| Kurt,
In most rooms, 38% from the back wall is far too close. Even in a 20 ft. deep room, 38% would be 7.6 ft. from the back wall. Since we always have to deal with a 1/4 wavelength null from the back wall, we need to do the math and see what frequency has a quarter wavelength of 7.6 ft., and see if that's an acceptable frequency for a null. So we divide the speed of sound by 4 and divide the answer by 7.6, and we get 37 Hz as the null frequency for 7.6 ft. That's a little tough to take, since 37 Hz approximately the frequency of the low D on a 5-string bass guitar (plus there's a lot of kick drum energy around that frequency). The range from 30 - 40 Hz really is something you need to hear accurately.
On the other hand, if you have the luxury of a 26 ft. deep room, say a mastering room, the situation changes, as 38% of 26 ft. is 9.88 ft. If you do the math on 9.88 ft., the 1/4 wavelength null moves down to about 28 Hz, which is low enough to be less consequential for bass accuracy. I really prefer to be 12 ft. off the back wall if possible, which moves the null point down to the threshhold of inaudibilty (23 Hz).
-Wes |