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8.3 - THE PROXIMITY EFFECT

Pressure-gradient microphones produce an increase in bass frequency response when the sound source is relatively near the microphone. The amount of bass increase is directly proportional to the percentage of the bi-directional component in the directivity response equation. Omnidirectional microphones have no proximity effect, while bi-directional microphones produce a maximum of bass boost. Care must be taken when measuring the frequency response of pressure-gradient microphones to make the measurement at a distance where the proximity effect will not invalidate the bass frequency measurements – this requires approximately half a wavelength between the sound source and the microphone under test^[13]. Compare the frequency response curves in Figure 69 to see the difference in response at various distances.


Figure 69 - Bass Frequency Response Measured at Various Distances
© response curve courtesy of David Josephson



For measurements to be valid this would imply a distance of about 8.5m at 20Hz, 3.4m at 50Hz or 1.70m at 100Hz. This also applies to the distance to the surrounding surfaces during measurement i.e. the walls of the measuring room. Microphone frequency response curves are normally drawn down to about 50Hz, but few of these curves have been measured at 3.40 metres - the majority have been measured at 1 metre or in some cases even less. If frequency response characteristics for pressure- gradient microphones showed some indication of measurement distance, then an estimation at least of any bass boost due to proximity effect could be made.

To understand the origins of this proximity effect it is necessary to re-examine the response of a pressure-gradient microphone but under both plane wave and spherical wave conditions. The differentiation between these two limit situations is in fact just a matter of scale, because any small sound source in free field conditions will produce spherical wave propagation. However our analysis of the proximity effect for a microphone takes into account the relation between the size of the microphone and the distance from the microphone to the sound source – sound coming from a certain distance (>2 metres) will be considered as an approximation to a plane wave, whereas sound coming from a source very much closer, can be considered as generating a spherical wave front. Of course this assumes that the size of the sound source itself is small.

Sound wave pressure is inversely proportional to the distance from the sound source – sound at 5cm from the sound source will be 10 times louder than sound at 50cm from the sound source. The decay of sound pressure with distance is shown in Figure 70 – we can see that sound pressure decreases very much more rapidly with distance near the sound source (in spherical wave front propagation) compared with further away (in plane wave propagation).

We need to analyse the sound pressure at the front and the back of the diaphragm of a pressure-gradient microphone but this time we must compare the response within spherical wave propagation conditions and plane wave propagation conditions. As described in section 4.2 (Pressure Gradient Acoustic Coupling), the pressure difference is determined by the apparent path length difference between the sound reaching the front and the back of the diaphragm – this is valid in plane wave propagation conditions. However in spherical wave propagation conditions the sound arriving at the back of the diaphragm will be attenuated proportional to the apparent path length difference.


Figure 70 - Decay of Sound Pressure as Distance Increases

In Figure 71 we can see the pressure-gradient function in plane wave propagation conditions, whereas Figure 72 shows the pressure-gradient function in spherical wave propagation conditions. The lower curve (dashed line) in Figure 66 represents the attenuated sound pressure at the back of the diaphragm.


Figure 71 - pressure-gradient function
in plane wave propagation conditions



Figure 72 - pressure-gradient function
in Spherical wave propagation conditions


If we compare these two diagrams (Figures 71 & 72), we can see that the medium and high frequencies remain almost unchanged, whereas there is a considerable increase in pressure-gradient for low frequencies in spherical wave propagation conditions. This explains the bass frequency boost observed when the sound source is close to the microphone.

Certain specialized microphone designs can turn this bass boost characteristic to their advantage. The well known commentators microphone shown in Figure 72 designed by the BBC is a typical illustration of this phenomena used to improve the noise rejection characteristic of the microphone. In most commentary situations such as a ring-side boxing commentary or a motor racing commentary, the ambient noise level is very high. In using a bi-directional microphone very close to the mouth, it is possible to incorporate a high pass filter into the microphone design so as to re-establish a flat frequency response for the commentator’s voice. At the same time the filter reduces considerably the level of any surrounding ambient sound in the bass frequencies – around 20dB attenuation at 200Hz. In this type of microphone the precise distance between the sound source and the microphone is critical in order to produce a calibrated flat frequency response of the commentator’s voice – the microphone therefore usually includes a spacing bar or lip-guard which must touch the upper lip.


Figure 73a - Coles 4104 BBC Commentator’s Microphone
photo and response curve (courtesy of Coles) from the Rycote Microphone Data


This approach to the reduction ambient sound (and acoustic feedback in the presence of sound reinforcement), is also a standard technique used in the design of the majority of vocal mics or general purpose hand-held microphones (Figure 74).


Figure 73b - low frequency roll-off
to compensate for proximity effect
response curve by courtesy of Schoeps

The bass frequency filter is usually incorporated into the microphone housing. However as the distance between the sound source and the microphone is impossible to control, the bass boost is usually only partially compensated. However a good singer soon learns to use this to his advantage, using the microphone at arms length for the louder voice levels, and approaching the microphone in the softer passages to obtain a warm bass boost sound in the more intimate moments in the song. Vocal mics and general purpose hand-held microphones must also be designed to minimize ‘popping’ from air-flow generated by the mouth – this is called anti-plosive protection and will also tend to minimize handling noise transmitted through the microphone casing.

[13] 1999 – "A Brief Tutorial on Proximity Effect" by David Josephson, Josephson Engineering, San Jose, CA, USA 1999 – 107th AES Convention, New York, USA – Preprint 5058