Date: Wed, 16 Feb 2005 14:19:44 -0800 (PST)
Author: Marc Kossover
Subject: Re: Keyboard for Physics of Sound Class
I ran into an interesting demonstration when using a
very cheap keyboard synthesizer.
The keyboard had a headphone output which I connected
to the microphone input of a power amplifier. The amp
was then connnected to a large full range speaker with
the cover removed.
I was playing various notes so that the students could
see (and feel!) the response in the speaker. For low
pitched notes, it was easy to see how the fundamental
frequency was the same for the notes over different
voice while the overtones varied. It was pretty cool.
The keyboard had a voice called "sine wave" and I
tried it as well. As I expected, it didn't have any
overtones at all, so it was a perfect baseline.
The cool thing happened when I played the C two
octaves below middle C with the C-sharp two octaves
below middle C.
Not only could everyone easily hear the beat
frequency, the could see it on the speaker cone as
well! You could see the beat frequency change as the
notes became farther apart. Additionally, you could
see how notes that were in whole number ratios would
move the speaker cone in simple patterns while
non-whole number ratios made the cone go crazy.
Other voices worked, but the effect was much less
I left that school, and I have never found another
keyboard with a sine wave voice. If you can find one,
it is pretty cool.
Marc "Zeke" Kossover
The Jewish Community High School of the Bay
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From MLowry@D115.ORG Wed Feb 16 18:25:57 2005