atn: Wind players

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atn: Wind players

Postby CS_TBL » Tue Aug 01, 2017 2:19 pm

I have a rather silly question I think, but Winnie the Pooh with me.

Just imagine there would be an air compressor that is absolutely constant, no fluctuations in air pressure. As tight as an atomic clock. Next we connect that air compressor to a kind of silicon mouth with lips and those lips are able to e.g. blow a wind instrument. Flute, oboe, trumpet, tuba etc. Now my question, with a constant air pressure, would the produced sound be as constant as an old rompler's static waveform? Without any subtle changes in the waveform?
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Re: atn: Wind players

Postby vicd » Tue Aug 01, 2017 5:25 pm

I'd think there would be some initial period: since the moment you provide the airflow into your "tube model input opening", and until the pressure wavefront reaches the end of the tubing. Then it would stabilize into something steadily self-oscillating, with constant airflow and equal pressure throughout the tube.
Since the moment it reaches that state, I can't think of anything major that would affect the waveform, provided no other parameters are changed. Well, maybe just the residuals and "reflections" of the initial nonequilibrium state? but these should decay really fast.

PS - I'm no big thing in gas dynamics, so don't take the above for granted :roll:

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Re: atn: Wind players

Postby meatballfulton » Tue Aug 01, 2017 7:03 pm

I agree there would be some sort of transient then it should settle into a steady state.

Maybe ask someone who works on pipe organs...
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Re: atn: Wind players

Postby clubbedtodeath » Wed Aug 02, 2017 10:37 pm

I suspect this can never happen, for three reasons.

Firstly, when the air passes through the flute/oboe/etc., the instrument will start to vibrate in multiple modes, all of which interact each other. This interaction sets up a complex series of (strictly) non-repeating vibrations in a slightly-imperfect structure, which means your sound will never be the same.

Secondly, the flow going through the flute is going fast enough that it will always transition to turbulent flow. Turbulence is the chaotic, non-repeating flow patterns (vortices/etc.) that evolve in air when its moves fast enough (a Reynolds number over 5000, say), and especially when it interacts with a solid boundary. These are irregular, even at a very, very small level, with hairpin vortices that are generated where boundary flow goes from laminar to turbulent.

Thirdly, movement of air into the instrument only comes through application of a pressure gradient. It will be almost impossible to get constant flow from constant (additional) pressure applied at the inflow, as the air flow further in the instrument will also dictate how that pressure accelerates the air. For instance, when you start blowing, that pressure has to accelerate the air inside the instrument. Once the air is moving though, it is not moving completely steadily: it will be turbulent. Therefore the resistance (drag) is always changing, and so the response to the pressure gradient is different.
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