A Brief Tutorial on Proximity Effect
Most directional microphones use the pressure gradient method to produce the desired directivity. Sound is sensed at two or more points on the microphone body, and the difference is used to favor sounds coming from some directions while reducing sounds coming from others. Microphones intended for a single purpose can be optimized to have the desired response at a certain distance, but the frequency response will always change with distance due to the fixed spacing between the sound sensing ports of the microphone. Over the years, there have been several approaches taken to reduce this proximity effect. This paper will discuss the basic physics of sound pressure and pressure gradient in the near and far field, common methods used in microphone design to achieve directivity using pressure gradient, and the practical results for the microphone user. Two main points will be discussed in detail, which are often missing from the literature on this subject. The first is that proximity effect is not only an issue for close sources but also for distant ones in many cases. While the international microphone characterization standard IEC 60268-4 suggests measurement of frequency response in the plane wave conditions (or the far field), very few microphone makers actually report this information. The second point is that proximity effect occurs only where there is pressure gradient: a ideal cardioid microphone has no proximity effect at 90 degrees. Differences between theory and reality will be illustrated with near- and far-field measurements of a number of dynamic, ribbon and condenser microphones.
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