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Introduction to acoustic treatment

A detailed introduction to acoustic treatment; why it is necessary to have good studio and control room acoustics to achieve a good recording.


If you were opening a commercial studio, then you would be engaging an experienced studio designer to put together a scheme for you. Some studio owners have lived to regret the decision to go it alone! But even so, a little knowledge about acoustics is very useful, and we home and project studio owners normally don’t have sufficient finance to contract out the design. It is my philosophy on studio acoustics that perfection is unattainable - which should go without saying - but that you can go at least three quarters of the way towards perfection for a fairly reasonable amount of time and money. The law of diminishing returns is working in our favour since it is at the lower end of the scale that the returns are at their highest.

Anyway, enough of the philosophy, now for the practicality. There is only one good kind of sound in a studio, a sound that emerges from the speakers, travels past the engineer’s head, and then is either absorbed or breaks up into a zillion vanishingly small reflections. Unfortunately, any flat surface will reflect sound to some degree and any strong reflections will interfere with the direct sound from the speakers causing an uneven frequency response and at the same time distract the engineer from what he/she is meant to be listening to. The frequency response problem is obviously undesirable, but the reason why certain types of reverberation may be undesirable needs further explanation.

Reverberation, the 'dying away’ of a sound, is natural to music. Most music is designed to be performed in a reverberant space - military band music being an obvious exception. When you make a recording, you need to be able to judge how much reverberation is on the recording. Excess reverberation in the room will colour your judgment. Another problem with reverberation in the studio control room is that the frequency response of the room itself may not be flat. More than likely, there will be more reverb at bass frequencies. This will make you think that a recording is more bassy than it actually is. What the engineer needs is to be able to judge accurately the sound he is getting. He will then apply his experience to judge what the recording will sound like in a domestic listening room. (I should say that it is not a good idea to record in domestic acoustics on the grounds that a domestic setting is where the recording will be played. Typical living rooms vary considerably in their acoustic qualities. Recordings should be made in rooms which are acoustically as neutral as possible).

Now you know the problems, what are the solutions? Well the simplest solution, which will go a long way towards improving your recordings - while not solving all the problems mentioned above - is having carpet on the floor and thick curtaining or more carpet on the walls. By 'thick curtains’, I mean thick material and enough of it to hang in very loose folds. Thick curtaining hung with a 50% drape (percentage drape represents the relationship between the width of curtain material and the width of the wall) will absorb around half of the sound energy that falls upon it. At low frequencies it doesn’t work quite as well, but it will definitely have a beneficial effect. Curtain material can be quite expensive so your best source might be a secondhand curtain shop (some people seem to like changing their curtains often, and can afford it!) or jumble sale.

If you want to go further, then Rockwool is the stuff. Rockwool is a fibrous mineral material that builders use for heat insulation (and is usually available from builders’ merchants). It is also very good for absorbing sound, and studios use it by the truckload. It needs to be supported by a timber frame and covered with material, but it will absorb nearly 90% of the sound that hits it, and work reasonably well at lower mid frequencies too. Unfortunately, using porous absorption alone inevitably has the effect of reducing reverberation at high and mid frequencies, but leaving the bass end pretty much as it is. The room will now sound dull, and you may be tempted to cut bass on your recordings when it isn’t really necessary. To cut low frequency reverberation a different type of absorber is used - a panel absorber. These are really easy to make since they can be just hardboard on thin plywood mounted on a frame against the wall, as in Figure 3.1 The thicker the panel and the greater the air gap, the lower the range of frequencies over which the absorber works, regardless of the area of the panel. If you put some Rockwool in the air gap, then the absorption works over a wider range of frequencies. You would need an expert to make calculations for you to do the acoustic treatment absolutely properly, but as a rough guide the BBC make modular absorbers similar to simple panel absorbers out of 3mm hardboard with air gaps of about 100mm to 200mm. The greater the area of wall covered with such panel absorbers, the more low frequency energy will be absorbed and you wouldn’t go far wrong if you made several panel absorbers with different depths and fixed them around the walls of the room. If you fill them with mineral wool and you drill a series of small holes in the panel then they will absorb mid and high frequencies too. Whether or not you drill the holes, an absorbent filling broadens the range of frequencies over which the absorber is effective from high absorption over a narrow band of frequencies, to a lower degree of absorption over a wider band.

I know one question you are asking just about now. Why is he talking about absorbing sound when a lot of studios are currently going for 'live’ rooms and spaces? The answer is that in a small home studio, you are likely to get a better result in a fairly dead acoustic. Reverberation may be beneficial. It certainly will be in the recording area, and a little will help in the control room too. But it certainly works a lot better in large spaces rather than small domestic rooms. Whether the studio is small or large, the one thing reverberation must be is diffused. As an example consider the difference between a mirror and a sheet of white paper (Figure 3.2). They both reflect most of the light that falls on them, but the mirror reflects in straight lines, forming an image, whereas the paper scatters the light in all directions. It is the same with sound, but specular reflections have the undesirable effects that I mentioned earlier. Diffuse reflections do not. The trouble is that it is not so easy to diffuse sound properly as it is to absorb it. Sound can have long wavelengths, so for a surface to be acoustically rough (as the paper is optically) it needs to have roughness with large dimensions. An easy way of providing a combination of absorption and diffusion is to build some bookshelves and put on them all those impressive volumes you bought but never quite got around to reading. The books are reasonably good absorbers, and their varying dimensions provides an acoustically rough surface for good diffusion of whatever energy isn’t absorbed. One last point before I move on: however you do the acoustic treatment, try as much as possible to make it symmetrical about the left/right axis, otherwise the uneven reflections will tempt you to unbalance the stereo image in the speakers to 'compensate’. It may sound right in your room, but it will be wrong on the tape.

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By David Mellor Tuesday February 1, 2000

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