Sound Insulation

Sound travels more readily through solids than it does through air, as demonstrated in old Western movies by the fellow with his ear to the railway track giving advance notice of oncoming trains. This has practical implications when soundproofing a room.

Air is actually the best insulator of sound, so if we're concerned about external sound getting into the studio or our own sounds leaking out, we need to trap air between solid layers in the walls, floor and ceiling, to provide good sound insulation. Regardless of the materials used for the inner surfaces, any struts or batons joining them to the external walls, floor and ceiling, will themselves transmit sound.

Optimal sound insulation can be achieved by floating a room within the room on tennis balls. This way there is very little solid connection between the inner and outer rooms, but a lot of insulating air. In fact anything which traps air will dampen sound transmission, so foam acoustic tiles, or even cardboard egg boxes, glued or tacked to the walls and ceiling, and thick carpets on the floor, will help.

Audio 101

Bearing in mind that sound is made up of travelling compression waves, let's look again at our sine wave model, in which the horizontal axis represents time.











The speed at which sound travels through air is a constant, 343 m/s at 20°C and at sea level, although this value varies slightly according to altitude and temperature.

Let λ be the wavelength (in metres),
let f be the sound frequency (full waves per second, Hertz),
and let c be the speed of sound (a constant) measured in metres per second.

λ = c / f

With this simple formula we can work out the wavelength of a sound, so long as its frequency is known. Or we can work out the frequency of a sound having a particular wavelength.

When setting up a recording studio it is important to take precautions against standing waves, which is when the wavelength of a sound coincides with the dimensions of the room, in which case the entire room vibrates at the frequency of that particular wave. The frequency of the standing wave can be predicted by first measuring the room (in metres) and then applying the formula above.

Unless precautions are taken, mixing down sounds with a standing wave in the studio will produce a mix which will sound completely different when played back in another room, as the standing wave will have been compensated for on the mixing desk, and that same frequency will be sadly lacking in the actual sounds that were mixed down.

We can reduce the effect of standing waves by breaking up parallel surfaces. I've found that wall-mounted shelves lined with books of different sizes (and whatever else comes to hand) is a tidy way of breaking up flat walls. Heavy curtains also help. Once you've done the physical work, using a parametric equalizer on your amplifier should take care of the rest. To set it up, listen to a CD of something you know really well and adjust the equalizer until it sounds just right. The next time you mix down a track you won't be compensating for standing waves!