Effective & Ineffective Absorbers
Now you've probably read about Technical Editor Hugh Robjohns and I sticking acoustic foam onto studio walls, but before you rush off and do this it is important to understand the limitations of such materials. While a thick black cloth placed over a mirror will absorb all the visible light of its reflection, a piece of acoustic foam will only be acoustically 'black' at high frequencies, becoming increasingly transparent at lower frequencies. In very general terms, a piece of four-inch-thick foam stuck directly onto the wall is only really effective for frequencies above about 200-300Hz, whereas a piece of two-inch foam is only effective above about 400-600Hz. In other words, if you halve the thickness of the foam, the frequency above which it is effective moves up by an octave.
Once you appreciate this, you can see immediately why attempts to deaden walls with carpet don't work too well — carpet is relatively thin and so only soaks up very high frequencies. This leaves the low and mid-range frequencies unaffected, giving a dull and boxy result. You can probably now also understand why acoustic foam would need to be several feet thick to be effective at bass frequencies, which is why foam is rarely the most practical solution for bass absorption.
Although foam is quite convenient, it's not the only option. You can also use one or more layers of high-density Rockwool insulation slab, as long as you cover it with fabric to keep those nasty irritating fibres out of the air. Rockwool is just as effective as foam, if not more so, but it does require more in the way of DIY skills.
One very useful tip is that spacing foam or Rockwool absorbers away from your wall by a few inches makes them more effective at low frequencies. For example, spacing four-inch-thick foam four inches from the wall is almost as effective as using double the thickness of foam in the first place! It can help in this instance to mount the foam or Rockwool onto a board with large holes or slots cut out of it to let the sound through — radiator fret panel is good for this, but not pegboard. This board can then be hung in the right place. Another quicker solution is to simply glue high-density foam cubes to the back of an acoustic-foam panel to act as spacers. I've found that spray carpet adhesive is as good as anything for fixing foam.
Another way to deal with high-frequency reflections is to break them up so that they become less disruptive. This is called 'diffusion', and requires a very uneven surface with protrusions and wells several inches in depth. Although commercial diffuser designs are available with precisely calculated profiles, putting shelves at the back of your room and part filling them with books, manuals, CDs and bits of unused gear also works quite effectively!
http://www.soundonsound.com/sos/feb06/articles/studiosos.htm
