Tip #5:
Microphone Performance in the Reverberant Sound Field

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The reverberant sound field is defined as all sound reaching the microphone that is reflected off walls, ceilings, floors, and other reflective surfaces.

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In the reverberant sound field, the null of the pickup pattern cannot be used to control offending sound sources. However, a unidirectional pickup pattern, compared to an omnidirectional  pattern, will provide improved gain-before-feedback and lower ambient sound pickup. In this situation, the narrower the pickup pattern, the better the performance, with the hypercardioid being the best, followed by the supercardioid, followed by the cardioid.

As a gauge of performance, a microphone pickup pattern has an associated directivity index (D.I.). The greater the index number, the narrower the pickup pattern.

Pattern

Omnidirectional
Cardioid
Supercardioid
Hypercardioid
Directivity Index

1.0
1.7
1.9
2.0

dB of Reverb Rejection

   0 dB
4.8 dB
5.7 dB
6.0 dB

Example: The reverberant field efficiency (D.I.) of a supercardioid is 1.9 times better than that of an omni a 5.7 dB improvement. This means that the supercardioid picks up 5.7 dB less reverb than an omni when both mics are in a reverberant sound field.

Question: Compared to an omni in the reverberant sound field, how much improvement in gain-before-feedback can be realized by replacing the omni with a cardioid?
Answer: 4.8 dB.

Question: If an omni in the reverberant sound field is placed 1 foot from a person speaking, how far away could a hypercardioid be placed from the person speaking and yield the same result?
Answer: 2 feet.