Proximity effect (audio)
Encyclopedia
The proximity effect in audio is an increase in bass or low frequency response when a sound source is close to a microphone
.
whose mechanical
movement is converted to electrical signals (via a magnetic coil, for example). The movement of the diaphragm is a function of the air pressure difference across the diaphragm arising from incident sound waves. In a directional microphone, sound reflected from surfaces behind the diaphragm is permitted to be incident
on the rear side of the diaphragm. Since the sound reaching the rear of the diaphragm travels slightly farther than the sound at the front, it is slightly out of phase
. The greater this phase difference, the greater the pressure difference and the greater the diaphragm movement. As the sound source moves off the diaphragm axis, this phase difference decreases due to decreasing path length difference. This is what gives a directional microphone its directivity.
In addition to the angular
dependence described above, the response of a directional microphone depends on the amplitude
, frequency
and distance of the source. These latter two dependencies are used to explain the proximity effect.
of the sound. (This frequency dependence is offset by damping
the diaphragm 6 dB
per octave
to achieve a flat frequency response but this is not germane to the proximity effect so nothing more will be said about it here). The point to be made regarding the frequency dependency is that the phase difference across the diaphragm is the smallest at low frequencies.
of the distance from the source (it is sound intensity level that drops as the inverse of the distance squared, for those familiar with the inverse square law), the amplitude of the sound will be slightly less at the rear of the diaphragm as compared to the front of the diaphragm. Since the pressure difference due to the amplitude component is dependent only on the amplitude differential with respect to the two sides of the diaphragm, it is independent of frequency, per se.
The properties of the amplitude component that are applicable to the proximity effect are that the contribution to the pressure difference is small and independent of frequency. At large distances between the source and the microphone, the amplitude component of the pressure difference is negligible compared to the phase component at all audio frequencies. As the source is brought closer to the directional microphone, the amplitude component of the pressure difference increases and becomes the dominant component at lower frequencies (recall that the phase component is relatively small at the low frequencies). At higher frequencies, the phase component of the pressure difference continues to dominate for all practical distances between source and microphone.
The result is that the frequency response of the microphone changes; specifically, it increases at the low frequency (bass) end, as the audio source is brought closer to the microphone. This is the proximity effect as it pertains to audio.
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...
.
Technical explanation
Proximity effect is distortion caused by the use of ports to create directional polar pickup patterns, so omni-directional mics are not affected. Depending on the mic design, proximity effect may easily result in a boost of up to 16 dB, usually focused below 100 Hz. Vocalists tend to like proximity effect since it fattens up their voice, but a constantly varying bass boost can wreak havoc on headroom and carefully set levels. Proximity effect is sometimes referred to as "bass tip-up."Angular dependence
To understand how the proximity effect arises in directional microphones, it is first necessary to briefly understand how a directional microphone works. A microphone is constructed with a diaphragmDiaphragm (acoustics)
In the field of acoustics, a diaphragm is a transducer intended to faithfully inter-convert mechanical motion and sound. It is commonly constructed of a thin membrane or sheet of various materials. The varying air pressure of the sound waves imparts vibrations onto the diaphragm which can then be...
whose mechanical
Mechanics
Mechanics is the branch of physics concerned with the behavior of physical bodies when subjected to forces or displacements, and the subsequent effects of the bodies on their environment....
movement is converted to electrical signals (via a magnetic coil, for example). The movement of the diaphragm is a function of the air pressure difference across the diaphragm arising from incident sound waves. In a directional microphone, sound reflected from surfaces behind the diaphragm is permitted to be incident
Angle of incidence
Angle of incidence is a measure of deviation of something from "straight on", for example:* in the approach of a ray to a surface, or* the angle at which the wing or horizontal tail of an airplane is installed on the fuselage, measured relative to the axis of the fuselage.-Optics:In geometric...
on the rear side of the diaphragm. Since the sound reaching the rear of the diaphragm travels slightly farther than the sound at the front, it is slightly out of phase
Phase (waves)
Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...
. The greater this phase difference, the greater the pressure difference and the greater the diaphragm movement. As the sound source moves off the diaphragm axis, this phase difference decreases due to decreasing path length difference. This is what gives a directional microphone its directivity.
In addition to the angular
Angular
The angular is a large bone in the lower jaw of amphibians and reptiles , which is connected to all other lower jaw bones: the dentary , the splenial, the suprangular, and the articular...
dependence described above, the response of a directional microphone depends on the amplitude
Amplitude
Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
, frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
and distance of the source. These latter two dependencies are used to explain the proximity effect.
Phase difference
As described above, the phase difference across the diaphragm gives rise to the pressure difference that moves the diaphragm. This phase difference increases with frequency as the difference in path length becomes a larger portion of the wavelengthWavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
of the sound. (This frequency dependence is offset by damping
Damping
In physics, damping is any effect that tends to reduce the amplitude of oscillations in an oscillatory system, particularly the harmonic oscillator.In mechanics, friction is one such damping effect...
the diaphragm 6 dB
DB
DB may refer to:In science and technology:*Decibel , a logarithmic unit of measurement in acoustics and electronics*Dubnium , a chemical element*DB connector, a size of D-subminiature electrical connector...
per octave
Octave
In music, an octave is the interval between one musical pitch and another with half or double its frequency. The octave relationship is a natural phenomenon that has been referred to as the "basic miracle of music", the use of which is "common in most musical systems"...
to achieve a flat frequency response but this is not germane to the proximity effect so nothing more will be said about it here). The point to be made regarding the frequency dependency is that the phase difference across the diaphragm is the smallest at low frequencies.
Amplitude difference
In addition to phase differences, amplitude differences also result in pressure differences across the diaphragm. This amplitude component arises from the fact that the far side of the diaphragm is farther from the sound source than the front side. Since sound pressure level decreases as the inverseMultiplicative inverse
In mathematics, a multiplicative inverse or reciprocal for a number x, denoted by 1/x or x−1, is a number which when multiplied by x yields the multiplicative identity, 1. The multiplicative inverse of a fraction a/b is b/a. For the multiplicative inverse of a real number, divide 1 by the...
of the distance from the source (it is sound intensity level that drops as the inverse of the distance squared, for those familiar with the inverse square law), the amplitude of the sound will be slightly less at the rear of the diaphragm as compared to the front of the diaphragm. Since the pressure difference due to the amplitude component is dependent only on the amplitude differential with respect to the two sides of the diaphragm, it is independent of frequency, per se.
The properties of the amplitude component that are applicable to the proximity effect are that the contribution to the pressure difference is small and independent of frequency. At large distances between the source and the microphone, the amplitude component of the pressure difference is negligible compared to the phase component at all audio frequencies. As the source is brought closer to the directional microphone, the amplitude component of the pressure difference increases and becomes the dominant component at lower frequencies (recall that the phase component is relatively small at the low frequencies). At higher frequencies, the phase component of the pressure difference continues to dominate for all practical distances between source and microphone.
The result is that the frequency response of the microphone changes; specifically, it increases at the low frequency (bass) end, as the audio source is brought closer to the microphone. This is the proximity effect as it pertains to audio.