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Proximity EffectMost microphones that have a pressure gradient component have a correction filter built in to fix the low frequency problems of the natural response of the diaphragm. In many cases, this correction works very well, however, there is a specific case where the filter actually makes things worse. Consider that a pressure gradient microphone has a natually rising frequency response because the incoming pressure wave arrives at the front as well as at the rear of the diaphragm. Pressure microphones have a naturally flat frequency response because the rear of the diaphragm in sealed from the outside world. Also, consider a little rule of thumb that says that, in a free, unbounded space, the pressure of a sound wave is reduced by half for every doubling of distance. The implication of this rule is that, if you're very close to a sound source, a small change in distance will result in a large change in sound level. At a greater distance from the sound source, the same change in distance will result in a smaller change in level. For example, if you're 1 cm from the sound source, moving away by 1 cm will cut the level by half, a drop of 6 dB. If you're 1 m from the sound source, moving away by 1 cm will have a negligible effect on the sound level. So what? Imagine that you have a pressure gradient microphone that is placed very close to a sound source. Consider that the distance from the sound source (say, a singer's mouth...) to the front of the diaphragm will be on the order of millimeters. At the same time, the distance to the rear of the diaphragm will be comparatively very far - possibly 4 to 8 times the distance to the singer's lips. Therefore there is a very large drop in pressure for the sound wave arriving at the rear of the diaphragm. The result is that the rear of the diaphragm is effectively sealed from the outside world by virtue of the fact that the sound pressure level at that side of the diaphragm is much lower than that at the front. Consequently, the natural frequency response becomes more like a pressure transducer than a pressure gradient transducer. What's the problem? Well, remember that the microphone has a filter that boosts the low end built into it to correct for problems in the natural frequency response - problems that don't exist when the microphone is close to the sound source. As a result, when the microphone is very close to the source, there is a boost in the low frequencies because the correction filter is applied to a now natually flat frequency response. This boost in the low end is called proximity effect because it is caused by the microphone being in close proximity to the sound source. There are a number of microphones that rely on the proximity effect to boost the low frequency components of the signal. These are typically sold as vocal mic's such as the Shure SM58. If you measure the frequency response of such a microphone from 1 m away, then you'll notice that there is almost no low-end output. However, in typical usage, there is plenty of low end. Why? Because, in typical usage, the microphone is stuffed in the singer's mouth - therefore there's lots of low end because of proximity effect. Remember, when the microphone has a pressure gradient component, the frequency response is partially dependent on the distance to the diaphragm. Also remember that, for some microphones, you have to be placed close to the source to get a reasonably low frequency response, whereas other microphones in the same location will have a boosted low frequency response.
Next: Acceptance Angle Up: Microphones - Directional Characteristics Previous: Pressure Gradient Transducers Contents Index Geoff Martin 2006-10-15 Click here to purchase the entire book in PDF format. |