How Does Sound Cancelling Headphones Work?

active noise cancellation

• active cancelation uses a microphone to listen to the sound, and then make an "opposite" sound by pushing air in the other direction. these opposite sounds are added to the music coming through the headphones.
• by the time it gets to your ear, the combined air from the original (ambient) sound and the opposite sound created by the device cancel out (mostly), so you hear more of your music with less outside noise.
• the music quality is a bit less because the music has extra sound added on top, but there's no more danger to your ears than listening to regular music.

sound waves and opposite waves

• sound wave , with peaks and valleys, coming out of a speaker. now add a second speaker playing the exact same sound. if the peaks of the first sound wave line up with the peaks of the second sound wave, the overall sound gets louder. if the peaks of the first wave line up with the valleys of the second sound wave, they cancel each other out.
• noise cancelling headphones have a microphone to "hear" the sound around you. they take what they "hear" and produce a sound that's the same, but shifted, so the peaks line up with the valleys. this cancels out the sound around you.
• it does remove the sound
• sound is pressure, alternating above ambient (positive) and below ambient (negative). when you play the opposite waves, the positive cancels with the negative and results in zero.
• so it does remove or "flatten" the wave.

push and pull of air

• sound waves are made up of crest and troughs (loudness and silence). the loudness comes from the air getting pushed and pulled by the speaker. now imagine what would happen if you could somehow set up another speaker that detects the sound and quickly emits the opposite. so if it detects and loud pushing sound, it'll emit an equally loud pulling sound. they would cancel each other.
• that's what a noise cancelling system does. it has a sound detector that converts sound into an electrical representation. that electrical signal is fed into an inverter which converts it into the opposite electrical signal. this signal is then fed into a speaker to turn it back into sound. since it's the opposite sound to what was detected, they cancel each other out.
• this system only works because electricity is far far faster than sound, so the inversion process and the sound emission process are almost instantaneous, so they're able to cancel the original sound before it travels away.
• sound is a push and pull of the air around your eardrums. active noice cancellation listens, and pushes while ambient sound pulls, and it pulls when the ambient sound pushes, thus cancelling out. it's not damaging to your ears, because it's literally stopping sound from moving your eardrums, same as if you were in a quiet room

digital signal processing

• digital signal processing , so let me just add a little bit more technical detail.
• the moving, vibrating of air has frequency. the noise, outside sounds consist also of various frequencies. the microphone picks up the sound, and then a fourier transformation is done on it. it's one of the basic mathematical transformations. the result will be the frequency spectrum of the sound, the frequencies of which the sound consist of, as well as the phase for it.
• by rotating the phase and adding every frequency together (creating a new sound), you get the inverse of the original noise. now if you add the noise and its inverse together, they both cancel each other out (with an error margin of course),reducing the overall outside noise.
• since you are adding some "noise" to the music, you degrade its quality somewhat.

active and passive cancellation

• noise cancelling headphones do two things. they isolate the noise with the padding around your ears and muffle it a bit. like what your regular headphones do. the second thing is that they have a microphone built into each side that records the abient noise and inverts the signal in real time. constant sounds with constant bandwidth disappear ( like jet engine sounds in a plane). irregular sounds with varrying bandwidth like talking are muffled and almost silenced because irregular sounds are hard to invert quickly.
• active noise cancellation works better on annoying constant white noise, like airplane engine noise.
• there is another way to dampen sound, usually used for loud environments like shooting ranges or industrial environments. the earmuffs you wear are actually passive, meaning that even without the system turned on, they are like normal earmuffs with a high noise reduction rating. however they use microphones to transmit sound to your ear but they limit the maximum amount of sound they allow.
• so at a shooting range, you can talk normally to people but when a gun shot goes off the sound is either cut off, or the sound is allowed through, but at safe volume.
• it’s like watching tv where they just turn the volume down on really loud harmful noises.

noise isolation and terminology

active noise cancellation and noise cancelling

noise cancelling and active noise cancellation (anc) mean exactly the same thing. both terms refer to the way headphones (or earbuds) use smart electronics to cancel out background and surrounding noises. this is done with mics and algorithms: the mics pick up the sounds from your environment, and the algorithms devise clever ways to cancel them out before you hear them.

passive noise cancellation

passive noise cancellation (pnc) or ‘noise isolation’ are when the physical design of your headphones seals out unwanted noise. the materials used - and the way in which the headphone ear cups cover your ear, or the earbuds sit in your ear - create a seal that reduces the noise you hear from your environment.

noise isolation

it’s worth noting that ‘passive noise cancellation” is a bit of a misnomer because unwanted external noise is dampened rather than cancelled. that said, the damping is pretty effective.

