Can ordinary carpet be useful as a form of acoustic treatment in either my recording space or my control room. If so, is any type better than another, and is there anything I should avoid?
Henning Jakobsson, via email.
SOS Editor In Chief Paul White replies: Carpet isn't all bad, but please don't stick it directly to large areas of wall in the hope that it will cure all your acoustic problems: this approach will make things worse. It is, however, OK to use carpet on the floor of a control room or recording space if you want to, and if you can leave it loose in the recording space (or use rugs), it will give you a choice of working over a highly reflective solid floor or not — you can simply take it up when you wish, to add a little life to things like drums and acoustic guitars.
The acoustic traps you see hanging on studio walls are usually only effective at mid and high frequencies, with a typical foam panel losing efficiency rapidly below around 300 to 400 Hz. The frequency range they absorb is determined both by their thickness and by their distance from the wall; the thicker the absorber and the further it is spaced from the wall, the lower the frequencies it can absorb.
To make sense of this statement, consider what happens when a sound wave meets a wall. When travelling in free air, the air molecules move back and forth according to the vibrations set up by the sound source, but when they encounter a solid surface, they can no longer move and so the energy of that moving air is converted to a region of changing pressure. Porous absorbers can only do their thing if they are placed where the air is moving, as their role is to rob those moving air molecules of energy by converting it to heat, through frictional losses. Maximum air movement occurs at a distance, a quarter of a wavelength from a wall, which at 1kHz is roughly 85mm from the surface. Some sound approaches at an angle and so encounters a greater effective thickness of absorber, which means that in real life an absorber of this type might be useful down to 300Hz or below.
If you stick carpet directly to a wall, it isn't thick enough to affect more than the very highest frequencies, thus throwing your whole room out of balance! The lows and mids will continue to bounce around as before, and only the extreme highs will be soaked, leaving you with a boxy, dull-sounding room. However, if you hang the carpet a distance from the wall, it will start to have a more beneficial effect. Providing the carpet is porous (not one with a sealed rubber backing), it can be used to cover a cheap absorbing material such as medium density Rockwool (mineral wool) slab 50 to 100 mm thick, to create an effective mid/high absorber with an attractive cosmetic finish that also prevents the mineral wool fibres from escaping. Just make up a simple wooden frame, fit the mineral wool and then glue the carpet to the frame. Spacing this from the wall by 50mm or so will make it more effective at lower frequencies.
You can also make cylindrical traps to stand in the corners of your room or to stack around an instrument you are trying to record, by making a cylinder of porous carpet 300mm or so in diameter and then stuffing the inside tightly with Rockwool loft insulation. A strip of thin plywood glued behind the join using impact adhesive will hold the two ends of the carpet together. You can cover the ends of the traps with fabric to hide the Rockwool and to keep the fibres from escaping or, alternatively, use a pair of large, empty sweet tins to form each end of the roll. Not only will this help your cylinder stay in shape but you also get to eat the contents!
Yet another trick you can do if you happen to have some spare solid rubber-backed industrial carpet is to create what's known as a limp-mass bass trap. These can be placed away from a wall, by at least 150mm, or hung across corners. The principle here is that the rubber backing forms a seal that prevents air from passing through the carpet, so the moving air tries to move the carpet along with it instead. However, being both heavy and well-damped, the carpet robs the sound wave of energy, again converting it to a minuscule amount of heat. The larger this 'curtain' and the heavier the carpet, the more effective it will be, and there's no reason not to double up on the thickness. Professional studio installations use the heavier and more costly mineral-loaded barrier matt (dead sheet) for this application, but if you have access to a cheap supply of suitable carpet it will do a reasonable job. What makes it through the first layer will be further attenuated by the second layer if you do decide to double up. While the carpet's surface will dry up high-frequency reflections to a useful degree, you may need mid-range absorbers elsewhere in the room or in front of the trap, just to keep the decay time more or less balanced across the audio spectrum.