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We've learnt that when you produce sound with a pipe, there's a standing wave in it.

I'm confused about two things.

  1. What happens when you blow air out of your mouth? Are the particles moving through the air or is it just an energy transfer like a wave?

  2. If it is the former, then how does blowing into a pipe produce sound? Where does the initial progressive wave that produces a standing wave come from?

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    $\begingroup$ If no particles came out when you blew out, the human respiratory system would not work, so you can rule out it just being a wave. You can look up Helmholtz resonance, it explains the sound produced by blowing across a bottle top for example. $\endgroup$ – CDCM Aug 27 '17 at 12:51
  • $\begingroup$ You might also look up how terrifying the noise of a glass bottle can be, when dropped from a height, as in the early days of military aircraft $\endgroup$ – user167453 Aug 27 '17 at 13:11
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Blowing air directly into the end of a pipe does not produce any sound (well, hardly any!) or a standing wave.

In musical instruments, either there is a vibrating reed that generates the sound (e.g. clarinet, saxophone, etc) or the air stream is aimed at a sharp edge and the vortex pattern around that edge supplies the energy to maintain the standing wave (e.g. recorder, flute, organ pipes, etc).

If you want to produce a standing wave tone by "blowing into a bottle", you don't actually blow into the bottle but across the open end. The vortices are generated when the air stream hits the edge of the bottle neck. Since the edge of the neck is not very "sharp", the tone produced by blowing a bottle is not very loud or stable compared with a better-designed wind instrument.

The details of how this works get complicated, but one important parameter is the time it takes the blown air stream to travel across the opening in the pipe (while being affected by the changing pressure and velocity of the standing wave), compared with the time for one cycle of the standing wave frequency. If you blow too slowly, you don't get a softer tone, but nothing at all. If you blow harder in a controlled manner, you can excite a harmonic of the fundamental standing wave frequency. That is part of the playing technique for instruments like the flute.

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