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Re: 2 source interference w/ sound outdoors



We're planning to try again this year the demo with 2 speakers & a
sine wave generator in a field. Last year it didn't work very well,
there was a building about 50 yards downrange from the speakers,
and relefections might have been too great, but we also found that
whereas a pair of cheap midrange speakers gave a weak
interference pattern, a pair of expensive two-way speakers (which
were louder and thus should have been better) didn't work at all. Is
it possible that crossover circuits in two or three way speakers
would somehow muddy the waters?

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Gary Hemminger


I have used such a demonstration off and on since the early 70's with 9th
graders on up. It does work better when there is a VERY BIG space in the
direction the speakers are pointed and 2-way speakers do seem to reduce the
"depth" of the nodes. Because I initially wanted a demonstration, my
initial choices (frequency and speaker separation) were designed to result
in alternating between a node and an anti-node on a typical sized step
along a line 3 - 4 meters away, but parallel to the line between the
speakers. With the college students we use some old AR4x speakers and
cannot get away from having buildings in the distance, but the nodes can
still be discerned well enough to mark their locations on the pavement and
make measurements which enable a calculation of path length diffs which
agrees fairly well with the setting of the signal generator. It's been a
while since I've done it, but a colleague, Willy Smith, will be doing it
this week. We'll report the results.

Driving the system hard could also muddy the waters, whatever system it is,
unless it is extremely expensive.

Dewey


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Dewey I. Dykstra, Jr. Phone: (208)426-3105
Professor of Physics Dept: (208)426-3775
Department of Physics/MCF421/418 Fax: (208)426-4330
Boise State University dykstrad@email.boisestate.edu
1910 University Drive Boise Highlanders
Boise, ID 83725-1570 novice piper: GHB, Uilleann

"As a result of modern research in physics, the ambition and hope,
still cherished by most authorities of the last century, that physical
science could offer a photographic picture and true image of reality
had to be abandoned." --M. Jammer in Concepts of Force, 1957.

"If what we regard as real depends on our theory, how can we make
reality the basis of our philosophy? ...But we cannot distinguish
what is real about the universe without a theory...it makes no sense
to ask if it corresponds to reality, because we do not know what
reality is independent of a theory."--S. Hawking in Black Holes
and Baby Universes, 1993.
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