microphones and algorithms

• in truth, wired and wireless noise cancelling headphones both use active noise cancellation (anc) to cancel noise in the exact same way. the only difference between wired and wireless headphones is how they connect to your listening device.
• the theory behind noise cancellation is all to do with the fact that one set of sound waves can be ‘flattened’ by equal and opposite waves going in the other direction.
• all waves come in peaks and troughs: the peak is the high part and the trough is the low part. (note: the frequency with which those peaks and troughs repeat dictates how high-pitched or low-pitched the sound is. a loping bassline has a low frequency, while hi-hats and snare drums have high frequencies.)
• here’s the clever bit: you can cancel a peak with a trough, and you can cancel a trough with a peak.
• active noise cancellation does the exact same thing to sound waves. the anc system in a pair of wireless noise cancelling headphones records the sound waves created by unwanted noise, then creates an equal and opposite sound wave to flatten it out.
• the result: you won’t hear what you don’t want to hear. with no peaks and troughs left to register as sound, those unwanted sound waves simply…vanish.

how the system knows what to cancel

• so, noise cancelling headphones cancel out sound waves with other sound waves. but how does the active noise cancellation (anc) system know which sounds to cancel?
• all noise cancelling headphones use mics to ‘listen’ to the sounds around you. basically, the mic or mics record the outside noise and send this recording to the anc system in the headphones. the anc system then plays opposing sound waves to cancel out the unwanted stuff.
• the location of the mics on your noise cancelling headphones tells you what kind of anc system you have - and each system works slightly differently.

feedback active noise cancellation

• a feedback anc system has the noise cancelling mic (or mics) placed inside the ear cup or earbud (for in-ear noise cancelling headphones).
• detect external noise with a very slight delay in some frequency ranges.tell the anc system what the mic has ‘heard’.produce an opposite sound wave to cancel out the unwanted noise.‘listen’ to what you’re hearing to check that the noise cancellation is accurate.
• because a mic placed inside the ear cup or earbud can detect what your ear is hearing when you wear the headphones, the feedback system can check its own effectiveness. if external sounds become varied, the system is more able to adapt and keep the noise cancellation working effectively.
• on the other hand, a feedback anc system doesn’t cancel out midrange frequencies (like speech) very quickly. that mic inside the ear cups we just mentioned? it has to wait for sound to travel through the headphone’s physical body before it can detect it.
• so there’s a slight delay between the moment a noise is made, and the moment the mic picks it up. the sound can’t be cancelled until the mic has ‘heard’ it, which means you might hear the noise before the anc system has dealt with it.

hybrid active noise cancellation

• if a pair of noise cancelling headphones have a mic (or mics) outside the ear cup or earbud, in addition to the mic (or mics) inside, then it’s using a hybrid anc system.
• react quickly and accurately to external soundsreact to a wider range of frequencies
• a hybrid anc system can more quickly recognise and mask a wider range of frequencies because the internal and external mics work together.
• the external mic (or mics) ‘hear’ the sounds in your environment as soon as they are made. the internal mic (or mics) then ‘listen’ to what your ear is actually hearing, just like in a feedback system.
• if anything from the outside world appears to have leaked into the mix, the hybrid anc system corrects itself to give you the best possible listening experience.

pitch levels and external sound

• noise-cancelling technology cannot wholly isolate sound due to the air trapped between your audio device and eardrum. however, the air is essential for you to hear music.
• suppose you removed the trapped air; the net effect is silence, no matter what volume your music is on. why? there would be no medium for the sound to travel from the audio device to your eardrum.
• noise-cancelling works best when the ambient noise is low or medium in pitch. bass is regarded as a low-pitched sound, whereas treble is a high-pitched sound.
• an aeroplane engine produces a low-pitched, constant sound meaning that the sound you hear remains consistent.
• when someone speaks, sound waves will scatter based on the volume and energy created by every word and sound produced. unlike an aeroplane engine, your speaking pitch changes frequently.

sounds usually masked

continuous engine noise

yes

background traffic sounds

yes

conversational murmur

yes

dog barking

no

car horn

no

loud shout

no

hearing and volume

• damage your hearing . with noise-cancelling headphones, people don't feel the need to crank them because there's hardly any outside noise. no cranking means less damage to your ears, so noise-cancelling headphones can actually be beneficial. :)
• and no, unless the sounds involved are very loud, noise cancellation shouldn't damage your ears any more than the original sound might have.
• i would have to guess that people using noise cancelling headphones generally listen to them at lower volumes (likewise for just plain over-ear headphones versus earbuds) and that they are therefore generally safer for hearing
• by wearing active noise canceling headphones in environments where inaudible pitches may affect your hearing, you could inadvertently be protecting yourself. just driving, or being on an aircraft, or mowing a lawn or operating a cordless drill are all acts where having any kind of sound damping is probably a good idea.