The proximity effect in audio is an increase in bass or low frequency response when a sound source is close to a directional or cardioid

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 public ...

.Proximity Effect in Directional Microphones
Shure Pro Audio Technical Library Proximity effect is a change in the frequency response of a directional pattern microphone that results in an emphasis on lower frequencies. It is caused by the use of ports to create directional polar pickup patterns, so omni-directional microphones do not exhibit the effect (this is not necessarily true of the "omni" pattern on multipattern condenser mics, which create the "omni" pattern by summing two back-to-back cardioid capsules, which may or may not share a common backplate.) Proximity effect can be viewed in two ways. In some settings, sound engineers may view it as undesirable, and so the type of microphone or

microphone practice There are a number of well-developed microphone techniques used for recording musical, film, or voice sources or picking up sounds as part of sound reinforcement systems. The choice of technique depends on a number of factors, including: * The w ...

may be chosen in order to reduce the proximity effect. On the other hand, some microphone users seek to intentionally use the proximity effect, such as

beat boxing Beatboxing (also beat boxing) is a form of vocal percussion primarily involving the art of mimicking drum machines (typically a TR-808), using one's mouth, lips, tongue, and voice.

singers in hip hop music.

Technical explanation

Depending on the microphone design, proximity effect may result in a boost of up to 16 dB or more at lower frequencies, depending on the size of the microphone's diaphragm and the distance of the source. A ready (and common) example of proximity effect can be observed with cardioid dynamic vocal microphones (though it is not limited to this class of microphone) when the vocalist is very close to or even touching the mic with their lips. The effect is heard as a 'fattening up' of the voice. Many radio broadcast microphones are large diameter cardioid pickup pattern microphones, and radio announcers are often observed to employ proximity effect, adding a sense of gravitas and depth to the voice. Proximity effect is sometimes referred to as "bass tip-up."Proximity Effect
music technology glossary

Angular dependence

To explain how the proximity effect arises in directional microphones, it is first necessary to briefly describe how a directional microphone works. A microphone is constructed with a

diaphragm Diaphragm may refer to: Anatomy * Thoracic diaphragm, a thin sheet of muscle between the thorax and the abdomen * Pelvic diaphragm or pelvic floor, a pelvic structure * Urogenital diaphragm or triangular ligament, a pelvic structure Other * Diap ...

whose

mechanical Mechanical may refer to: Machine * Machine (mechanical), a system of mechanisms that shape the actuator input to achieve a specific application of output forces and movement * Mechanical calculator, a device used to perform the basic operations of ...

movement is converted to

electrical Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by ...

signals 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'' ...

(via a

magnetic coil An electromagnetic coil is an electrical conductor such as a wire in the shape of a coil (spiral or helix). Electromagnetic coils are used in electrical engineering, in applications where electric currents interact with magnetic fields, in de ...

, for example). The movement of the diaphragm is a function of the

air pressure Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The Standard atmosphere (unit), standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equival ...

difference across the diaphragm arising from incident

sound waves In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid. In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' by the ...

. In a directional microphone, sound reflected from surfaces behind the diaphragm is permitted to impinge 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 or phases may refer to: Science *State of matter, or phase, one of the distinct forms in which matter can exist *Phase (matter), a region of space throughout which all physical properties are essentially uniform * Phase space, a mathematic ...

. 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 An axis (plural ''axes'') is an imaginary line around which an object rotates or is symmetrical. Axis may also refer to: Mathematics * Axis of rotation: see rotation around a fixed axis *Axis (mathematics), a designator for a Cartesian-coordinate ...

, 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 The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of amplit ...

frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...

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

wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...

of the sound. This frequency dependence is offset by

damping Damping is an influence within or upon an oscillatory system that has the effect of reducing or preventing its oscillation. In physical systems, damping is produced by processes that dissipate the energy stored in the oscillation. Examples incl ...

the diaphragm 6 dB per

octave In music, an octave ( la, octavus: eighth) or perfect octave (sometimes called the diapason) is the interval between one musical pitch and another with double its frequency. The octave relationship is a natural phenomenon that has been refer ...

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 Sound pressure or acoustic pressure is the local pressure deviation from the ambient (average or equilibrium) atmospheric pressure, caused by a sound wave. In air, sound pressure can be measured using a microphone, and in water with a hydrophone ...

decreases as the

inverse Inverse or invert may refer to: Science and mathematics * Inverse (logic), a type of conditional sentence which is an immediate inference made from another conditional sentence * Additive inverse (negation), the inverse of a number that, when ad ...

of the distance from the source (it is

sound intensity level Sound intensity, also known as acoustic intensity, is defined as the power carried by sound waves per unit area in a direction perpendicular to that area. The SI unit of intensity, which includes sound intensity, is the watt per square meter (W/m2 ...

that drops as the inverse of the distance squared, for those familiar with the

inverse square law In science, an inverse-square law is any scientific law stating that a specified physical quantity is inversely proportional to the square of the distance from the source of that physical quantity. The fundamental cause for this can be understoo ...

), 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. 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.

References

{{Reflist Acoustics