[From Bill Powers (930427.2045)]
Back a little early -- forcast was for 20-30 degrees F tonight,
too cold for camping. So we looked at the sand dunes and came
home. Nice trip anyway.
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Bruce Nevin (9304227.1232) --
Gary Cziko has already relayed some comments on your noise
cancelling device; mine are probably superfluous, but anyway:
Suppose you install a speaker having a large membrane just ahead
of the microphone (or mount the microphone in a hole in the
speaker). With fast electronics you could cause the speaker cone
to just cancel the sound pressure arriving at it, using a control
system of the type you suggest. This would prevent the sound wave
from traveling through the speaker cone, so transmitted sound
would be absent at the back (downstream) surface of the speaker,
over a circular area the same size as the speaker.
Now do this the easy way. Cut out a circle the same size as the
speaker from a sheet of 1/4-inch steel. Sound waves reaching this
disk will be reflected back in the direction from which they
came, and immediately behind the disk there will be a region of
zero transmitted sound, a sound shadow. So you have achieved the
same effect without any electronics or moving parts.
Alternatively, you could make the disk of a thick piece of sound-
absorptive material which would not reflect the sound, but simply
convert it into heat.
However, sound will still pass around the disk, and some of it
will diffract around the edges and reflect from other surfaces,
so sound waves will still reach into the sound shadow of the
disk. The only way to prevent this is to make the disk so large
that sound-waves diffracting around the edge have lost most of
their amplitude by the time they travel to the centerline of the
sound shadow. And even then, at some distance behind the disk,
the converging diffracted waves will reinforce and produce a
sonic hot spot -- the principle of the Fresnel "zone plate.".
With an electronic feedback system you could make the loudspeaker
look like anything between a totally reflecting disk and a
totally absorbing one. But the result will be no different from
using a disk of thick steel or of a sound-absorber. If you
imagine using the passive disks in the application you have in
mind, you can draw your own conclusions about the usefulness of
the idea.
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More comments on posts from the past few days tomorrow.
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Best,
Bill P.