Acoustic Feedback
Definition
Acoustic feedback (the Larsen effect) is the howl or sustained tone that occurs when a microphone picks up the output of a loudspeaker it feeds and the loop gain reaches unity at some frequency. The frequencies that ring first depend on the room, the positions of the microphones and loudspeakers, the microphone polar pattern and the equalisation. A narrow EQ cut at a ringing frequency raises the gain available at that frequency by roughly the depth of the cut, until another frequency starts to ring. Shure estimates that precise parametric equalisation can add about 4 to 6 dB of gain before feedback.
In the field
Monitors are usually rung out before the show with each microphone at its performance position, by raising the gain until a frequency rings and cutting it with a narrow filter. Doing it with the performer in place gives a more representative result, because the body changes the reflections around the microphone.
Sources
- Shure, « Feedback: Fact and Fiction » : l'égalisation paramétrique précise peut apporter 4 à 6 dB · shure.com
- Shure, « How to Control Feedback in a Sound System », facteurs : distances, directivité, égalisation · shure.com
- Audio feedback (Wikipedia) : boucle micro-enceinte, gain de boucle unitaire · en.wikipedia.org