Gain before feedback

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In

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.[1]

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.[2]

Directional microphones are used in live sound to maximize GBF. Directional microphones with

foldback monitors (stage wedges). Directional loudspeaker systems may also be used to increase GBF.[3][4]

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.[3][1]

The sound system operator can use

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.[6]

Hearing aid

A hearing aid incorporates a miniature microphone and a very small speaker driver, and various conditions may increase or decrease the amount of gain that can be applied to the microphone signal before feedback. A well-fitted hearing aid has more GBF than one that is loose. The shape of the earmold is a factor, with larger and heavier designs capable of delivering greater GBF. Hearing aid designs deliver gain in increasing steps depending on the severity of the patient's hearing loss; the range spans from 10 to 65 dB of gain. To prevent feedback with the highest amounts of gain, such designs require the tightest-fitting earmolds with no venting, and the deepest penetration of the ear canal to place the speaker driver as close as practical to the eardrum.[7]

See also

  • Control Theory ยง Stability

References

  1. ^ a b c "Microphone Techniques: Live Sound Reinforcement" (PDF). Shure Educational Publication. Shure. December 2007. p. 36. Archived from the original (PDF) on March 3, 2011. Retrieved May 2, 2011.
  2. ^ "How-to: Miking Acoustic Instruments". Shure Notes for Houses of Worship. Shure. Archived from the original on July 22, 2011. Retrieved May 2, 2011. Every time the number of microphones doubles, the Potential Acoustic Gain of the sound system decreases by 3 dB.
  3. ^ a b c Nave, Carl R. "Rod". "Increasing Potential Acoustic Gain". HyperPhysics. Georgia State University. Retrieved May 2, 2011.
  4. ISBN 0-88188-900-8
    .
  5. ^ Hargraves, Kent (April 28, 2011). "Lose The Feedback: Improving Gain Before Feedback in Worship". StudyHall. ProSoundWeb. Retrieved May 2, 2011.
  6. ^ a b Troxel, Dana (October 2005). "Understanding Acoustic Feedback & Suppressors". RaneNote. Rane Corporation. Archived from the original on May 7, 2015. Retrieved May 2, 2011.
  7. ISBN 1-58890-103-3
    .

External links
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