Proximity effect (audio)

The proximity effect, also known as proximity effect, proximity effect or proximity effect, is an over-emphasis (higher sensitivity) of the low frequencies with directional microphones ( pressure gradient ) in the near field of a sound source. Together with the usual technical bass boost, which actually serves the purpose of compensating for the weakness of recording Druckgradientenmikrofonen at low frequencies, creating a dull, unnatural sound. This bass boost is usually undesirable, but can also be used as artistic design of the sound, for example, to generate a full singing voice.

Occurrence of the effect

The proximity effect occurs when a pressure gradient is close to the sound source. Sealing means within about one wavelength. As for low frequencies, ie large wavelengths, the near field is more extensive than for high frequencies, the low frequencies are overemphasized.

Double diaphragm for microphones that consist of two opposite pressure gradient transducers, the proximity effect occurs, albeit reduced, even when they are electrically connected together so that the pair of pressure as a microphone has an omnidirectional characteristic. A pair of pressure microphones, which is connected together, so that the pressure gradient is effective, shows the effect.

In contrast, pressure microphones interconnected as a microphone array, so technical signal delays that compensate for the different distance to the sound source, the effect does not occur.

In use, the effect can be avoided

• By a greater shooting distance;
• By the directional microphones with cardioid characteristic is held perpendicular to the main direction, because it is insensitive to the pressure gradient acts as a pressure microphone;
• By electronic filtering (bass attenuation).

Competing explanations

According to some sources close to the sound source strong curvature of the spherical sound wave is the cause. Does such a strongly curved wavefront to a pressure gradient, the sound pressure difference is created ( the sound pressure difference) between the front and back in two ways: In the far field caused by the different long distance the sound wave to the front and put back to back must one for frequency proportional pressure difference. This path difference is increased by the curvature of the wavefront. This results in an increase of the pressure gradient and the membrane is more deflected. This mechanism does not explain the occurrence of the effect for small diameter of the microphone capsule, nor the observed frequency dependence.

In the far field of the sound source sound pressure and particle velocity "in phase"; in the near field of the sound source, sound pressure and particle velocity up to a maximum of 90 ° out of phase. It must be said that pressure gradient depending on the moving mass and stiffness of the membrane on the particle velocity are sensitive and that for spherical sound waves, the sound velocity as the pressure gradient in the near field with 1 / r ² decreases ( in the far field with 1 / r, as the sound pressure in the near and far field).