Re: Noise cancellation

[brian whatcott <inet@INTELLISYS.NET>]

> Steve Richardson writes:
>
> I am curious about noise canceling done in machinery. I have heard that they
> isolate the waveform, etc and send in interference to cancel noise and/or
> vibrations. I have also heard of this being tried near airports (which I
> find hard to believe). Does anyone have experience and/or information
> relating to this?

A web search on noise cancelling is not likely to be the highlight
of your day.   There appears to be at least one franchise featuring
similar products with similar text, repeated on numerous URLs.
Cell phone noise cancelling is a drug on the market, it seems.

Then there are the headphones. From Bose on down....

But here are just three URLs (from 300) with some academic
interest or content:

 http://www.ti.com/sc/docs/psheets/abstract/apps/spra337.htm

 http://sbec.abe.msstate.edu/1999/abstracts/032.html


http://www.spd.eee.strath.ac.uk/~interact/AF/aftutorial/interaction/speech.h
tml

(This URL is so long that your mail copy will probably see it
folded to two lines, in which case your hot link will probably
 fail until you massage the URL back to life without line
punctuation.)

Briefly, cancelling a one dimensional sound at one position is easy.
(This is the headphone situation)
Cancelling a two dimensional field at some plane is possible
(This is the fuselage skin application)
Cancelling a sound field in an extended three dimensional space
is all but impossible.

 The control of phase and direction is harder at increasing
frequency, so cancellers do best at lowest frequencies.


brian whatcott <inet@intellisys.net>
Altus OK