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Gain Before Feedback

WIKI – Gain Before Feedback

In live sound mixing, gain before feedback (GBF) is a practical measure of how much a microphone can be amplified in a sound reinforcement system before causing audio feedback. In audiology, GBF is a measure of hearing aid performance. In both fields the amount of gain is measured in decibels at or just below the point at which the sound from the speaker driver re-enters the microphone and the system begins to ring or feed back. Potential acoustic gain (PAG) is a calculated figure representing gain that a system can support without feeding back.

Live sound

In live sound mixing, GBF is dependent on a wide variety of conditions: the pickup pattern (polar pattern) of the microphone, the frequency response of the microphone and of the rest of the sound system, the number of active microphones and loudspeakers, the acoustic conditions of the environment including reverberation and echo, and the relative positions of the microphones, the loudspeakers, the sound sources and the audience. Each doubling of the number of open microphones (NOM) reduces the PAG by 3 dB.

Directional microphones are used in live sound to maximize GBF. Directional microphones with cardioid and hypercardioid pickup patterns are designed with reduced sensitivity to the rear (cardioid) or to an angle between the side and the rear (hypercardioid). Such microphones are aimed such that their pickup pattern is weakest in the direction of the loudspeakers. This is especially useful in the presence of foldback monitors (stage wedges). Directional loudspeaker systems may also be used to increase GBF.

The distance from the sound source to the microphone is a critical element of GBF. Greater GBF is obtained with the performer closer to the microphone; an instance of the inverse-square law. If the performer reduces the distance to the microphone by half, the PAG is increased by 6 dB while the environmental sounds remain relatively the same.

The sound system operator can use equalization to change the frequency response of a microphone or loudspeakers system to increase GBF. The frequency which first begins ringing or feeding back is identified by the operator, and a notch filter or parametric filter is engaged to reduce the overall level of that frequency. This process is repeated several times to identify and reduce the level of further feedback frequencies.A graphic equalizer can be used for the same purpose but with somewhat less precision. Automatic feedback suppressors automate and speed the process of identifying and reducing feedback frequencies. A small amount of pitch shift applied to the signal can increase GBF, as can the addition of a few milliseconds of straight delay. The latter will increase the number of feedback frequencies while reducing the frequency range within which they occur, but it will slow the rate at which feedback grows. In practice, adding straight delay to a signal improves GBF.

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    • Splay
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    • All About Loud Speakers
      • ht-audio.com Speaker Basics
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      • Equalization / EQ
      • Gain
      • Gain Before Feedback
      • Hearing
      • Hertz
      • High Pass Filter / HPF / HP
      • Inverse Square Law
      • Low Pass Filter / LPF / LP
      • Microphones
        • Omnidirectional
        • Cardioid
        • Hypercardioid
        • Supercardioid
        • Bi-directional or Figure 8
        • Shotgun
        • Subcardioid
      • Precedence Effect
      • Sound Pressure Level / SPL
    • FFT theory
      • FFT – fast Fourier transform
      • Transfer Function
  • Hardware Based
    • Stand Alone FFT
      • Audio Precision
      • Meyer Sound – SIM3
      • NTI – Minilyzer ML1
      • SRS – SR1
  • Bob McCarthy
  • Essentials
    • Essentials – 101
    • Essentials – 102
    • Essentials – 103
    • Essentials – 104
  • Special Mentions
    • Alexander Yuill-Thornton II
    • Bob McCarthy
    • Bruce Jackson
    • Don Pearson
    • Jamie Anderson
    • Jean Baptiste Joseph Fourier
    • John Meyer
    • Meyer Sound
    • Rational Acoustics Smaart
    • Sam Berkow
  • Mentors
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    • Bob “606” McCarthy
    • Charles Orville Bowles
    • Topper Sowden
    • Craig Doubet
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    • Harry Brill Jr.
    • Howard Page
    • Jim van Bergen
    • Kelly Prince
    • Toby Guynn
    • True Taylor
    • Tom Young
  • Blog
  • Random noise
  • training
    • AFMG – SysTune training
    • Meyer Sound – Education
    • Rational Acoustics – Smaart training
    • SynAudCon – Synergetic Audio Concepts
    • L-Acoustics training
  • library
    • Audio Precision – Audio Measurement Handbook by Bob Metzler
    • JBL Pro – Sound System Design Reference Manual
    • Sound Systems – Design and Optimization by Bob McCarthy 3rd Edition
    • Meyer Sound Design Reference – Bob McCarthy
    • Sound System Engineering – Don Davis, Carolyn Davis, Eugene Patronis, Jr., Pat Brown
  • Forums
    • Metric Halo – SpectraFoo Forum
    • Smaart Live Users – Facebook group
    • Rational Acoustics – Smaart Forum
  • Mic accessories
    • Mic Clips
    • Mic Stands
  • Software
    • Bruel & Kjaer – FFT 7770
    • Dayton Audio – OmniMic V2
    • Easra
    • Electroacoustics Toolbox
    • LAUD – replaced by Praxis
    • LinearX Systems Inc.
    • Praxis
    • mlssa
    • REW – Room EQ Wizard
    • SATlive
    • Smaart 7 DI
    • Smaart 8
    • SpectraFoo Complete
    • SpectraPLUS
    • SysTune
    • FuzzMeasure
    • WaveCapture
    • TrueRTA
    • ARTA
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  • Ios
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  • contact
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    • ACO Pacific
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    • Audix TRM40 – discontinued
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    • Dayton Audio EMM-6
    • Dayton Audio UMM-6 usb
    • dbx RTA-M
    • DPA 4007
    • Earthworks M50
    • Earthworks M30
    • Earthworks M30BX – self powered
    • Earthworks M23
    • ISEMcon EMX-7150
    • Josephson C550H / C550L
    • MBHO 550EL
    • Nady CM100
    • Peavey PVR 2
    • Presonus PRM1
    • Rational Acoustics – RTA 420 with calibration file
    • Schoeps MK-2 capsule
    • Sound First – SF101a
  • Wavelength Calc
  • RF based measurement
    • Lectrosonics TM400 & TM400A
    • Line 6 XD-V75TR
    • Beyer TG MM1w
    • Zaxcom TRX742 plug-on transmitter
    • Lectrosonics HMa / Venue 2 or HM / Venue 1 rig
  • DSP platforms
    • BSS London BLU
    • QSC QSYS
    • Media Matrix
    • Biamp
    • Meyer Sound – Galileo Galaxy
    • dbx Driverack
  • Real Time Analyzer – RTA “RIP”
    • Audio Control SA-3052 stand alone RTA unit
    • Crown RTA 2 – discontinued
    • dbx RTA 1 – discontinued
    • Rane RA 30 – discontinued
    • Rane RA27 RTA – discontinued
  • RF measurement, coordination & optimization
    • RF Explorer – handheld RF scanner
    • RF Venue RF Explorer Rack Pro – RF scanner
    • Clearwaves – RF Scanning and coordination software (PC)
    • Touchstone – RF scanning software
    • Touchstone Pro – RF scanning software (PC or Mac)
    • Vantage – RF scanning software for Mac OS
    • IAS (intermodulation analysis system) – RF coordination software (PC)
    • Intermod Assist – RF coordination software (PC)
    • Lectrosonics – Wireless Designer
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