Sound waves

[John Delacour]

At 11:22 PM -0500 12/5/01, Stephen Birkett wrote:

>First, sound is organized movement of molecules in an elastic medium. In
>this context _all_ waves in an elastic medium are sound, and this has
>nothing to do with whether anyone can hear them.
>
>There is no technical distinction between certain types of waves 
>being sound and other not sound. They are all sound. That is what 
>sound is - waves in an elastic medium.

At 12:48 AM -0500 12/6/01, Stephen Birkett wrote:
>_All_ vibrations in an elastic medium are sound, by definition. It has
>nothing do do with whether we can hear them directly because they can
>interact with our eardrum, or whether they are primarily transverse,
>longitudinal, or otherwise. They are all sound in one form or another. If
>we're clear with our use of terminology then communication is enhanced.
>
>Au contraire...the way the soundboard works is not the issue.  It's
>illogical use of the term "sound" in simultaneous and different contexts.


At 11:22 PM -0500 12/5/01, Stephen Birkett wrote:
>There is also often an attempt to distinguish the vibrations of an elastic
>medium which is exposed to air, from the vibrations induced in the air by
>the interaction. Both of these are technically sound waves, since both are
>vibrations in an elastic medium. The fact that the air can interact
>directly with our ear, while the soundboard cannot do so makes absolutely
>no difference. They are both sound.

And I thought _I_ was being categorical :-).  Now that point is 
clear, to my satisfaction at least (since that's the way I see it 
too), then we have another apparently confusing term to define and 
that is "wave", but first it's worth pointing out that "the speed of 
sound" in a medium and a medium's property as a "conductor of sound" 
are widely used by engineers and have never to me been in the least 
confusing.  I know of no other expressions to describe these two 
phenomena.

Now I presume the primary meaning of "wave" is the the wave we see on 
lakes and seas, "a ridge or swell on the surface of a liquid (as of 
the sea) having normally a forward motion distinct from the 
oscillatory motion of the particles that successively compose it". 
Already that definition goes far beyond most people's apprehension of 
a wave, which is generally perceived as the purely visual aspect of 
the wave, of a wavy line, of a girl's wavy hair, of pennants waving 
in the breeze.  This naive conception of the wave is reinforced in 
school physics classes by the graphic representation of certain 
phenomena in the "sine wave" on a cartesian plane.  I knew a man who 
worked as a telephone engineer, gone through his training and spent 
at least 15 years as a competent electrician and installer.  We got 
to talking one day about alternating current and it soon became clear 
that he visualized the current of electricity through a wire as a 
sine wave, that the stuff was actually wiggling its way back and 
forth through the wire.  He was quite nonplussed when I suggested 
this was not an accurate picture.

The above Webster's definition will surprise people who have not 
learned it a school, and maybe still surprise them if they have 
forgotten it and are then reminded of it, since it it quite contrary 
to our perceptions and preconceptions.  It would probably take Euclid 
or Aristotle quite a while to get their heads round it.

Since sound travels as a 'compression wave' through, or in, a medium, 
and in no other way, and since this sort of wave bears no resemblance 
at all to the waves of the sea, except when graphically illustrated 
in a cartesian plane -- i.e. it cannot rationally be visualized as 
anything similar to a "wave" -- then we need to get a right picture 
of these oscillations in our various media.

I'll stop there and let someone else contradict or clarify or 
otherwise add to this step in my understanding, emphasizing that the 
soundboard at this stage need not be at issue.

JD