Recording - June 2011 - 31

room, you can achieve just about the same effect for this particular resonance just by moving your speakers toward and away from the rear wall until you find the place where the low-end response is best. This type of room eq also becomes pretty much useless above a few hundred Hertz, because you soon get to frequencies where a wavelength is so short that moving your head a few inches changes the room response. However, the good news is that dealing with the high-frequency problems in the room is much easier than dealing with the low-frequency ones. High frequencies Fixing high-frequency problems is relatively inexpensive. Foam will readily absorb high frequencies, and bookshelves and stacks of random (broken?) equipment will readily diffuse it. You can buy expensive diffusers that look nicer than stacks of old junk and are a little more effective, but old junk is remarkably useful in this regard, and books have the added benefit of being something to read. When inexpensive foam absorbers became available in the 1970s, lots of studios began damping everything down until they were nearly anechoic above 500 Hz. You’d go in and they seemed totally dead, which made them unpleasant to perform in, and resulted in recordings that would become lifeless without some added reverb. However, the deadness allowed better isolation between different microphones used at the same time, and at the time it seemed like isolation was the holy grail of the recording studio. It turns out, though, that the sacrifices required to get that isolation resulted in a lot of recordings that were kind of flatsounding. On top of that, most of these rooms were very dead at high frequencies, but they still had plenty of low-frequency reverberation and plenty of lowfrequency standing wave problems. The goal of the enlightened studio designer today is to make rooms that are not too live and not too dead, and rooms that have about the same degree of liveness at all frequencies. Useful tricks There are a lot of useful tricks out there for improving the sound of a room. One very useful one is to make the walls and ceiling cocked inward or outward slightly. This breaks up the room modes because the room reflections don’t just come back to their source but are reflected at a slight angle. This is a relatively inexpensive thing to do when the room is built and it results in an incredible improvement in the low end. However, it’s an expensive thing to retrofit into an old room.

Barring that, bass traps will deal with the low end by absorbing and dissipating the low frequencies. However, the nice thing about the bass traps is that you also get high-frequency absorption at about the same level, so unlike most other room treatments that require careful engineering, you can just keep throwing bass traps into the room and for the most part the room will stay balanced and the low-end control will keep improving the more you throw in there. The ease of use and the fact that you don’t need so much careful design engineering makes them a serious win. Diffusion also is one of those things that you really can’t hurt having too much of; diffusers are just irregular surfaces that reflect sounds back at different angles. They are useful down to the frequency at which the irregularities are a quarter wavelength long (and that usually means above 1 kHz for the kind of sizes you normally see). Some very careful work has gone into optimizing commercial diffusion panels so you get the most random reflections down to the lowest possible frequency given the available space, but you can make your own panels out of random blocks (or use a mixture of different-sized books or old junk on a shelf) that will still be remarkably effective. Now, a lot of the traditional acoustical measurement techniques like measuring the reverb time of a room become useless in small rooms, because the problems you encounter in small rooms are just too different from the ones you encounter in large rooms. Too many acoustical books are geared toward working with theatres and concert halls and neglect the rather different problems that we encounter every day in smaller studio facilities. [Brian Gadson talks about this in his guest editorial at the end of this issue—Ed.] The ears have it I think the most useful thing that any recording engineer can do is simply to get a sense of when a room sounds good and when it does not. You should be able to clap your hands and hear the sound decay evenly, with no slaps or boinging sounds. A bass should sound about the same in every part of the room and all the notes should be about the same level when you play a Jaco Pastorius record without any one note standing out or disappearing. If you know when a room sounds good, you’ll know when a room sounds bad, and knowing this is the main key toward dealing with room problems. Once you can hear it, then you can find out what to do about it. Scott Dorsey (dorsey@recordingmag.com) is a recording engineer and lifelong student of good audio in Williamsburg, VA.
RECORDING JUNE 2011

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Recording - June 2011

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