PART VII Acoustics and Sound Reinforcement at 10 kHz is more logical and air absorption is not a factor. When necessary to err, then err on the side of a little extra rolloff DON'T EQUALIZE FOR HEARING LOSS Many times there is a tendency to attempt to adjust the amplitude response of a sound system to make it the inverse of the hearing-loss curve. This is not a good idea for several reasons: 1. Young people with normal hearing will be annoyed. 2. Older people have made mental compensation for the gradual onset of the loss and would also be annoyed. They usually desire the overall level higher. 3. Available high-frequency drivers would have their distortion increased noticeably with such a boost. PROXIMITY MODES The microphone proximity effect, traditionally referred to in the technical literature, is the effect of increased bass response in the microphone as the talker gets closer to the unit. This remains true of most unidirectional microphones today and is often effectively used by trained performers to enhance their otherwise weak bass tones. Since the advent of sound system equalization, however, we have become aware of still another effect of the proximity of large bodies (performers) on a typical cardioid microphone. That is the increased tendency to feedback at some key midrange frequency where the system is otherwise stable until the microphone is approached. You can use your hands cupped around the microphone to bring the system into feedback and can adjust the level of feedback by "playing" the microphone. In adjusting the appropriate filter, care should be taken not to carry the adjustment too far. The idea is to correct the tendency of the microphone to cause instability when it is approached by the performer and not to remove all tendency toward feedback even when the microphone is com- pletely encircled by a closed hand. TEF analysis has shown that this instability is caused by "comb filters" produced by reflected sound from the performer. 550 One classic example was Dan Seals who is a left-handed guitar player and was having trouble with acoustic feedback whenever he turned to his left at the microphone. He allowed us to make a meas- urement with the same setup as when he was performing. Figure 18.33 shows our measurement. The guitar reflection and the hat brim reflection combined acoustically at the microphone to cause a genu- ine excess gain problem. When he turned to the left, the body of his guitar reflected the left monitor towards the microphone and his hat brim reflected the right monitor to the same place. When Dan Seals saw the measurement, he said, "We have met the enemy and they is us!" CHECKING MICROPHONE POLARITY Surprisingly , one minor checkout prior to equalization time that often is overlooked is the poling of the microphones in a multimicrophone system. The old way was to arbitrarily assume that the first Upper trace is Dan Seals with cowboy hat and guitar Lower trace is open microphone FIGURE 18.33 Effects of surfaces on feedback.