Gain Before Feedback (GBF)
Definition
Gain Before Feedback (GBF)
Gain Before Feedback (GBF) is the maximum amplification a sound reinforcement system can achieve before acoustic feedback (howling) occurs. GBF depends on the acoustic coupling between speakers and microphones, speaker and microphone directivity patterns, room reverberation, and the number of open microphones. SonaVyx measures GBF by analyzing the system transfer function loop gain.
GBF = -10 × log₁₀(N) + 10 × log₁₀(Q) + room absorption factor, where N = number of open mics
How It Is Measured
GBF is measured by gradually increasing system gain while monitoring the transfer function for the first frequency where loop gain approaches 0 dB (unity). SonaVyx detects incipient feedback as narrow spectral peaks with rising Q factor, providing a real-time GBF estimate without reaching full oscillation. The system gain at which the first frequency hits 0 dB loop gain defines the GBF for that configuration.
Practical Example
A podium microphone and ceiling speaker system has a GBF of only -4 dB — feedback occurs before useful amplification is achieved. SonaVyx analysis reveals the omnidirectional ceiling speaker sends strong energy directly back into the cardioid microphone. Replacing the ceiling speaker with a directional column speaker aimed away from the mic increases GBF by 10 dB to +6 dB, providing ample amplification margin.
Factors Affecting GBF
The primary factors are: speaker-to-microphone distance (more distance = higher GBF), speaker directivity (more directional = less energy toward the mic), microphone directivity (cardioid rejects rear-hemisphere sound by 15-25 dB), room absorption (more absorption = less reverberant buildup), and number of open microphones (each doubling reduces GBF by 3 dB per the NOM equation).
NOM Rule
The Number of Open Microphones (NOM) rule states that GBF decreases by 10 × log₁₀(N) dB when N microphones are simultaneously open. Two open mics lose 3 dB, four lose 6 dB, eight lose 9 dB. Automatic microphone mixers that gate unused channels help maintain GBF by reducing the effective NOM to only the actively speaking channels.
Improving GBF
Practical strategies to increase GBF include: using directional microphones positioned close to the sound source, using directional speakers aimed away from microphones, adding acoustic absorption to reduce room reverberance, using automatic feedback suppression as a safety net after manual ring-out, and reducing the number of simultaneously open microphones through automatic mixing.
Try It Now
Maximize gain before feedback — guided workflow in SonaVyx