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Re: [Phys-l] amplifier



I have tried a simple sound wave cancelation demo and always been
defeated. One of my colleagues tells me has done a simple demo outside:
one tone played on two large speakers in the middle of a field -- easy
to walk to different positions and go from constructive to destructive
interference -- but he has not been able to do the demonstration in a
classroom and attributes the failure to reflections off walls.

However switching the leads on one speaker would give an easy
comparison: A or B.

Sounds good! ;-) Thanks for the nice idea. I also look forward to
other contributions to this thread.

My own current sound demos: I use Mathematica on my laptop to generate
stereo sounds which I play through the classroom PA system, vary the
left & right channels separately, play different harmonics, different
superpositions of harmonics, the sum of two close frequencies while
varying one (beat frequency demo). For interference I play the sum of
two waves of the same frequency, but incrementally change the phase
between them. When the phase difference gets to 180 degrees there's
silence. Note that I'm playing the sum through both speakers driven by
my computer and an external amplifier, so the cancelation is occurring
early in the process! I've also tried playing one tone on the left
speaker and a phase-shifted same frequency tone on the right. It
produces a weird rippling sensation as the phase difference changes. I
had hoped that every person in the room would experience constructive
and destructive interference of the sound waves (at different times) as
the phase difference gradually changed, but we get nothing so clear-cut.

All the best,

Ken Caviness



-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of Anthony Lapinski
Sent: Wednesday, April 02, 2008 8:16 AM
To: tap-l@lists.ncsu.edu; phys-l@carnot.physics.buffalo.edu
Subject: [Phys-l] amplifier

I'm teaching about sound and want to reverse speaker wires (red/black)
to
show constructive/destructive interference of sound waves (music) with
two
speakers/microphones and an oscilloscope. In the past I used cassette
tapes, an old boom box (volume turned up), and old speakers. I would
reverse the (red/black) wire connection (via alligator clips) for one
speaker while the music played. A bit cumbersome, but it worked.

I now have some songs on the computer (iTunes) and wish to repeat the
demo. However, the old stereo speakers (connected via the earphone
jack
on
the computer) are not loud enough. The new computer speakers are very
loud, because of the amplifier built in, but I don't want to strip the
wires on them.

So I need an "inexpensive" amplifier to boost the loudness from the
computer (1/8 "earphone jack) to my old speakers. Does anyone know
where I
could purchase such an item? I assume Radio Shack might have some
stuff.

Any better ways to do this demo?

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