Why is noise cancellation difficult?

[George]

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.

 

[Kryten]

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.

 

[Richard H.]

George wrote:

Anyway, it occurred to me at some point that I
needed to design a noise cancellation system for
my bedroom.

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.

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

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