Question of the Week

How do noise cancelling headphones work?

Sat, 4th Jun 2016

Part of the show Your Home In 2050

Question

Mark Cobb asked:

How do active noise cancelling head phones work? If they play back loud ambient noise could it be dangerous to our ears?

Answer

Fanny Yuen  put this question to Trevor Cox, professor of acoustic engineering...White headphones from the 1970's, model

How do active noise cancelling headphones work? If they playback loud ambient noise, could they be as dangerous as listening to loud noises?

Fanny - Thanks Mark. I called up Dr Trevor Cox, Professor in Acoustic Engineering at the University of Salford to cut the rumble on this question.

Trevor - Well normal ear defenders that you might wear to protect yourself from damage from noise, normally have just a sort of cup which gets in the way, sort of physically block sound from the outside world into your ear canal. But what active noise control headphones do is they actually use electronics. So they have a little microphone on the outside that picks up the noise you donít want and on the inside of the cup theyíve got a little loudspeaker which plays the inverse of the sound and that cancels out the noise.

And I can give you an example here. Iíve taken a recording of the International Space Station and Iíve shown you what it might sound like inside active noise control headphones. So what will happen is youíll first of all here this noise which is all the machinery going one, and then I fade in the inverse of that noise on top of the original and youíll hear how it cancels out the original sound and virtually gets rid of itÖ

How the active control headphones remove the sound is by a process of interference. So if you can imagine a sound wave, you know, you often see them as a squiggling sine wave, a series of peaks and troughs. Just think of the ripples on top of a pond where youíve got peaks and youíve got troughs across the pond. What happens is youíve got that original noise and thatís got that sort of wavy pattern, and then youíve got the inverse of - literally imagine flipping it over so where there was a peak in the original, thereís a trough in this new sound that youíre trying to produce to cancel things out.  And when you add a peak to a trough you get nothing because the cancel each other out and it does that through the whole sound and thatís how it removes the noise and cancels it.

Fanny - So if you play two noises itís going to be twice as loud but, if you play one sound and then invert it, so that the peaks and the troughs cancel each other out, then actually you get no noise at all. And you wonít hurt your ears?

Trevor - Well yes, if you imagine you were just in your kitchen and you put a kettle on, and you put another kettle on, itís obviously just going to get louder, thatís because all the peaks and troughs are randomly arranged. When one kettle is making a sound it has no relationship to whatís going on in the other kettle. But, if I was to take a recording of the first kettle and to invert it, so to flip around the peaks and the troughs, and play it out of a loudspeaker next to the kettle, then these peaks and troughs would exactly align and, if you do it properly, you get nothing. Of course, with active control headphones there's a little bit left over because nothingís perfect.

Fanny - A big thanks to Professor Trevor Cox from the University of Salford.

Next time weíre going to untangle our tentacles with this question from Android:

Iíve heard that octopus are colourblind, so how can they so reliably match their background colours when they camouflage themselves?

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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: https://en.wikipedia.org/wiki/Noise-cancelling_headphones 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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