Re: [Phys-l] sound advice

[John Denker <jsd@av8n.com>]

On 04/09/2008 11:43 AM, kyle forinash wrote:
> I'm at the end of teaching a non-major course on the physics of sound 
> for the first time and I'm running one class short of material. I've 
> done everything I can think of. Any suggestions for the last class?


The following might or might not be suitable, depending on what
sort of groundwork has been laid.

It is fun to have a collection of Helmholtz resonators, including
some tall and skinny with a short neck, plus some short squat
Chianti bottles with long necks.  

--->  Ask the students to predict which ones will sound a high
note and which will sound a low note when you blow on them.
Unless you have covered Helmholtz resonators in some detail,
the guesses will be wildly wrong.  (Hint:  with a modest amount
of preparation you can find a short-neck and long-neck bottle
with exactly the same resonant frequency.  One has a big L and
small C, while the other has a small L and big C.)  

  Tune them by pouring in water in the usual way.  If you want 
  the tuning to be permanent, pour in epoxy.


==============

As Jeffrey S. suggested, go out in a field ... but rather than
use a drum, use a slap stick.  That produces something that much
more resembles a delta function.  The reason for that is: ...

In addition to doing the speed-of-sound measurement, listen to the
/timbre/ of the sound as a function of distance.  What sounds like 
a snap! up close becomes a quick whistler farther away (because
of dispersion) and then becomes a boom farther away (because of
attenuation of the high frequency components).

  The wave equation for /planar/ sound waves is non-dispersive, but
  in polar coordinates the wave equation is quite noticeably 
  dispersive.

  This comes as a shock to many students.  It challenges their working
  definition of what a "wave" is.  The idea that a wave "moves along 
  without changing its shape" doesn't work when there's dispersion.

========

Also outdoors:  Find someplace on campus where there is a set of
evenly-spaced architectural features ... which you can use as a
_diffraction grating_ for sound.  A big set of steps will sorta
do, but it is much nicer to have an array of features that run
/vertically/ so that by walking horizontally you can scan the
different frequencies in the diffraction pattern.  (The analogous
scan for steps would involve moving vertically, which is less
convenient.)

Use the slap-stick to create the incident sound;  the diffracted
sound will be an almost-musical tone, with a pitch that depends
on the angle.

The next time you're walking across campus, you can survey the
various buildings by snapping your fingers and/or clapping your
hands every so often.

  This has real-world applications for bats, blind people, et 
  cetera.  By listening to echoes, is possible to perceive not 
  only the distance to an object, but also to perceive something
  about the nature of the object.  Perceiving a flight of steps
  is super-easy;  with more work you can learn the sound of a
  tree.  (A deciduous tree sounds a lot different in winter!)

===================================

Of course if the students have never heard of resonance, dispersion, 
attenuation, or diffraction these demos are lost on them.

But IMHO it is worth saying at least something about dispersion.  Even
non-majors should realize that gunshots up close do not sound like
gunshots far away.

The hands-on speed-of-sound measurement is important.  I once had to
wade through reams of eyewitness testimony concerning the crash of
TWA 800.  Several witness said they "heard the boom and looked up
just in time to see the missile hitting the aircraft."  (Hint:  the
aircraft was at 13,000 feet, in addition to being 9 miles offshore.)

This should tell you something about the reliability of eyewitness
testimony in general.