[pete]

Dick Pierce wrote:

RichD wrote:
What is the physics/acoustics of a wind instrument?

It's easy to see how a percussive surface vibrates, and
induces acoustic waves at the same frequency.
Ditto a string.

But pushing air through a trumpet (sax, etc.), then out
the bell - how does that create sound?

It doesn't. Simply pushing the air through smoothly merely
moves the air.

And in particular,
how does the valve action produce controllable wave
sequences (a/k/a music)? It's just air on air, I'm at a
loss to explain it.

What you're missing in the trumpet, the sax (and oboe,
clarinet, bassoon, trombone, tube, sackbut, serpent,
etc.) is that there is a physical vibrating mechanism
that interrupts the flow of air. In the case of brass
instruments, such as the trumpet, it's the vibrating
lips of the performer. In the case of reed instruments,
it's the single or bouble reeds.

The rest of the instrument is essentially an acoustical
filter and impedance matcher. The filter portion enhances
those components of the very "buzzy (wide-band, very complex
waveform) nature of the lip-reed or real-reed needed to
give the instrument it's characteristc sound, while at the
same time the length of the vibrating air column "pulls"
the reed closer to the desire note by resonance, and the
bell at the end provides a better acoustical mtch wth
the surrounding air and increases its efficiency.

The MORE interesting question is when you DO push aire
through some instruments, like the flute or recorder or
pipe organ, how does THAT work.

Well, in a somewhat analogous fashion. These instruments
all depend upon producing a thin sheet of air, which has
some turbulenace in it. The chaotic nature of the resulting
flow might initially flow more into the tube than out and
thus slightly pressurizing. That pressure wave travels to
the end of the tube (at the speed of sound, not surprisingly)
and, whethet the tube is open or closed, some of it is
reflected back down and when it gets to the point where it
started (the "mouth"), it opos the sheet out, thich sends a
slight evacuation wave on the same trip. The round-trip time
is largely dependent on the length of the tube, so the the
longer the tube, the less frequent the flip-slop occurs, and
the lower the note: the shorter the tube, the quicker the
round-trip time, the faster the flip-flop, and the higher
note.

This will be on Friday's quiz.

On an ocarina, it's the combined surface area
of the open holes, which determines the pitch.

--
pete