From: "Sir Charles W. Shults III"
Subject: Re: Sound cancellation technology?
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
Date: Thu, 26 Dec 2002 20:57:47 GMT
NNTP-Posting-Date: Thu, 26 Dec 2002 15:57:47 EST
Organization: RoadRunner - Central Florida
This is a simple way to explain it.
Imagine that you have a pool of water and you drop a small pebble into it.
You see that it will produce a ring of expanding ripples. Now, imagine that you
are one meter away and your task is to cancel those ripples by dropping other
stones. You can see that if you make "negative" ripples, they will sum with the
other ripples to make spots where the overall local pattern can be cancelled
However, since you are not at the source of the ripples, you have that
positive curvature working against you- all the waves coming your way "bow out"
at the front, and the waves you send to cancel them also "bow out" at the front!
Therefore, it is only poorly that you can cancel the ripples, and only at a very
Now, let's apply three dimensions to the equation, as well as long and short
Suddenly this problem explodes in complexity- it becomes literally thousands
of times more difficult. Instead of waves confined to a simple two dimensional
surface, we suddenly have complex, three dimensional "volumetric" waves. And we
are still confounded by two problems- the outward bowing wavefront from a noise
source (plus its complex reflections to make things worse) and a new problem,
the fact that low frequency waves are far easier to counter than fast, high
If we use a simple inverted phase amplifier, we can do an admirable job on
low frequency sounds, and in particular if we have multiple microphones and
multiple cancellation speakers. But high frequency sounds create real problems
because our cancellation sounds, like any sound, has a finite velocity. The
effective cancellation volume becomes more and more difficult to create. This
is because due to the speed of sound, the collisions of sound and anti-sound in
midair results in a volume dependent on frequency.
The velocity of sound in air at sea level turns out to be about 74
microseconds per inch. For a 20 hertz bass rumble, this gives a wavelength of
about 676 inches. But for a 2 kilohertz midrange sound, this wavelength is 6.76
inches, and for those 8 kilohertz squeaks, it is 1.69 inches. In other words, a
bass sound has lots of room for you to work with- you can spend 10 or 20 feet
doing your cancellation thing and most all of the sonic energy will be countered
But for voice frequencies you only have a bit over 6 inches to work, meaning
that by the time the sound reaches the microphone, it will have changed before
your canceling waveform can catch up with it. And you can forget high
Now, one solution is to create a "sonic hologram" that matches the offending
source and uses special speakers to adequately match the incoming wavefronts-
sort of like phase conjugators or synthetic aperture radar. But you can imagine
that this gets really expensive quickly and it also takes complex speakers,
microphones, and software, and it it generally not good on things it is not
So sound cancellation is not a fiction, it does exist. But it has real
limitations and knowing some physics can explain why that is so.
My robotics, space and CGI web page - http://home.cfl.rr.com/aichip