From: DPierce@world.std.com (Richard D Pierce)
Subject: Re: Loud Music, Noise cancelation systems?
Organization: The World Public Access UNIX, Brookline, MA
Date: Sun, 24 Jan 1993 23:39:07 GMT

In article <C1DIz4.9C2@news.cso.uiuc.edu> rmg53668@dcl-nxt53 (Ryan Martin Grant) writes:
>>>: I've read (in Popular Science) about new tech where sound is analysed
>>>: in real time using chips and an opposite wave is generated to cancel the
>>>: signal.
>>>I can see that sort of thing getting very popular when DSP and ADC
>>>technology get cheap enough. Cancelling a sound field rather than a
>>>single source would require more channels, but if it's cheap...
>>>Another reason is that if you use some form of adaptive filtering (the
>>>easiest way to do it?) you end up with a very general plug-and-go
>>>sort of arrangement. No complicated tuning procedures etc, it does
>>>it by itself. Also, the system could be used to make sound as well.
>>And then the next improvement, of course, would be a system that could be
>>used both to cancel and reproduce sound at the same time.  Imagine the end
>>user's surprise when he wonders why his 200 watt amplifier begins clipping
>>for no apparent reason on a soft musical passage, and suddenly he realizes
>>that his system has been cancelling out the sound of a helicopter flying
>>directly overhead.  :-)
>
>>Portable stereos could have the sound cancelling features built in as well,
>>so that you could take your jambox to the movies and set it up to cancel
>>the people who won't stop talking.
>
>>I can see it now - the marketing types can call it Sound Forcefield (tm).
>
>These dreams are very similar to my own, but I was wondering if any 
>rec.audio wizards could bring up any hard facts for/against this technology
>ever going mainstream.
>

There's a couple of fundamental reasons why it can't possibly be the dream
system you hope it would be. Most fuindamental among them is the fact
that, at the very best, you can provide cancellation over a volume whose
dimensions are comparable to the shortest wavelength your interested in
cancelling. For example, if your interested in providing cancellation over
a volume, say, 2 meters in diameter (enough to take in two people), then
the shortest possible wavelength has to be substantially longer than two
meters. That implies a maximum frequency of all of 40 Hz or so. If your
willing to confine the region to something the dimension of inter-aural
spacing, (about 15 cm or so), you'll be lucky if it works as high as 500
Hz. Want complete cancellation at 20 kHz? then be prepared to have the
"sweet spot" confined to all of a couple of millimeters or so. Not real
practical, eh.

If, on the other hand, you could provide cancellation at the source, then
the closer you get, the higher the frequency you can reliably cancel, same
rules as above.

Another problem is that the cancellation generator is going to have it's
own field that will have to be cancelled, else you'll hear its residual.

Note that these are fundamental issues, and not technological ones.
Finding the DSP horsepower to even take care of the simple cases of
knowing precisely what needs to be cancelled is, at the present time,
pretty tough, as the people who are doing DSP based realtime loudspeaker
correction are learning the hard way.

The technique works fine for mufflers where you can characterize the
emmissions precisely but for less well described circumstances (a
loudspeaker in a random room playing who-knows-what with the listener who
is both the receiver and a piece of the room acoustics moving about) is
turning out to be a real tough nut to crack.

Cancel it at the source. Don't bother with DSP. Use a pair of wire cutters
instead. It's cheaper. Even selling your house and moving is cheaper.
-- 
|                Dick Pierce                |
|    Loudspeaker and Software Consulting    |
| 17 Sartelle Street   Pepperell, MA  01463 |
|       (508) 433-9183 (Voice and FAX)      |
Newsgroups: rec.audio