Shure Apps Tech Tip: Microphone Distance Factor

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What is the Distance Factor for a microphone? In brief, it means that a directional microphone may be placed farther away from a talker than an omnidirectional microphone and still produce similar audio results. This assumes two microphones of equal quality and sensitivity.

As an example of the Distance Factor, let's consider a simple application: recording a talker's voice in a meeting room. Through experimentation, an omnidirectional mic is found to produce an acceptable recording when placed 2 feet away from the talker.

Acceptable recording = minimal level of background noise in relation to the talker's voice level. Rule of thumb: the talker audio should be at least 20 dB louder than the background noise.

Now try a cardioid microphone in place of the omnidirectional. The Distance Factor for a cardioid is 1.7. This means the cardioid may be placed 1.7 times the distance of the omnidirectional and produce the same audio quality. In this example, the cardioid may be located 3.4 feet away (2 feet x 1.7) from the talker and produce an acceptable recording. The Shure KSM141 is the perfect microphone for this experiment as it can be switched from omnidirectional to cardioid.

Next, try a supercardioid mic in place of the omnidirectional mic. The Distance Factor for a supercardioid is 1.9. So it may be placed 3.8 feet away (2 feet x 1.9) from the talker and produce an acceptable recording.

Then, put a hypercardioid microphone in place of the omnidirectional. The Distance Factor for a hypercardioid is 2.0. It may be placed 4 feet away (2 feet x 2.0) and produce an acceptable recording.

Finally, try a shotgun microphone in place of the omnidirectional microphone. The Distance Factor for a typical shotgun is 3.0, which allows the microphone to be placed 6 feet away (2 feet x 3.0) from the talker and produce an acceptable recording.

Remember that the Distance Factor is a multiplication function that directly relates to the audio quality obtained with an omnidirectional mic in a given acoustic environment. If an omnidirectional mic must be used at 1 inch from the talker for acceptable results in a noisy setting, then a hypercardioid mic must be used at 2 inches for the same results... not, not, not the 4 feet mentioned in the previous example above.

IMPORTANT: The increase in Distance Factor for a directional mic is due to its greater rejection of ambient (background) noise, not due to any increase in sensitivity to the desired sound source. In other words, the directional mic does NOT reach out and grab the sound emanating from the talker's mouth. Really, it does not…

When a mic is placed farther from the talker, more amplification is necessary to maintain the same output level. In a public address application, it is loudspeaker positioning that often dictates microphone location and overrides the Distance Factor in determining the maximum distance from microphone to talker.


SUMMARY OF DISTANCE FACTOR
Omnidirectional = 1 Cardioid = 1.7 Supercardioid =1.9
Hypercardioid = 2.0 Shotgun = 3.0