OK, now I'm going to talk about acoustic treatment of a recording room!

The first question you must ask yourself, is if the room is actually big enough to support a nice live acoustic environment in the first place. I'd say anything less than 300 sqare ft, preferably with a high ceiling, is pushing it. You can still go for a semi-acoustic space in a smaller room with standard ceiling height, but the smaller the room, the deader it needs to be to avoid coloring the recordings too much.

If the "room" is barely more than a vocal booth, then you need to KILL it completely! Line walls and ceiling with 4 inches of glass fiber (or your preferred damping material), and put a rug on the floor!

Such a booth will STILL color recordings in the lower midrange and upper bass because of air resonances between parallel walls and the only thing you can do to keep that in check, is to keep the microphone relatively close to the sound source.

You can do all your recording in such a booth and use a convolution reverb to simulate a live environment. Few will notice the difference in the end result, but if you have the space for a proper live or semi-live recording room, go for it! It's MUCH nicer to make music in!

Acoustics in a room has a tendency to take care of itself and turn out great, as soon as you take care of the problem areas. There are three things you need to watch out for:

1. Air resonances in the bass and low midrange.
2. Lack of diffusion in the mids and highs.
3. A nice, even damping from the low mids to the highs.

The first is handled by a 2-step procedure. FIRST you make sure the proportions of the room are as irregular as possible. An assymmetric room with a funky shape is actually best, but if you need to have an ordinary rectangular room, at least make sure the porportions aren't too simple.

A 15x15x15 ft room will produce the exact same air resonances in the x- y- and z-directions, which leads to booming resonances, with deep non-responsive zones in between. Such a room is practically impossible to get to sound good!

A room with irrational proportions is best, like 11x23x31 ft. The key is that there are no common small number factors in the proportions, that ensures that the air resonances hit different frequencies.

The SECOND thing to do, is to add bass traps. Probably!

If the inner shell of the room is made from a relatively pliant material such as wood or gypsum board, then it may already have sufficient damping in the low-mid range. If it is made from concrete or bricks, then it absolutely needs bass traps though, or you'll get an uncontrolled bass response in the room.

A bass trap is simply a wide "pipe" of arbitrary cross-section shape, stuffed with glass fiber, closed in one end and open in the other, and that has a length of 1/4 of the wavelength of the frequency you're intending to "trap".

There's little need for complex calculations here, because the primal air resonance mode between floor and ceiling for instance, is 1/2 wavelength, so you simply need a bass trap of half your ceiling height to damp that air resonance. Sound moves a little slower in glass fiber wool, so you can reduce that by 10 % or so (The exact dimension are not at all critical, the bass trap has a wide absorption peak).

So, you simply build bass traps with slightly less than half the dimensions of the room, and mount them with the open end facing the corners for maximum efficiency! Just play bass notes on an instrument and add more of them until the bass response in the room is smooth!

I promise you, recording a bass amp in a room with this treatment is a MUCH more pleasant experience than in an untreated concrete room!

Next, we deal with the matter of diffusion (and diffraction, for the pedantic). A good-sounding room needs the sound to bounce around in a chaotic fashion, but parallel walls make it bounce back and forth instead. This is the evil we must cure here!

If the room doesn't have parallel walls, and has a sloping ceiling, then you're more or less golden already - the sound will change direction for each bounce, and the result will be quite chaotic. But when did you last see such a room?

There are several ways to break up the static reflection pattern in a rectangular room.

One is to add dimensional irregularity. A normal living room has furniture, and that actually helps a lot. Mount wooden boxes of different sizes on the walls, or buy expensive ready-made diffusing panels for the purpose. Just make sure that the boxes vary in depth (how far they stick out from the walls). It's preferable if they have a slightly slanted top, so the sound that hits them isn't reflected in the same axis, but even fully rectangular blocks will diffuse sound a bit, because of diffraction around the edges.

The other way to break up reflection patterns, is to put up spread patches of damping material in the room, which also provides diffraction around the edges.

Break up ALL big areas of parallel walls (and the ceiling) with irregular combinations of boxes and glass fiber patches of different sizes, until the room has a beautiful smooth decay sound! Clapping your hands is the best ways to hear the room response.

The amount of damping patches required is of course depending on how "live" you want your recording space. It's best to add those first until the room has the length of response you seek, and then add diffusing boxes until you're satisfied with the smoothness of the response. Then check if there is still a problematic resonance in the bass region, if so, add more bass traps to control it.

Moving all of this around for a bit before you screw it to the walls may be a good idea, it's OK to experiment!

Remember that you only need to fix ONE of two parallel walls, because you'r just breaking up the bouncing pattern here. Or you can fix half of it on one side, and half on the other. It is usually a good idea to leave one wall untreated somewhere, that give more flexibility when recording - some things may sound best recorded close to the diffuse wall, some at the untreated wall.

The only thing left to explain, is that the damping patches must be THICK, or the room will be boomy in the upper bass/lower midrange. I'd hang 4 inch thick glass fiber patches at a 4 to 8 inch distance from the wall and ceiling, to make sure they do wide range absorption. Thin damping patches only work at higher frequencies.

This all sounds very complicated, but it's really not, because you already have the perfect instrument to tell you if something sounds good or not - your ears!

Just be systematic and change things little by little until you're happy with the result, and you'll be ... well .. happy with the result!

That's how to create a nice acoustic space to record music in! It may surprise some that this much care is needed, since music sounds fine in any old room, doesn't it? The difference is, you're RECORDING the room's quirky behavior here, and all irregularities in the recorded room response will be added to the listening room response. That's why a recording room needs to be so well-behaved acoustically.

Final note: Make your room as live as possible if you're mainly recording acoustic instruments, and more semi-live if you're mostly going to record rock bands.

I'll speak of the mixing room next!