Gain Before Feedback (GBF)
Definition
Gain before feedback (GBF) is the maximum gain a sound-reinforcement system can apply to a microphone before the microphone-loudspeaker loop starts to ring. It increases when the microphone is closer to the source (halving the distance gains about 6 dB), when directional microphones are aimed away from the loudspeakers, when ringing frequencies are cut with narrow filters and when fewer microphones are open. Each doubling of the number of open microphones (NOM) reduces the available gain by about 3 dB. GBF is therefore the practical limit on the level of a live microphone.
In the field
The loss follows 10 × log of the number of open microphones, so going from four to five open microphones costs about 1 dB, and going from two to four costs about 3 dB. Muting unused channels, or using gates and automatic mixers where appropriate, recovers that margin; reducing eight open microphones to two recovers about 6 dB.
Related lessons
Sources
- Shure, « Gain before feedback with multiple microphones » : chaque doublement du NOM coûte 3 dB · service.shure.com
- Shure, « Feedback: Fact and Fiction » : distance micro-source, égalisation, nombre de micros ouverts · shure.com
- Gain before feedback (Wikipedia) : PAG/NAG, effet de la distance et du NOM · en.wikipedia.org
- ProSoundWeb, « Microphone Feedback Facts… And Fiction » : diviser la distance par deux fait gagner 6 dB · prosoundweb.com