Tue. Nov 30th, 2021

Let’s inside noise cancellation

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.

Noise Cancellation in a nutshell

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.

Turning waves into compressions and rarefaction

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. 

The example of removing the noise from traffic

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.

Issues with noise cancellation insulation

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 2nd issue: frequency

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 3rd issue: Adding in the music

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.

By Ahsan