How do noise-cancelling microphones work?

-January 15, 2018

If someone is in a really noisy environment and has to be heard over an intercom or radio communications channel in spite of all that noise, one aid for accomplishing that is called a noise cancelling microphone.

The operating principle is essentially the following:

Figure 1 Noise cancelling microphone and a far away sound source

There are two identical microphone elements inside the microphone case which are connected in series with each other and are phased as shown. Any sound coming from far away arrives at the two elements as a pair of virtually identical acoustic excitations. Each excitation is picked up by its element which then generates an electrical output signal while the other element generates the identical but inverted output signal. Because of their phasing arrangement, the two output signals buck each other. They cancel each other out and there is suppressed electrical output in response to that far away sound.

Figure 2 Noise cancelling microphone and a nearby sound source

However, if the acoustic excitations arise from a nearby sound source and they each arrive off axis, the acoustic stimulations of the two elements are unequal and their electrical outputs only partly cancel. As a result, there is a significant electrical output signal in response to that nearby sound.

The underlying assumption is that a nearby sound source is a human being who is holding the microphone by hand or who is wearing a microphone and headset assembly of some kind. While there is always some critical axis which would yield the forgoing cancellation for a nearby sound source too, the chance of that nearby sound source actually being exactly on that axis is vanishingly small. Therefore, in practical terms, it just isn't going to happen that way.

[See how speech recognition software works with microphones to enhance performance: Alexa, can you hear me now? Advanced low power voice interface technology]

Also, while "cancellation" is a strong word and perhaps "suppression" might be more technically correct, the colloquialism of "cancellation" is in common usage and microphones made like this come pretty darn close to it.

For example, microphones made this way were used for the two crewmen in the F/A-18 fighter jet for cancelling out the background of jet engine noise.

Think about overcoming that decibel level.

John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).

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