Moving Microphone Closer to Source

It is sometimes hard to convince a performer that one should move closer to the microphone rather than farther away if the system rings from feedback. Moving the source closer to the microphone and decreasing the amplification improves the chances of getting an adequate sound signal to the listener. The improvements obtainable by close-miking can be modeled fairly accurately from the simplified amplification system because the source is typically close enough for the microphone to be in the direct sound field where the inverse square law holds. When the performer doubles the distance to the microphone, their signal into the microphone drops by 6 decibels while the input from reverberant sound and noises in the auditorium do not change appreciably. Therefore holding the microphone closer not only strengthens the signal to the microphone but also increases the signal-to-noise ratio and the potential acoustic gain.

Moving Source Closer to Listener

The fact that the source-to-listener distance D₀ appears in the expression for potential acoustic gain just practically factors in the size of the auditorium into the system design. In the P.A.G calculation the most meaningful value for D₀ is usually that to the most distant listener. Varying D₀ in that calculation is not particularly useful - if you decreased D₀ by a factor of 2 without changing the other parameters, you would find that the P.A.G. decreases by 6dB. But since that point is half the distance from the source, the unamplified sound is 6dB higher so it doesn't represent an actual loss of sound intensity.

Varying D₀ for systems with multiple speakers might help model their contributions. You would like to get about the same sound level to all listeners. If the sound is brought up to an ideal level at the back of the auditorium, it may be too loud for those who are closer. Sometimes this problem is addressed by using a high central cluster speaker which has horn-type speakers with a more directional pattern than ordinary cone-type loudspeakers. If the horn is pointed toward the distant listeners, then the drop-off of the sound off-axis can keep the amplified sound from being excessive for the closer listeners.

Using More Directional Microphones

Directionality in both microphones and loudspeakers can be used to advantage to increase the potential acoustic gain. The relaxing of the omnidirectional assumption in the simplified amplification system can yield significant increases in useful gain. The most common microphone pattern used in general auditorium systems is the cardioid pattern illustrated at left. It gives higher useful gain if the peak of the pattern is pointed toward the source, but if the microphone is held straight up it becomes effectively omnidirectional and the increase is lost. Assuming that the cardiod pattern is directed toward the source, then if the sound feedback from the loudspeakers is at an angle of 90° with respect to that direction, you can get 6 dB more gain. The microphone discriminates against sound at 90° by 6 dB and by an even greater amount if the angle is greater than 90° .

Higher gain can be obtained with unidirectional microphones, but it is more difficult the keep the narrow microphone pattern centered on the source.

Using More Directional Loudspeakers

Directionality in both microphones and loudspeakers can be used to advantage to increase the potential acoustic gain. It is more difficult to get narrow directional patterns with loudspeakers, but various types of horn loudspeakers achieve signigicant increases in useful gain plus the reduction of echoes and a more uniform sound level over the audience. Another approach is to use a vertical column of loudspeakers to spread sound in the horizontal plane, achieving some directionality.

Notch Filters

When a non-uniform frequency response of an amplification system/room combination causes the system to "ring" before giving adequate amplification, the potential acoustic gain can sometimes be improved with the use of notch filters to take out the offending frequencies as illustrated below. Shown is the use of two notch filters to discriminate against the two most prominent peaks in the frequency response.

Often several such tunable notch filters are packaged together and marketed as "feedback suppressors" . Sometimes such feedback suppressors are used to smooth out the roughest spots before undertaking a full-fledged equalization of the sound system.