Gain before feedback
Encyclopedia
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 decibel
s 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.
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. Advanced techniques such as the addition of a few milliseconds of straight delay or a small amount of pitch shift
can increase GBF.
before the system feeds back. 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 dB 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
.
Live sound mixing
Live sound mixing is the art of combining and processing a number of audio signals together to create a "mix" that the audience or performers at a live show hear. There can be a variety of different mixes required, depending on the performance requirements...
, gain before feedback (GBF) is a practical measure of how much a microphone
Microphone
A microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. In 1877, Emile Berliner invented the first microphone used as a telephone voice transmitter...
can be amplified in a sound reinforcement system
Sound reinforcement system
A sound reinforcement system is the combination of microphones, signal processors, amplifiers, and loudspeakers that makes live or pre-recorded sounds louder and may also distribute those sounds to a larger or more distant audience...
before causing audio feedback
Audio feedback
Audio feedback is a special kind of positive feedback which occurs when a sound loop exists between an audio input and an audio output...
. In audiology
Audiology
Audiology is the branch of science that studies hearing, balance, and related disorders. Its practitioners, who treat those with hearing loss and proactively prevent related damage are audiologists. Employing various testing strategies Audiology (from Latin , "to hear"; and from Greek , -logia) is...
, GBF is a measure of hearing aid
Hearing aid
A hearing aid is an electroacoustic device which typically fits in or behind the wearer's ear, and is designed to amplify and modulate sound for the wearer. Earlier devices, known as "ear trumpets" or "ear horns", were passive funnel-like amplification cones designed to gather sound energy and...
performance. In both fields the amount of gain
Gain
In electronics, gain is a measure of the ability of a circuit to increase the power or amplitude of a signal from the input to the output. It is usually defined as the mean ratio of the signal output of a system to the signal input of the same system. It may also be defined on a logarithmic scale,...
is measured in decibel
Decibel
The decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...
s at or just below the point at which the sound from the speaker driver
Speaker driver
A speaker driver is an individual transducer that converts electrical energy to sound waves, typically as part of a loudspeaker, television, or other electronics device. Sometimes the transducer is itself referred to as a speaker, particularly when a single one is mounted in an enclosure or as...
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
Foldback (sound engineering)
Foldback is the use of rear-facing heavy-duty loudspeakers known as monitor speaker cabinets on stage during live music performances. The sound is amplified with power amplifiers or a public address system and the speakers are aimed at the on-stage performers rather than the audience...
(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
Inverse-square law
In physics, an inverse-square law is any physical law stating that a specified physical quantity or strength is inversely proportional to the square of the distance from the source of that physical quantity....
. 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
Equalization (audio)
Equalization is the process commonly used in sound recording and reproduction to alter the frequency response of an audio system using linear filters. Most hi-fi equipment uses relatively simple filters to make bass and treble adjustments. Graphic and parametric equalizers have much more...
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. Advanced techniques such as the addition of a few milliseconds of straight delay or a small amount of pitch shift
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 are called "pitch shifters" or "pitch benders".-Pitch/time shifting:...
can increase 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 signalAudio signal
An audio signal is an analog representation of sound, typically as an electrical voltage. Audio signals may be synthesized directly, or may originate at a transducer such as a microphone, musical instrument pickup, phonograph cartridge, or tape head. Loudspeakers or headphones convert an electrical...
before the system feeds back. A well-fitted hearing aid has more GBF than one that is loose. The shape of the earmold
Earmold
An earmold is a device worn inserted into the ear for sound conduction or ear protection. Earmolds are anatomically shaped and can be produced in different sizes for general use or specially cast from particular ear forms. Some users specify how hard or soft they want their mould to be, an...
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 dB 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
Ear canal
The ear canal , is a tube running from the outer ear to the middle ear. The human ear canal extends from the pinna to the eardrum and is about 35 mm in length and 5 to 10 mm in diameter....
to place the speaker driver as close as practical to the eardrum
Eardrum
The eardrum, or tympanic membrane, is a thin membrane that separates the external ear from the middle ear in humans and other tetrapods. Its function is to transmit sound from the air to the ossicles inside the middle ear. The malleus bone bridges the gap between the eardrum and the other ossicles...
.
External links
- HyperPhysics: Potential Acoustic Gain. Dr. Carl R. "Rod" Nave, Georgia State University.