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Old 15th June 2020
Lives for gear
David Rick's Avatar
There's no need to put a baffle in the mic's null, esp. for vocals. For a guitar amp, you might do it when tracking a band because you're trying to fence off amp radiation from other parts of the room. But, generally speaking, you want to figure out the path of the strongest first-order reflections and place the gobos to exclude them. When I place a PIB behind the mic, it's because I'm using a figure-eight or omni pattern.

When you look at the pictures of my overdub room, understand that the PIB's are to either side of the singer, and the singer's back is to the Art Panel over the couch. (I usually pile some blankets or pillows on the back of the couch to cover the remaining wall area.) So I've neutralized three major reflections, and there's not a direct reflection from above because it's a vaulted ceiling.

Below 150 Hz, the thickness of the absorptive material doesn't matter as much as the fact that those long wavelengths will diffract around the baffle anyway. It doesn't make sense to think of ray acoustics at low frequencies; you need to think about how to dampen the room resonances. My use of PIB's for the old "closet trick" takes advantage of the extra space behind the panel to put a frictional loss element in a region of higher velocity for some of the axial room modes. That makes a big difference, even though the absorptive batting isn't very thick. (The clothing in the closet probably helps a bit, but there's less loss near the back because the air velocity is going to zero.)

David L. Rick
Seventh String Recording