Re: making sound waves visible

[William Beaty <billb@ESKIMO.COM>]

On Mon, 13 Sep 1999, Leigh Palmer wrote:

> Before you guys go hog wild let's think about what you are saying:
> you are suggesting these displays represent sound *waves* when they
> really represent thepassage of a sound *signal*, not unlike a shock.

I have to disagree.  A shock wave is defined as a sound which is so "loud"
that it affects the medium and alters the propagation velocity.  Low
frequency (inaudible) sound or propagating changes in pressure are not
shock waves.

What the students display in these demonstrations is the envelope of a
complicated AM/FM sound signal.  Since air is fairly linear, group
velocity is about the same as phase velocity (otherwise a pistol shot
would sound like a "chirp.")

Sound waves extend in frequency all the way to zero (e.g. pressure
changes caused by weather.)  The propagation velocity is fairly
independent of wavelength when wavelength >> interatomic spacing,
or so I imagine.  Maybe I can find a plot of velocity vs. frequency for
STP...

> Audible sound waves are far too short to represent in this way, and
> they vibrate far too rapidly for anyone's flapping arms.

True.  If I yell out a word periodically, the students can only flap their
arms in time with the AM/FM envelope of the sound, not in time with the
fundamental/overtones wave.  Their flapping represents incoherent acoustic
energy pulses, not phased-locked sound waves.  Also, we should be careful
not to apply a human bias to the physics, and be careful not to say that
100Hz waves are different than 20Hz waves because the former is audible
and the latter is not.



> Be careful with your terminology, please; the students are listening.

Suppose we transmit 5Hz periodic impulses by whacking a couple of wood
blocks together.  The frequency spectrum then is broadband: a comb-like
spectrum with spikes at the whacking-period.  In that case the students
can flap their arms in time with the genuine sound waves (although in that
case any coherent phase-lock with a single-frequency sinusoidal signal is
no longer an
issue.)


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