Noise Cancelling in Software?

doc_verma wonders: "There are directions to build noise-cancelling devices in hardware, but what would it take to create noise cancelling in software? Since computers have a speaker-of-sorts and can possibly have a microphone, why not take the input from the mic, reverse-phase it via software, and output it through the speaker? A noise-cancelling feature would be great to run on servers in a rack. It would also be a great app to run on your laptop when you are on a plane."

Too slow

[QuietRiot]

Software would generally be too slow. A DSP-based sound card could potentially be programmed to do phase-inversion and time-slew (to account for distance) but an ordinary desktop machine would generally need to be very fast and incorporate real time extensions in the kernel. This is the reason you generally find hardware-only solutions (and in headphones for the reasons mentioned in a peer post). Of course I could be way off....

This is possible with two specific caveats.

[stvangel]

The biggest issue is to only have one specific sound source you're trying to muffle. You have to place the microphone as near to the sound source as possible, and the location of your speaker will depend on how long it takes to process the signal. It will also be highly directional unless you rig something up with multiple speakers all aimed in different directions. You will also need to be a certain distance away to allow the reverse-phase sounds to merge and cancel each other out.

The other issue is that it will only work on low pitched sounds, the lower the better. The slower the frequency, the more time you have to do the math and output your opposite signal. High frequency sounds you can pretty much forget about being able to process.

One use for this might be to muffle a single exhaust pipe from a loud motor, but you won't be able to get it to work for general noise. The reason it works in Noise-cancelling headsets is that the microphone is located at the destination of the sound (your ear), rather than at the source. The closer you can get it either end, the better it will work. It's not really worth doing it in Software

One interesting thought about quieting individual fans or motors is to use an Adaptive algorithm. Because most of these devices make a consistent repeating sound, it would be interesting to be able to anticipate the sound source over time and broadcast an estimated calculation before the current sound is even processed and use the processed result to tweak the future estimation. This would work on only reasonably consistent sources and there would be issues with feedback and unexpected noise changes.

Re:Real-time sound

[munpfazy]

"you're" should of course be "your," to satisfy the grammar fanatics out there.