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Re: _Parasaurolophus'_ crest and how much we can't deduce from it.



Peter Buchholz (Stang1996@aol.com) wrote:

> I really don't see what difference making the pipes the same size
> and shape of _P._'s crest will make.

I'm glad Peter wrote the message from which I excised this quote
because I've been wanting to complain about this thread too.  I
confess that I haven't gotten around to reading Weishampel's published
reports (presuming there are any) of his PVC Parasaurolophone, but
I've heard many popularizations, and they all seem to misunderstand a
couple of very important points.  The one which Peter brought up:

> Although, size and shape does control the range and vague tone
> qualities, the real test as to what the thing sounds like is in the
> mouth-piece.

is one of them.  _Parasaurolophus_' crest may well have acted like a
woodwind or brass instrument, but we really don't have enough
information to reconstruct the sounds it might have generated.
Presumably analogous musical instruments are basically waveguides, and
_Parasaurolophus_' crest can certainly be analyzed as such.  However,
in order to understand the sounds such a waveguide would create, you
need to know the properties of the material out of which it was made
and the nature of the structure that initializes its vibrations.

The basic principle determining how a waveguide will behave is that
waves (acoustic waves in this case, but the principles are
generalizable to other types of waveguides) will propagate through the
waveguide only via patterns of constructive interference.
Simplistically you can imagine two walls of a box and the air in
between them -- if you get the wall on one side of the box vibrating
it will push air back and forth across the box and cause the other
side of the box to vibrate ("resonate") as well.  The speed of sound
is fixed by the pressure, density and temperature of the air, and that
determines the amount of time it will take for the pressure wave
caused by the first side's vibrations to reach the second side.  The
box will only resonate if the pressure waves striking each wall hit
the wall with the proper timing (just like you can only get a child on
a swing set to go higher if you push on them as they're moving away
from you).  That part of the story is well captured by Weishampel's
PVC pipe, but there are two other wrinkles which are not.

In an enclosed structure, the waves are propagated not only via the
air between them (e.g. like that between my two hypothetical walls
above), but also by the vibrations in the walls themselves.  Imagine a
wave travelling around the circumference of a clarinet -- as one part
of the wall vibrates it causes the adjacent parts to vibrate and so on
and so on...  The waves travelling in opposite directions around the
circumference have to interfere with each other constructively just
like the air going across the diameter would.  The speed at which the
wave travels around the sides of the waveguide is determined by the
material properties of the waveguide's walls.  That aspect was NOT
necessarily captured by Weishampel's PVC pipes because you'd expect
that the real tube was comprised of the bone (which we know something
about) and some sort of mucosal membrane (that we know considerably
less about).  If you don't believe this makes a difference, think
about how you can tell when someone has a cold.

The other wrinkle is that waveguides potentially allow more than one
interference pattern to propagate at the same time.  The strength (for
simplicity read "volume") of each pattern is established by the manner
in which the patterns are created.  That's what Peter was talking
about with regards the mouthpiece.  The patterns set up by the
initially vibrating surface determine the pitch and timbre of any
resulting sound.  Think of the fact that a bugle can play many
different "notes" even though the mechanics of the waveguide are
fixed.  The difference comes about from the fact that different
vibrational patterns are established in the bugle by the bugle
player's lips.  Again, not knowing what the larynx (or syrinx?) of a
_Parasaurolophus_ was like, we can't reconstruct that aspect of its
sound-generating potential either.

Hypothesizing that any hadrosaurs' crest was a cavity resonator
suggests some things about their biology.  But with current data we're
a *long* way from knowing what the animals might have sounded like
were we there to hear them.  (At this point the philosophers in the
crowd might wish to conclude that they made no sound because there was
no one there to hear it!  If a _Parasaurolophus_ honks in the forest
and there's no one around... ;-)

-- 
Mickey Rowe     (rowe@lepomis.psych.upenn.edu)