Does room geometry matter in an absorptive/dead room?

[blunderfonics]

Hi everybody. This is my first post, so I’ll include a little background. For the last 3 years I have been operating a small studio out of a room in a rehearsal complex. I was lucky enough to move into a space that had been previously built out as a studio by the former tenants. Although the room was less than ideal, it has served my purposes well enough to keep pursuing this crazy business venture. Unfortunately, the building that housed the complex has been sold to make way for a luxury condo hi-rise (what else?). In the next couple of months, I will be moving into a new, yet unseen space, but this time the build-out will be up to me. I’ve come here to absorb as much information as possible before I start. Hopefully with a little research I can start off on the right foot with the new room.

We all know that parallel walls are bad for listening and tracking rooms as they contribute to standing waves when the sound bounces back and forth between opposing walls. What happens if those walls are well treated with absorption so as to minimize all that bouncing around…does this negate or minimize the problems caused by parallel walls? I realize that any absorptive treatment is not going to stop all sound 100% across the spectrum so the geometry still matters to some extent, but what if the room is absorptive enough so that the resonances are attenuated well below any direct sound in the room?

I ask because, most buildings are built with right angles everywhere. Basic building materials are also rectangular by design. When building a recording space within one of these rectangles with all those rectangular building materials, you would maximize your available floor-space and materials if you build your rooms as smaller interlocking rectangles. Like most folks, I am working with a budget and am trying to maximize it any way I can. Wasted material and space is wasted money if it doesn’t sound significantly better in the end.

I’m primarily thinking about this with an eye towards the construction of iso-booths, but I am also wondering if this would apply to a small control room. I’ve read in a couple of different threads that when a control room is constrained below a certain size, it would be better to make it’s walls absorptive as reflections reach the listener too soon after the initial source regardless of the techniques we might employ to delay their arrival.

Sorry to be so wordy. I love the site, and look forward to your thoughts.

[len-morgan]

I disagree that "wasted space is wasted money." If you give up a little floor space so that the walls can be splayed, you're likely to end up needing less treatment for the room. You have to remember that all of that absorbtion takes up space too.

Additionally, panel absorbers and such will be seen by the people in the Iso rooms so they need to be finished well to have a professional look. That also takes money. Angleing the walls a little won't cost any more in terms of finish.

Just because the rooms you start with are rectangular, that doesn't mean that the walls you build inside of them have to be the same.

That's my 2 cents' worth.

len

[knightfly]

We all know that parallel walls are bad for listening and tracking rooms as they contribute to standing waves when the sound bounces back and forth between opposing walls.

This is a common misconception; actually, ANY shaped room will have standing waves. The things non-parallel walls and ceilings "fix" are flutter echo and (if done right) first reflection points, so that you can use only the amount of absorption you need to achieve the degree of "liveness" you WANT (and to balance/smooth out modal problems), not the degree it takes to kill these problems with absorption instead of with geometry.

HTH... Steve

[blunderfonics]

Thanks for your comments and for correcting my misconceptions. I think I understand.

So just to clarify: I plan on building a small iso-booth or two designed primarily for separating loud amplifiers from the drums during basic tracking of a typical rock band. For this application I would like these booths to have as close to zero liveness as possible. That being the case, parallel walls are not so much of a problem as I will be trying to kill all reflections. I'm guessing overall dimensions would still need to be evaluated for the best overall modal response and subsequent addressing of any problem areas, correct?

In the case of a small control room, where some amount of liveness would be desireable, I would still need to still endeavor to avoid paralell surfaces whenever possible.

[len-morgan]

When you said "iso-booths" I was thinking vocals not amps. That changes things quite a bit. I would think you could build "inside out" walls with LOTS of absorbtion. Amps don't really care about looks and don't mind if their a little cramped. I'd pick a maximum height for an amp/speaker cabinet then build that (small), really THICK (maybe 3 or 4 layers of drywall) and put a cabinet on top of it for storing mics and cables and such. You'd really maximize your floor space. Then just run a (well calked) cable connection or two from the inside to the outside of it (like instrument in, microphone out) and be done with it.

len

[blunderfonics]

I would like to use them for vocals as well if the design constraints allow, but isolating guitar/bass amps would be their primary function. In any case they would still need to be built as dead rooms, albeit with a little more elbow-room and panache if they were to be used for the occasional vocal.

The idea of short iso-cubbies with storage space above is interesting. I'll have to mull that over while I'm playing with various layout possibilities. Thanks.

[knightfly]

"inside out" walls might not be a bad idea, but you need to be careful to maintain mass-air-mass where isolation is necessary; John's original idea was (I believe) for an entire room, where there was already another mass in place - all that's different is that the wallboard goes on the other side of the studs so you can use the stud cavities for holding insulation in place as ABSORPTION.

Since then, it's been found that insulation INSIDE the wall is ALSO necessary for best performance/isolation (about 10 dB worth of difference)

You're right about overall dimensions of a booth making a difference - generally, odd numbers of feet in all surfaces tends to work better but it's best to calculate for each set of dimensions - then, your absorption doesn't also have to compensate for modal dips as much... Steve

[bpape]

Just to be clear, having non parallel walls will not require any less absorbtion to reach the appropriate RT60 time for a space. What it CAN help with is:

- help minimize flutter echo
- help focus some early reflections away from the mix position in a control room

No matter what the shape is, there is a certain amount of absorbtion required to achieve an appropriate decay time.

[knightfly]

Basically that's what I said earlier, except that I refrained from using the term "RT-60", because a small room will never reach critical distance so technically won't have a true reverberant field... Steve

[blunderfonics]

"inside out" walls might not be a bad idea, but you need to be careful to maintain mass-air-mass where isolation is necessary...

Since then, it's been found that insulation INSIDE the wall is ALSO necessary for best performance/isolation (about 10 dB worth of difference)

If I were to go this route in a 2 leaf system where one of the walls is inside out and the other is standard:

a) Would the insulation on only one side of the cavity effectively damp the airspace or would I need to add insulation to the airspace side of the inside-out wall.

B) Would I need to increase the distance between the studwalls to make up for the decrease in airspace when the sheetrock is moved to the other side of the studs?

[knightfly]

You would still need insulation between the two masses - part of its reason for existence is to damp/absorb the air space itself. In addition, I don't believe it's possible to damp a wall surface too much. Ideally, walls should be levitated in heavy oil for best results... :=)

m-a-m calculations are based on the actual distance between masses, so yes the frame would need to be located further away from the first (outer) mass.

Also, absorption has a slight effect on speed of sound (not sure how much) so would slightly change m-a-m calculations for a wall with or without insulation inside. My calculator assumes standard household fiberglass insulation, with no option to change. (haven't decided to part with $1000 yet)

From a practical standpoint, it's easier to build a standard frame, insulate it, add wallboard, then add a light framework and fill THAT with absorbent. "inside out" was more of a band-aid for severly restricted space where good isolation/acoustics were needed. IF you have more space, you'll get a LOT better isolation with the wider air gap between leaves... Steve