Recording - June 2011 - 30

By Scott Dorsey

Over the last couple of years we’ve seen a number of magic digital room-correction boxes that claim to somehow flatten the response of your monitors, by measuring the room response and creating a digital filter with the opposite response. All you have to do is press a button. The idea sounds like a good one, and when you try these systems they make things sound different as soon as you turn them on. Sometimes it’s hard to tell if a system really sounds better or just different. Differences within a room The main problem with these systems is that the room problems are different from one part of the room to another, because the room problems are caused by standing waves. That is, sound reflects from one surface (wall or ceiling) to another and then back, and this causes peaks and dips in the room response at the frequencies where those distances are exact multiples of a wavelength. Sit in a bathtub and wave your hand back and forth... you will find that there are several different frequencies at which you can create standing waves in the tub. That is, the wave reflects off the wall, and returns in phase with the outgoing wave, so there are obvious peaks and troughs that don’t seem to move. Acoustics folks think of the reflection as being the result of an impedance discontinuity between the water and the side of the tub. The side of the tub is much more solid and less compliant than the water, and so the wave is reflected off. When the wavelength is exactly the length of the tub, or twice or three times the length of the tub, you get the peaks and dips in the tub happening at the same place with each pass of the wave.

If you do this in an Olympic swimming pool, you can’t make the same thing happen, because the wave dies out by the time it gets to the end of the pool. There are still frequencies at which it would occur, it’s just that they are very, very low and you’d have to excite the water very strongly. If you put a wooden box inside the pool, you’d get the same effect with the water reflecting off the side of the box. But, if you were to use a flexible box made of rubber, the wave would strike the box, the box would push on the water outside it, and the wave would continue to expand without being reflected back. This is because the impedance (or “softness”) of the rubber diaphragm becomes approximately the same as that of the water behind it. And that, in short, is how a bass trap works. Bass traps The whole notion of the bass trap is that it’s a gadget that provides the same impedance as the free air, so the wave goes into it and is not reflected back. Some people think traps are tuned to particular room frequencies but in fact the regular bass traps are broadband absorbers. (There are in fact some narrowly tuned absorbers called Helmholtz resonators; they are useful for fixing some very specific low-end problems, but unlike generic bass traps they need to be designed into a room when it’s built.) Traps stop the standing-wave problem in the first place, so you don’t have the peaks and nodes. If you can stop standing waves from forming, most of the low-frequency room issues go away. Magic eq boxes So, what about the magic eq boxes? Because the peaks and dips are located in different parts of the room, you cannot equalize the system to be flat everwhere in the room, so effectively if you fix the problem in one place you make it worse in another. This is why some of the eq systems are very specific about placing a microphone right in the sweet spot where you’ll be sitting in front of your mixer and speakers; they know they can’t make the whole room right, but they can possibly make a big difference exactly where you’ll be sitting. The one instance where the eq can be very helpful in general, though, is in dealing with the resonance that exists between the speakers and the rear wall. A number of “room compensation” systems out there from major manufacturers just look for the one big mode created there and try to eq it out. This is very effective since it results in a bass boost or dip that is the same throughout the room. Note that if you have the ability to move your speakers around your

Standing waves in a space. The lowest supported mode for this one dimension of a room, where a half wavelength fits into the space, is shown in red; the second mode, where a full wavelength fits, is shown in blue; and the third mode, with one and a half wavelengths, is shown in green. There will be modes like this for all three dimensions of any given room.
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RECORDING JUNE 2011

Recording - June 2011

Table of Contents for the Digital Edition of Recording - June 2011