Eli5 how noise cancelling in cars works


Given that the general noise in cars (background hum of an engine on a freeway) is fairly predictable could a tech exist that just plays a generic opposing sound in a regular car, such as an mp3 track. This would enable your average user to experience this through a car stereo

In: 2

Why would you play a pre-recorded audio when ANC technology is so cheap both from hardware and computing power requirements standpoint while also being infinitely more versatile?

This already exists. Here is a quick description of what Acura does. I am sure others do similar.


The problem is, the sound isn’t the same in every seat of the car, and with noise cancellation you need the cancellation to be taking place as close to the ear as possible or else propagation delays will ruin the effect.

Mann I got nearly all of the way through writing a super in depth reply to this (since I did my thesis on it) then completely lost it because I don’t know how to use the app on my new phone 😭 Rip I’ll try recap it

Short answer: Unfortunately it would be extremely difficult to get active noise canceling (ANC) working in a car without a LOT of setup. It definitely wouldn’t be possible to use a prerecorded track.

How ANC works:
(it’s worth noting that as the size of the space you’re trying to noise-reduce in grows, the complexity of the audio setup and processing you’re going to need grows pretty exponentially. A space like the inside of a car is HUGELY more fussy and difficult than, a pair of headphones.)

The basic idea behind ANC is that when two sound waves overlap with eachother, they will combine into a single wave whose intensity at any given point is just the sum of whatever it would have been for each original wave at that point (had they not combined). The way waves combine with each other is called wave interference.

Sound waves are made up of rapidly alternating sequences of areas of high pressure (known as peaks) followed by areas of low pressure (known as troughs). An ANC system needs to output a perfectly timed wave at exactly the right volume so that every time the external wave provides a high pressure peak, the ANC wave will provide a matching low pressure trough (and vice versa). So then when these two waves overlap eachother, they should (ideally) at every point be an exact negative of the other, therefore summing to a combined intensity of zero at all times = no sound 😎 ([a very basic ](https://i.imgur.com/Oh0Orm3.jpg) This is called destructive wave interference – the goal is essentially to completely neutralise and thus destruct an incoming wave.

A low frequency sound like road noise I think would have a frequency of, say, 200Hz, meaning that every 1 second, 200 pairs of peaks and troughs go past. With that many waves going psty every second, it’s essy to see how a generic recorded track has pretty much no hope in hell of ever matching perfectly up to the waves of external noise. The noise cancellation wave being output must perfectly match the background noise for noise cancellation to work. If the frequency is even slightly different. the wave they combine to make will normally be a fluctuating garble ([ref pic](https://i.imgur.com/CQeBGwy.jpg) that is at times louder than the road noise alone.

As a kind of worst case scenario to consider, imagine that your track happened to be exactly the same frequency as your road noise. If it started playing half a wave length too early or late (that’s 1/400th of a second lol), you would end up with every peak from your speakers matching up perfectly with a peak from the external wave (and every trough matching up with a trough also) = amplifying the road noise to be double as loud. (This is called constructive wave interference btw!) So ya rip even if given a physical copy of the track that would be able to completely neutralising the road noise, you would have just as high a chance of increasing the road noise as ypu would of minimising .

It’s difficult even for a proper ANC system to get everything running well. The systems use a contimuous microphone feedback loop to evaluate and test adjustments in real time. A typical ANC setup uses 4-6 microphones distributed throughout the car (often at various places around the ceiling) connexted up to an ANC controller (the “brain” of the ADC system ~ the countrol board that processes mic inputs and eventually creates and sends output streams to the speakers). The system has to be both extremely precise and extremely fast in order to react to noises in real time. The components all need to have extremely low latencies and transfer/conversion speeds. the microphones were impressively sensotive. all of the controllers I’ve looktd at also pacled some seriously beefy processing power ~ normally they were sampling input devices well above above micro (=100,000samples/second) though so that makes sense.

Ob I was also going to say something about speaker placement in normal cars not being very good for it either. The reason why there needs to be so many speakers and microphones working together is because a) the region of effectiveness for active noise cancelling is normally within a pretty narrow kind of wedge radiating out of the speaker. On either side pf the wedge there are regions constrictive wave interference (= volume amplication). and if you were using car speakers on the side doors. ypu’d probably only get a pretty narrow stripe pf noise reduction between the two speakers, then you’d get probably weird patches of loud and quiet throughtout the space. Most cars with ANC use 4 speakers set as far back into the corners as possible.

Ya anyway pretty much every part of in-car ANC that I saw seemed at least somswhat inaccessible in some way – everything ftom the insane techinal complexity of proceses through to the qualiy(cost) of hardware components makes it seem like it’s not a tech anywhere near ready for being adapted into a cheaper mass produced consumer producr any time soon