How Do Active Noise-Cancelling Headphones Work?

Noise-Cancelling headphones are designed to cancel or block out noise, and they fall into two basic categories. The simpler type use what is called passive noise reduction, and work by screening out noise with a barrier. They cover the ear completely, using a high-density foam or composite material to lower the level of sound pressure by as much as 25 or 30 dB.

Such a reduction is impressive, and can lower the noise of a motorcycle down to that of a loud conversation. Such headphones are large and bulky, and bear little resemblance to even the over-ear type of headphones used for listening to music, although for anyone who remembers the headphones of the 1970s and 1980s, they may strike a familiar chord.

When a 15-20dB reduction is not enough, the second method of sound mitigation comes into play. If passive noise reduction cannot do the job, then active noise reduction is the method of choice.  

This method of noise mitigation is based on the same sort of barrier that the passive types use, but then an electronic means of noise-cancellation is added. A spectrum of sound waves, tuned to the frequency of the outside noise, is generated. That signal is designed to be 180 degrees “out of phase” with the sound outside, effectively cancelling or silencing it.

To accomplish this, built-in microphones sense the frequency of the outside sound, and electronic components create its “opposite,” effectively cancelling that range of frequencies, protecting the user’s hearing.

The active type of headphone often also has the added benefit of being able to connect to an external sound source or two-way radio, adding to its usefulness on the job site.

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