Science Podcasts

Question of the Week episode

Sun, 5th Jun 2016

How do noise cancelling headphones work?

White headphones from the 1970's, model "C525". (c) PJ and Piko

Mark got in touch to find out how noise cancelling headphones work. Can they damage your ears by playing back loud background noise? Fanny Yuen spoke to Trevor Cox to cut the rumble...

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No. Noise cancelling means just that. alancalverd, Thu, 19th May 2016

It is true that noise-canceling headphones are optimized for reduction of noise in the human hearing range (approximately 50Hz to 20kHz for a child).

They do this by measuring and reducing the sound pressure in the ear canal. If they are working properly, there will not be dangerous levels of audible noise in the ear canal.

However if there were considerable amounts of ultrasound (above 20kHz) or infrasound (below 50Hz) in the environment, the headphones would probably not do a good job of canceling it.
The microphone, amplifier and loudspeaker coil will have a certain frequency response, which is not so good outside the audible range.
For digital devices, the Digital Signal Processor (DSP) will have a certain sampling rate, eg 44kHz (as used in CD players), and in this case, frequencies above about 20kHz will not be processed or canceled
Passive ear cushions will filter out some ultrasound, but will not be so effective at filtering out infrasound.

See: evan_au, Fri, 20th May 2016

An engineer described NC as follows in a way I found easy to understand. The goal is to stop the headphone housing from vibrating. Ordinarily, noise strikes and vibrates headphone material, which in turn vibrates the air on the air inside, and so sound passes to the ear. Where the headphone material not to vibrate, then no outside noise can pass through. Active noise cancellation stops the headphone housing from vibrating, by doing all it can to hold the material dead still.
When the pressure wave pushes, then the headphone housing pushes back on the sound. When the pressure wave sucks, the headphone sucks back in the opposite direction. The mic on the outside tells the active element when, and with what volume, to push and pull. The sound waves strike an immovable object and ideally all of it is reflected away, non passing through to the ear.
Erichiggs, Wed, 8th Jun 2016

The hearing function is quasi logarithmic hence the efficiency of the noise cancelling can be expressed in db how good are they ?.
syhprum, Sat, 11th Jun 2016

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