1. Home
  2. Electronics Design
  3. Why is noise cancellation difficult?

Why is noise cancellation difficult?

Last night, about 2:00 am, my neighbor's dog started barking, and basically kept it up, on and off, until dawn. The neighbors were out of town overnight, and apparently the dog suffers from "separation anxiety" when no one is at home.

Anyway, it occurred to me at some point that I needed to design a noise cancellation system for my bedroom. In theory, I could place a microphone outside the window, or maybe even ON the window, and then place a speaker inside and a simple amplifier that would output the inverse of whatever the microphone picked up. Then with a little tweaking, voila, a silent dog. Or passing airplane. Or ambulance. Or whatever.

But then it also occurred to me that noise cancellation must be more complicated than that - because it has taken so long for this kind of device to become available even in headphones. There must be problems - including maybe timing or phase considerations - that require special digital signal processing.

Does anyone here have experience with, or expertise in, noise cancellation design? What am I up against here?

Of course, I could just buy some headphones, but just wearing them would probably keep me awake too.

Reply to
George
Loading thread data ...

[snip]

I have three dogs... but I don't allow indiscriminate barking.

One thing I've found that's very effective in training dogs... a water pistol loaded with dilute household ammonia.

I can break a dog of chasing cars in three trips down the street ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.
Reply to
Jim Thompson

From the perspective of consumer noise cancelling headphones...

Perhaps by design, but while they're good at cancelling continuous noise (a la aircraft engine) they pass sounds like dialog. E.g., you can hear your travelling companion much better than they can hear you.

They'd probably just make the dog's bark clearer. :-(

Reply to
Richard H.

Hi George.

It is difficult to cancel such noise because sound doesn't travel in a direct path.

If the bark was a point source at your window, then an inverse sound at a loudspeaker at the same place might do the job.

However, the sound is a complex wave front by the time it hits your window, and you would need a planar sound source that could produce an inverse wavefront.

You could try getting the barks to trigger a very loud ultrasonic screech aimed at next door. Hopefully it will get a taste of it's own medicine, and learn to shut up.

If it is too dumb to learn, it might get even crazier.

Complain to the neighbours. If they don't do something like make a soundproof kennel, record and play back their dog's barking when they are in and you are off down the pub or away.

Reply to
Kryten

Exactly.

The fact that the noise signal is quite different in various locations.

Lotus Cars IIRC worked on active noise reduction using a car's existing stereo speakers. It sort of worked I think. It didn't *cancel* the noise though, just reduced it a bit.

You can buy active noise cancellation headphones from various suppliers. Used in industrial applications and also popular with some pilots.

Oh - Bose now make one for aircraft passengers too !

formatting link

Noise cancelling headphones are much easier since there's only a small space that needs to be noise cancelled and the headphones provide some direct noise reduction anyway. Even so - the effect isn't 100%.

I suggest you get some double glazing or whatever for the dog problem.

Graham

Reply to
Pooh Bear

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.