Klarinet Archive - Posting 000356.txt from 2001/02

From: Mary Driver <to3456@-----.com>
Subj: [kl] Combination tones
Date: Sat, 10 Feb 2001 17:31:50 -0500

I have followed the recent thread about combination
tones and difference tones with great interest. I am
going to unlurk for awhile and add my 2 cents to the
discussion. Since I only get the digest it is very
difficult for me to add clips so I am not going to
quote anyone directly.

It has been stated that the air is a linear medium and
does not generate difference frequencies. This is
correct. I will also add that the medium that carries
radio waves is also linear and does not generate
difference tones. In most any case I can think of
difference frequencies and harmonics are only
generated in the transmitter or the receiver.

Microphones, speakers and audio amplifiers are linear,
at least they are supposed to be. If an audio
amplifier generates a difference frequency then its
called intermodulation distortion. This is not to say
that all electronic equipment is linear. I would
expect that keyboards use some non-linear circuits in
their sound generators.

I do agree that the ear is non-linear. It is
logarithmic. In being logarithmic it can receive
audio energy over a much wider range than if it was
linear. However, I am convinced that the ear is not
non-linear in the way that would generate difference
tones. To put it in musical terms, if I hear a
trombone and a trumpet separately or together I can
still recognize them as a trombone and a trumpet.
There is no third tone. I would expect that if our
ears were sensitive to difference tones we would all
have headaches from the noise.

I agree with everyone who says beats are not
difference tones. They are changes in amplitude. As
two tones move closer together we do not become aware
of the beat until it reaches a frequency low enough
for the ear to detect the change in amplitude that is
generated. Also note that even though the ear hears it
as an amplitude change the total energy generated by
the two waves does not change. As to what the combined
frequency looks like on an oscilloscope, I can't say.
I've probably done that experiment some time in my
life but I can't remember what it looks like.

Thanks for letting me add my 2 cents worth.

Richard S.

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