Radio announcers use it to make their voice sound bigger and more impactful. The so-called proximity effect leads to a bass-boost in directional microphones. But why does it occur? We have asked our acoustic engineers.
What is the proximity effect?
The proximity effect occurs when the microphone is too close to the sound source. Imagine you are speaking into a microphone. The bass becomes stronger the closer you hold the microphone to your mouth.
The proximity effect is a double-edged sword. It can help to make your voice sound bigger (and "better"). But be aware, too much bass can make your recording sound muddy.
Does it occur on all microphones?
The proximity effect occurs in various degrees of intensity with different types of microphones (e.g., pressure gradient microphones) and polar patterns.
It does not occur at all with the omnidirectional pattern, and it is most pronounced with the figure-8 pattern.
The most commonly used pattern is the cardioid polar pattern, and it has a fairly strong proximity effect.
Directional microphones or pressure gradient microphones
Ok, let's get technical.
Directional microphones respond to differences between the pressure at the front and back of their diaphragms.
We generally refer to these as pressure gradient microphones.
When directional microphones are placed in close proximity to a sound source, the output signal is generally exaggerated at low frequencies.
We refer to this as the ‘proximity effect’, or ‘bass tip-up’ and it is the reason that radio broadcasters often have such ‘deep’ sounding voices or why kick drums sound so ‘boomy’ when recorded at close distances with directional microphones.
The extent to which this effect occurs is dependent upon the type of directional microphone used and its operating principle.
When the sound source is far away, the sound arrives in waves which are generally regular and flat in shape. We refer to these as ‘plane-waves’.
Under these conditions, the resulting ‘pressure gradient’ across the microphone diaphragm is smaller at low frequencies, since the wavelength is large compared to the microphone dimensions.
When the microphone is close to the sound source, i.e. the distance is comparable to the wavelength, the sound arriving looks more like a ‘spherical wave’. Under these conditions, the difference in pressure between the front and back of the microphone diaphragm becomes greater towards low frequencies.
For this reason, we experience an exaggeration of low frequencies when placing directional microphones close to sound sources.
Omnidirectional microphones or pressure-operated microphones
Most omnidirectional microphones respond to changes in pressure on only one side of the microphone diaphragm. We refer to these as pressure-operated microphones. These show no variation in low-frequency behavior when placed near a sound source.
Figure-8 microphones have the most pronounced proximity effect.
Directional microphones generally respond to the pressure gradient of a sound source.
Figure-8 microphones rely purely on the pressure-gradient principle and are therefore the most influenced by the proximity effect.
Cardioid microphones rely on a combination of pressure gradient and pressure operated principles and are therefore somewhat less influenced by the proximity effect.
The pressure gradient principle also relates closely to the particle velocity of air, while the pressure operated principle relates to changes in sound pressure.
Wind and wind noise effects typically involve fluctuations of particle velocity and microphones that operate purely with a pressure gradient principle are more sensitive to this, while microphones which operate mostly with a pressure-operated principle are less affected.
One way to combat the effects of wind noise is to try to reduce the particle velocity of air near to the microphone - for this reason, placing windshields and pop-filters on microphones can reduce the effects of wind and vocal plosives. They are reducing the movement of air close to the microphone capsule, improving the quality of your recordings.