Acoustics Primer

I just posted this over in the Gear section as a response to a specific question, but it lies buried deep in thread, and it took a bit of work to write, so I am also posting it here.

This is a specific example of a typical "bedroom studio"...the dimensions of your room may differ but the general concepts hold true for any smaller room with an eight foot ceiling.


Assuming that your ceiling is 8 ‘ high then the first four axial room resonance modes occur at the following frequencies (in Hertz):

1st 2nd 3rd 4th

8’ 71 141 212 283

12’ 43 87 130 174

13’ 47 94 141 188

They do not stop there, just keep multiplying the first number by whole number integers to find all the harmonics of these dimensions. These numbers represent the “standing waves” that will build up and completely screw up the low frequency response of the room. They cannot be overcome with equalization.

These lower frequencies are a real problem to deal with. You must choose a method for absorption of these powerful waves. They are most often treated with dense foam corner traps. This works and is relatively inexpensive. It is also easy to install.

Another solution is the membrane absorber. This involves placing a flexible membrane of known density (plywood, or compressed fiberglass) across a cavity of specific depth so as to create a type of “drum” that will resonate at the axial room mode frequencies. This will effectively absorb the low frequency energy and prevent the resonance from building up in the room. You can build or buy these.

You can build a simple broadband membrane absorber by screwing and gluing pieces of 1”x2” to the wall at less then 4” apart. Then take a 4’ wide piece of ¼” or 1/8” plywood and jam it in between the 1” x 2” ‘s, so that the plywood is forced to curve outward from the wall. Glue the joints or caulk them. You can lightly fill the cavity behind the plywood with fiberglass insulation; or if you prefer, cotton batting, wool is actually the best, but it tends to settle, as do all the natural fibers. The deeper the curve the lower the frequencies you will affect. The beauty of this construction is that the gently curving surface also makes a great midrange diffuser. Some people will also cover the surface with fabric to help absorb higher frequencies. Do not use heavy fabrics, as this will dampen the absorber’s membrane, lessening its effectiveness. You must cut little plywood caps for the ends and glue them on to seal the enclosure. These structures can be placed either vertically of horizontally. They look cool and they work. Build a bunch; don’t forget they can go on the ceiling too!

As a side note, bookcases full of books make rather effective low frequency absorbers, (and the random spines make pretty good diffusers). Overstuffed cushy sofas or chairs don’t hurt either.

The midrange and higher frequencies require a different approach, once we get over 500 Hz we require the judicious use of both diffusion and absorption, somewhere in the 50/50 range. Hard parallel surfaces are not a good thing. Reflections between them must be prevented.

Absorption can be of the foam variety. Once again easy to buy and easy to install. Heavy curtains qualify as high frequency absorbers, although they are probably not very effective below 1000 Hz. Carpet works.

Diffusion is the most neglected of the acoustic tools, yet when done properly it can make a big difference. Diffusion breaks up the sound waves, thereby preventing them from continuously bouncing back and forth and building up into “flutter” echoes, “standing wave’s” higher frequency equivalent.

There are products that are scientifically designed to diffuse a wide variety of frequencies. Stick ‘em on the wall and away you go. Yes, egg cartons will diffuse sound, if they are free give some a try.

In essence what you want is to change a smooth reflective surface into a rougher bumpier type of surface. Protuberances. You want lots of stuff that sticks out. This could be art, the above-mentioned bookcase full of books, or whatever breaks up the surface and adds dimension to the wall. You could randomly cut up a hundred pieces of 2”x2” to lengths between 1” and 6” long, and then glue them in an attractive geometric pattern to a sheet of plywood and hang it on the wall (or ceiling), a la Prince’s Paisley Park control room.

The real trick is balance. You don’t want to overdo the absorption. Dead rooms are not useful and are uncomfortable to be in. Better to err on the side of diffusion than absorption.

Chaz mentioned first reflections. Ideally the path between you and your audio monitors should be free of hard reflective obstacles. Obstacles between you and the tweeter will cause acoustic reflections, and create a series of ever changing comb filters which shift around the mid to high frequency spectrum as you move your head forward or side to side.

As to the distribution of these various methods about the room:

Bass trapping, as much as you can afford or take the time to build.

Absorption should definitely be on the wall behind the speakers at ear level, say the middle four feet of the wall. Above this should be diffusion. Below the strip of absorption should be bass trapping. The corners behind and above the speakers should be treated with bass trapping. The wall opposite the speakers should have bass trapping in the corners and lots of diffusion. The wall on either side should be a 50/50 mix of diffusion and absorption, and could include additional bass trapping along the upper corners.. If the floor is carpeted then put lots of diffusion on the ceiling, most especially above the mixing position.

R

I just posted this over in the Gear section as a response to a specific question, but it lies buried deep in thread, and it took a bit of work to write, so I am also posting it here.

An absolute "must read", for anyone, regardless of experience level.
Great Job Nprime.

-Donny

Thanks Rod!

I will have to read it a couple times to get some of the stuff, but It saved me from posting a redundent thread.
I've got it bookmarked

Thanks again
joe

Greetings Rod,

Thanks for the information, but I think the title is misleading, it should be something like "Primer on Small Studio Standing Waves"

If you edit the post to include a picture, it would be perfect!

Regards,
David

Rod,

Is a heavely texured wall going to make any kind of differance as far as diffusion?

Thanks
joe

ORIGINAL: zgraf

You can build a simple broadband membrane absorber by screwing and gluing pieces of 1”x2” to the wall at less then 4” apart. Then take a 4’ wide piece of ¼” or 1/8” plywood and jam it in between the 1” x 2” ‘s, so that the plywood is forced to curve outward from the wall.

Can't quite picture how the 4' piece of plywood is to be "jammed in". Can you post an illustration?

I'll second that

You jam the plywood in so that it bows towards you into a semi-circular shape. In other words you are creating a rounded suface that will deflect the waves in a direction other than towards the other wall. This helps to break up standing waves.

1/8" ply will do and it's even easier to bend.

R

Can be either way. It's best if you make them all slightly different actually...alternate small, medium and large spacings. You may have to cut the plywood for the smaller spacings. These can be both horizontal and vertical BTW. It's a great corner treatment as well, anywhere two surfaces meet (wall/wall or ceiling/wall). In this case you will make a quarter of a circle arc.

Thanks for the illustrations.

R

Joe,
Unless I misunderstood the question, my acoustics 102 prof, or The Master Handbook of Acoustics, your textured wall will probably only diffuse very high frequecies. How deep the texture is will correspond to the wavelength of the frequencies that will be affected. The frequency of 1", for example, is roughly 44,304 Hz. What the texture will probably do is slightly add to the mass of the wall, which will alter the absorption and STC characteristics. In terms of diffusion, you are better off adding the bookcase with random size books. Personally, I wish I had the space (and an understanding wife) to put up the absorbers nprime describes.

I'm expecting you to bend them so that they stick out 6" or more, even 12" for a full sheet of plywood. I assure you that it will diffuse down into the midrange frequencies that way.

You know, if you use a nice veneer these things can look very attractive with a coat of polyurethane on them.

R