How noise cancellation works

Noise cancellation has become much more common over the last 5 to 10 years, particularly with how many people now own some kind of noise cancelling headphones. Despite how widely it's used, I'd be willing to bet that most people don't actually understand how the technology works (myself included until recently). Technologies like Active Noise Cancellation (ANC) allow headphones to reduce external sounds without needing to turn up the volume. There's a bit more going on behind the scenes than simply blocking out noise.

Understanding sound waves

One of the key things to understand first and foremost is that sound travels through the air as pressure waves, which are created when something vibrates and causes nearby air molecules to move back and forth. This creates alternating areas of higher and lower pressure that travel through the air until they reach our ears. These pressure changes are what we perceive as sound.

How active noise cancellation works

Regular headphones are unable to prevent external sounds, which is why you can still hear them. ANC headphones however have a mechanism to detect surrounding noise and then have the speakers produce “anti-noise” to then cancel it out. This is called destructive interference, and it happens near the ear area. ANC headphones must also be able to adapt to changes in surrounding noise by constantly listening, and hence adjusting the sound waves they produce.

Different types of ANC

There are three kinds of noise cancellation:

• Feedforward ANC: This is where the external microphone on the outside detects the noise before it reaches the ear. Because it detects it early, it has more time to process the signal.
• Feedback ANC: This is where microphones are inside the earcup, and are used to monitor the sound which has already reached your ear in real time. It's best for steady and low frequency noise. Things like air-conditioning hums and rattles, and office machinery get have their noise levels significantly reduced.
• Hybrid ANC: This is a combination of both, using microphones on the outside and inside of the headphones. This allows the system to detect external noise while also measuring the sound reaching your ears, helping it cancel a wider range of frequencies more effectively.

Why it isn't perfect

Noise cancellation in general isn't perfect, and there are some noises which are more difficult to cancel out. Low-frequency noises are much easier to cancel. The waves have much longer periods, which means that the pressure changes happen more slowly. It gives the system more time to determine the opposing sound wave. That's why it works really well against things like air con, engine noise, and train rumbling. Things like speech and sudden noises are much harder though. Even though they may be quieter, it combines a mixture of frequencies that constantly change as someone talks. Because they're higher frequency, they have shorter periods, which means the pressure changes happen much faster. Sudden sounds like someone dropping something, or clapping when the plane lands, are difficult because they're unpredictable and happen very quickly.

Why headphone design matters

With this underlying technology, the design of the headphones becomes much more important. One thing in particular is microphone placement, since the microphones need to accurately detect the surrounding noise for the system to cancel it effectively. Processing speed is also important because even a small delay in producing the opposing sound wave can affect how well the cancellation works. Another factor is how well the headphones fit around your ears. A good seal helps physically block external sounds, particularly higher-frequency noises that ANC struggles to cancel. This is why even expensive noise cancelling headphones can perform poorly if they don't fit properly.

One thing you might have noticed if you own a pair of noise cancelling headphones is that sometimes they can produce a faint hissing sound when you turn them on. This can come from the microphones and the electronic processing involved for activation. Another side effect is a weird pressure-like sensation in your ears, similar to being on an airplane. This isn't caused by actual changes in air pressure, but it may be related to how ANC changes the low-frequency sounds we normally hear.

Where noise cancellation is used

Noise cancellation has been used in a lot of products. Sony, Bose, and Apple have incorporated this tech into many of their products for regular people. It's also used commercially such as aviation headsets, where the engine noise is a genuine concern. There are also some vehicles where microphones and speakers use this in order to reduce the cabin noise.

My take

I think the most interesting thing about ANC is that it produces more noise in order to reduce noise. The fact that headphones are able to constantly detect surrounding noise, process it, and produce opposing sound waves without us even noticing is pretty cool, and makes me appreciate the underlying tech more. It's also something which a lot of people take for granted, especially after prolonged use. Even though it's not exactly perfect (yet), it's a clever solution to an everyday problem.