live sound mixing Live sound mixing is the blending of multiple sound sources by an audio engineer using a mixing console or software. Sounds that are mixed include those from instruments and voices which are picked up by microphones (for drum kit, lead vocals an ...

, gain before feedback (GBF) is a practical measure of how much a

microphone A microphone, colloquially called a mic or mike (), is a transducer that converts sound into an electrical signal. Microphones are used in many applications such as telephones, hearing aids, public address systems for concert halls and publ ...

can be amplified in a sound reinforcement system before causing

audio feedback Audio feedback (also known as acoustic feedback, simply as feedback) is a positive feedback situation which may occur when an acoustic path exists between an audio input (for example, a microphone or guitar pickup) and an audio output (for exa ...

. In

audiology Audiology (from Latin , "to hear"; and from Greek , ''-logia'') is a branch of science that studies hearing, balance, and related disorders. Audiologists treat those with hearing loss and proactively prevent related damage. By employing vario ...

, GBF is a measure of

hearing aid A hearing aid is a device designed to improve hearing by making sound audible to a person with hearing loss. Hearing aids are classified as medical devices in most countries, and regulated by the respective regulations. Small audio amplifiers s ...

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 An electrodynamic speaker driver, often called simply a speaker driver when the type is implicit, is an individual transducer that converts an electrical audio signal to sound waves. While the term is sometimes used interchangeably with the ...

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 In geometry, a cardioid () is a plane curve traced by a point on the perimeter of a circle that is rolling around a fixed circle of the same radius. It can also be defined as an epicycloid having a single cusp. It is also a type of sinusoidal ...

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 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 Equalization, or simply EQ, in sound recording and reproduction is the process of adjusting the volume of different frequency bands within an audio signal. The circuit or equipment used to achieve this is called an equalizer. Most hi-fi eq ...

can be used for the same purpose but with somewhat less precision. Automatic

feedback suppressor A feedback suppressor is an audio signal processing device which is used in the signal path in a live sound reinforcement system to prevent or suppress audio feedback. Digital feedback reduction is the application of digital techniques to sound r ...

s automate and speed the process of identifying and reducing feedback frequencies. A small amount of

pitch shift Pitch shifting is a sound recording technique in which the original pitch of a sound is raised or lowered. Effects units that raise or lower pitch by a pre-designated musical interval ( transposition) are called pitch shifters. Pitch and tim ...

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.

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 In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The '' IEEE Transactions on Signal Processing' ...

before feedback. A well-fitted hearing aid has more GBF than one that is loose. The shape of the

earmold An earmold (also spelled; ear mold, ear mould or earmould) is a device worn inserted into the ear for sound conduction or hearing protection. Earmolds are anatomically shaped and can be produced in different sizes for general use or specially ca ...

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.

References

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External links

HyperPhysics: Potential Acoustic Gain
Dr. Carl R. "Rod" Nave, Georgia State University. Audio engineering Sound production technology Tone, EQ and filter