room, you also have a VELOCITY peak, and vice versa.

OK, so back to bass traps: The idea with pure absorption bass traps is that you want them in the high velocity area, since they are also velocity devices. Assuming you have a good bass trap at a point in the room where it will experience the velocity peak of the wave, then it will be really good at removing energy from the wave, turning it into low-grade heat. Now, once you take some energy out of the wave, you have reduced the TOTAL energy, not just the velocity energy. Sure, what you took out was in the form of velocity, but at some point further down the wave, where the energy types interchange again, you'll notice that the pressure energy is now also lower: once again, because the TOTAL energy in the wave is the combination of velocity energy PLUS pressure energy, at every single point.

So, in other words, by absorbing velocity energy inside the bass trap, that will show up as s reduction in pressure energy at the measurement mic. And since the "null" in pressure energy directly means that there is a peak in velocity energy at the same point, and the total energy is the sum of the two, which is now LOWER than it was, therefore the null will also be "lower". Let me put that in other words: the null was "BIG", because it took a lot of energy to create it, but sine the wave has lost energy, the null is now "smaller", meaning the there is LESS of a null there: the level is higher.

Long-winded rant to explain a simple phenomena. Hope I didn't confuse you even more!

The key to understanding the REW graphs is t realize that it represents what is happening at only one single point in the room. If you move the mic around to different points in the room, you will see very different things in the graphs. That's one of the techniques that I use to analyze problematic rooms: I take multiple measurements at carefully selected intervals and key spots around the room, then look at all that individually and in various combinations, to understand how the waves are moving around the room. That can get complicate, but you gain a MUCH better insight into the room behavior.

short version: how do the bass traps "know" to not bring that 123hz dip even further down??

Short answer: They don't!

Porous absorption bass traps are not tuned, so they don't affect individual frequencies differently. All they do is to absorb the energy of the waves as they pass through. They are slightly better at absorbing some frequencies and slightly worse at others, but there's not a huge difference for similar frequencies. So that trap that absorbs the 123 Hz dip will do a pretty good job on the dips at 157 Hz, and 203 Hz, and 98 Hz, and all others in between. All they do is absorb energy, and since the bass problems in small rooms is pretty much always due to "ringing", which is excess resonant energy beyond what is being produced by the speakers as direct sound, they are pretty good at dropping that WITHOUT affecting the direct sound.

Of course, all of this assumes that we really are talking about room modes, which are resonant issues. If we are talking about phase cancellation effects, such as SBIR, comb-filtering, reflections, and suchlike, then bass trapping won't make a lot of difference. Those need to be attacked in other ways.

And another "of course": you'll only see a reduction in ringing for those modal issues that are associated with the location where you put the bass trap! For example, if you your modal null at 123 HZ happened to be associated with the ceiling-floor axis, and you only have bass traps on the rear wall, then nothing will happen! You won't get any reduction (or very little), because that wave never "sees" the back wall, so it can't get absorbed there...

sorry if this sounds obtuse

Not nearly as obtuse as my explanation above sounds! I wrote that too fast, too tired, not feeling well, and taking strong medication, sew iph id duzent maek two mush senze, thenn yull kniew whie! I can try again tomorrow, after I get a good night's zleeeppppp......


- Stuart -

Quote:
sorry if this sounds obtuse
-----
Not nearly as obtuse as my explanation above sounds! I wrote that too fast, too tired, not feeling well, and taking strong medication, sew iph id duzent maek two mush senze, thenn yull kniew whie! I can try again tomorrow, after I get a good night's zleeeppppp......
- Stuart -

Not obtuse at all, Stuart!
Quite the contrary.. very helpful!

What I've learned so far:

1- Use Amroc (or similar software) to determine - if nothing else - the room sizes/ratios to stay away from. AND use as a guide to select best room ratio (or the closest to 'best' you can get, within given parameters.. size, shape, ceiling height, money..)

2 - User REW (or similar) to measure room after build, and with basic (previously designed) bass trapping, absorbers and diffusers in place

3 - Take REW info (graphs) back to Amroc, locate problem areas on the modes chart (that one peak at 124hz right in the listening position, for example) and use that to determine location of further treatments (i.e. btw floor/ceiling, btw front/back etc)

This, of course, is massively simplified.. but that's how it looks to me, at 'this point in my career'

I had a disconnect btw Amroc/Modes, on the one hand, and Room response curves/graphs on the other.
The inclination - for me - was to look at modes as PEAKS only and not both.
And thus, go looking for spots on the Amroc graphs with no modes - which is incorrect, in a way.

Onward!

OK - update / mini rant

Custom doors are INSANELY overpriced ($9700 for a "sound proofing door" including installation and shipping)
So, building my own.
(the deadbolt for said doors is $750, so i suppose i could skip that...)

Also.. Glass: I'll need a lot, and (yes, you guessed it - not cheap)

I found a local glass place when i built my last studio, and i checked in with them again (along with 8 others, nationwide) and they're still VERY reasonable.

They quoted me $230 for a 36" x 60" piece of PVB laminated glass at 3/8" thick. ($578 for the 5/8")
That seems like a really good deal to me.

Can someone confirm this?
Or care to chime in?

Cheers from sunny LA

Sounds about right!

Greg