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Old 14th April 2003, 02:52 AM   #1
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Default Blow it out your port.......

Guys,

I bought a penny whistle ($10.95) for my kids the other day,
and noticed that when blowing a note very softly, the pitch of the note is flatter than it should be. Would this effect be the same in a ported box being played at low levels?

cheers,
Pete McK
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Old 14th April 2003, 09:54 AM   #2
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Humm, might this be due to changing speed of sound? As you blow slower, air pressure will be higher (as opposed to a hard blow...remember the Bernoulli principle), humidity and temperature ought to be either higher or lower depending. Speed of sound is determined by temperature, density and pressure, I forget really what all affects it though..

In any case, I don't think this applies to a speaker box. A whistle is composed of a (usually) edge-tone generator, tuned to the adjecent resonant chamber. The pitch is determined by the shape of the chamber and the conditions of the air inside (the generator itself is wideband and needs to be tuned).
In the case of a speaker box, the excitation is closed-loop; changing the characteristics of the box (or air inside) will only change frequency response. It's notoriously difficult to frequency-modulate an audio signal, without digital processing or the Doppler affect.

Tim
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Old 15th April 2003, 01:10 AM   #3
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The speed at which you push air through could also (I think) change the temperature slightly...

I used to play in a band, and I learned that with woodwinds, you go louder and flatter, but with brass instruments, you go louder and sharper.

I don't quite understand these phenomena, though I suspect the flat/loud woodwinds have to do with a larger travel distance for the reed, and thus, more time to travel the distance...

In short, I have no real idea, but just some fake ideas.
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Old 15th April 2003, 01:20 AM   #4
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(hijack)

I saw this, and instantly thought "THAT'S A GREAT IDEA!!! Ping-pong ball launcher using high-excursion woofers!"

So now I'm going to talk to my dad about building such a thing...

(/hijack)
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Old 15th April 2003, 03:51 AM   #5
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I think manually operated steam whistles like on a locomotive are similar.
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