Active Noise Cancellation is a fascinating feature of high-end headphones that requires some rather
complicated engineering. Headphones such as these are able to eliminate or destroy unwanted noise
that is produced by the external environment, while simultaneously playing the desired music
or audio that is sent from your smartphone. In a nutshell, headphones such as these do
this by using a microphone to measure the unwanted noise produced by the environment
and then calculating an anti-sound wave. This anti-sound wave is added to the waveform of
your music or audio, and when the combined waveform is played through the headphone’s
speaker, the external noise is eliminated or canceled out, and just the desired audio remains.
That’s the basic principle, but there’s a lot of intricate engineering that underpins active noise
cancellation. In order to better understand the engineering, it’s easier to visualize sound waves
not as these sinusoidal patterns but rather as a set of traveling high-pressure zones,
known as compressions, and low-pressure zones, known as rarefactions.
Let’s say you live or work next to a highway. All the cars and trucks that zoom by generating
a sound with a waveform such as this, which is really just a sequence of traveling high pressure
and low-pressure zones. These compressions and rarefactions move through the air,
and when they hit your eardrum, you hear the sounds of the highway.
In order to produce an anti-sound wave for the highway’s noise, a sound wave must be generated
that is equal and opposite. By that, we mean that for every high-pressure zone from the noise,
a low-pressure zone needs to be generated, and for every low-pressure zone from the noise
a high-pressure zone needs to be generated. The noise and anti-noise meet, and when these
low-pressure zones and high-pressure zones meet or vice versa, they average out in that area,
thereby eliminating the sounds from the highway. There are a few engineering challenges with
this solution. First, the headphones or earbuds them-selves provide some passive noise insulation,
and therefore we need two microphones. One microphone is used on the outside to measure
the noise itself, and a second microphone is used on the inside of the head-phones
to measure the percentage of noise that’s getting through the sound insulating
properties of the headphones and into your ear. The second challenge is that it’s rather difficult
to perfectly time the anti-sound wave with the external noise. The noise from the highway has
hundreds of compressions and rarefactions every single second, so, if the anti-sound wave is off
by a few milliseconds, then the noise cancellation won’t work properly. Therefore, a high-powered
digital processor is used to measure and calculate the perfect anti-sound wave, while simultaneously,
the interior microphone is monitoring whether the active noise cancellation is in fact working
properly. Note that active noise cancellation is more effective for low frequency, repetitive
noises because they have only a couple hundred peaks and troughs, or wavelengths every second,
but high-frequency noises have thousands of wavelengths per second. If there’s even
the smallest inaccuracy in timing the anti-sound wave with the high frequency noise, the resulting
combination between the anti-sound wave and the high frequency noise will be ineffective.
The third challenge, managed by the processor and circuitry is that the anti-sound waveform
needs to be combined with the audio waveform that is sent from your smartphone. The audio plus the
anti-sound acts just like different instruments in a song and they combine to make a new sound which
is played through the single speaker. When the combined audio plus anti-sound waveform
and the noise from the environment mix, the anti-sound component of the combined waveform
and the noise from the environment cancel out, and we are left to hear just the desired audio.
We made a separate episode that goes over the different components of these headphones
and we’re working on future episodes that will cover other feats of engineering such
as Bluetooth communication, the antenna, and the micro-electrical mechanical system microphones.
This video was made possible by our sponsor
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Also, one quick note about this 3D model. These are the Apple AirPods 2, which don’t actually
have an active noise cancellation feature. In order to model the interior of these headphones,
we have to get detailed pictures of each component, which as you see can be rather
destructive. We have exterior models of these other rather expensive headphones which do provide
active noise cancellation, but no interior models of them. Sorry for the switch up which we hope
wasn’t too confusing. Furthermore, check out the creator’s comments in the English Canada Subtitles
where we discuss the difficulties of implement noise cancellation in a room.
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liking this video, and commenting below. This is Branch Education, Thanks for watching.