[RichD]

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? 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.

--
Rich

[M J Carley]

In the referenced article, RichD writes:

What is the physics/acoustics of a wind instrument?

The best place to look is probably Neville Fletcher's work:

http://www.phys.unsw.edu.au/jw/brassacoustics.html
--
Qualcuno era comunista perché abbiamo il peggiore Partito Socialista d'Europa.
Qualcuno era comunista perché lo Stato peggio che da noi solo l'Uganda.

Michael Carley: http://people.bath.ac.uk/ensmjc/

[Sam Wormley]

On 12/9/10 11:19 AM, RichD wrote:
What is the physics/acoustics of a wind instrument

http://hyperphysics.phy-astr.gsu.edu...es/clocol.html
http://hyperphysics.phy-astr.gsu.edu...opecol.html#c2
http://hyperphysics.phy-astr.gsu.edu...locol2.html#c1

[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

[Angelo Campanella]

"Sam Wormley" wrote in message
...
On 12/9/10 11:19 AM, RichD wrote:
What is the physics/acoustics of a wind instrument
http://hyperphysics.phy-astr.gsu.edu...es/clocol.html
This assumes a closed end, promoting only odd harmonics,

http://hyperphysics.phy-astr.gsu.edu...opecol.html#c2
This discusses closed and open ends, promoting odd-only as well as both odd
and even harmonics.

http://hyperphysics.phy-astr.gsu.edu...locol2.html#c1
This discusses the conical column as well, and I'm not sure I go along with
his rationales as to why it behaves differently.
I am told that clarinets and other tapered-bore wind instruments, do not
emit even harmonics in their timbre. This implies that those instruments
have the characteristics of a closed-end tube. My thought is that the
conical bore acts as a barrier or reflector for a sound wave reflecting from
the opening back into the bore because that closing bore impedes propagation
back up said bore toward the tiny mouthpiece opening..

Ange.

[David Nebenzahl]

On 12/9/2010 9:51 AM Dick Pierce spake thus:

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.

I'd always understood that wind instruments like flutes and recorders
work because the airstream gets split (by the fipple in the recorder).
But I still have no idea how this produces oscillation. Very mysterious.

You mentioned the complex waveform produced by a vibrating reed. Maybe
similar to bowed instruments like the violin, in which the sound
generator (a sticky bow repeatedly "grabbing" and releasing the string)
produces roughly a triangular wave. (Dunno what the waveform of the
sound that emerges from a violin actually looks like, though; presumably
the body of the instrument does some wave-shaping.)


--
Comment on quaint Usenet customs, from Usenet:

To me, the *plonk...* reminds me of the old man at the public hearing
who stands to make his point, then removes his hearing aid as a sign
that he is not going to hear any rebuttals.

[RichD]

Forgot about this one -

On Dec 9, "Angelo Campanella" wrote:
What is the physics/acoustics of a wind instrument
http://hyperphysics.phy-astr.gsu.edu...es/clocol.html

This assumes a closed end, promoting only odd harmonics,

http://hyperphysics.phy-astr.gsu.edu...opecol.html#c2

This discusses closed and open ends, promoting odd-only
as well as both odd and even harmonics.

This is what I don't get.
How do you get a reflection from the open end?
I can't picture that. It doesn't jibe with my (fading)
memory of studying transmission lines, in EM,
where you get standing waves, with a termination
at each end.

--
Rich

[RichD]

On Dec 9, Dick Pierce 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) 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.

That's news to me. Though watching Louis Armstrong,
it's easy to see -

So the 'vibrating lips' act as a wideband noise source?
Then the valves filter it, as I thought.

I wonder, is the performer able to control,
consciously or not , this lip vibe, to get particular
effects? Or is it continuous?

This will be on Friday's quiz.


Will you accept a fresh picked apple?

--
Rich

[Scott Dorsey]

RichD wrote:

This is what I don't get.
How do you get a reflection from the open end?
I can't picture that. It doesn't jibe with my (fading)
memory of studying transmission lines, in EM,
where you get standing waves, with a termination
at each end.

No, you DON'T get standing waves with a termination at each end.

If you short the end of the cable, you get an inverted reflection back.
If you leave the end of the cable open, you get a non-inverted reflection
back.

ONLY if the cable is terminated into an impedance equal to the characteristic
impedance of the line do you not get a reflection; the termination load
appears like additional cable and the signal goes quietly into it as if it
were so.

The same thing happens with an open or closed organ pipe and creates the
internal resonance of the pipe.

In a trumpet it gets interesting.... because the bell of a trumpet is
effectively an impedance-matching transformer....
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Angelo Campanella]

"Dick Pierce" wrote in message
...

Another way to look at is that a closed-end forces the
particle velocity to be zero, and the pressure to be
at a max, while an open end allows the particle velocity
to be at a max while the pressure is zero. That means that
a closed end will provide a pressure node and a velocity
anti-node while the open end provides a velocity node and
a pressure anti-node. In a standing wave, adjacent pairs
of nodes or antinodes are found a half wavelength apart, while
alternating nodes and antinodes are found 1/4 wavelength
part.

OK

So we can look at the combinations of the two: a pipe with
two open ends or two closed ends provides a matching set of
nodes, and can support standing waves of all multiples of
half wavelengths: 1/2 wave, 1 wave, 1 1/2 wave, etc.

Thus, assuming the speed of sound is about 1200 ft/sec, an
open or fully stopped pipe 6" long can support resonances
at 500 Hz, 1000 Hz, 1500 Hz, etc.

whoops...

1200 ft/second sound speed has 1200Hz sound as a wavelength of one foot.

600 Hz sound will have a wavelength of two feet, so a half-wave will be one
foot or 12".
1200 Hz sound , will have a half-wave (closed ends resonator) of one-half
foot or 6".
Harmonics of hat closed tube being 2400 Hz, 3600, 4800.

On the other hand, a pipe which is open at one end and stopped
at the other supports a node at one end and an anti-node at
the other supports resonance that are odd multiples of 1/4
wavelengths.

OK

The same 6" pipe, stopped at one end, open at
the other, support resonances at 250 Hz, 750 Hz, 1250 Hz, ......

A one-half foot length of tube closed at one end will have a quarter wave
resonance for a two foot wave, or 600 Hz.
Harmonics are at odd # of quarters, or 600, 1800, 3000, ......

And yet another way to look at the behavior at the end of an
open pipe is that it forms an acoustic inertance (acoustic
mass) whose magnitude is roughly proportional to the inverse
square of the diameter or the pipe, and is different if the
pipe is just hanging in free space vs terminated of "flanged"
in a wall of some sort. That inertance itself provides an
acoustical reactance at the end of the tube and can itself
cause some of the energy to be reflected back down the pipe,
just like an electrical "inertance" (aka inductance).

OK,

That accounts for there being enough inertance to cause a mismatch and
retention of the sound wave at this resonance frequency, leading to a
reinforcement of that frequency wave.

I'm more interested in the conical bore effect of a clarinet, where it
apparently offers a virtual impedance against waves traveling back up the
bore; that's my take on this phenomenon. I'm not sure how high a harmonic
should be to really encounter this back-impedance.

When examining an FFT spectrum of the radiated sound level of any given
clarinet note, I have noticed that the amplitudes of the "missing" harmonics
are not zero, but only diminished to varying degrees. And I can't help but
stress that if harmonic energy is already produced by blown reed action,
said energy must be accounted for. Ergo, it does become emitted into the
space outside the clarinet bore, but perhaps does not enjoy any reverberant
buildup as occurs for the favored harmonics.

Ange