XPost: sci.physics   
   From: dpierce@cartchunk.org   
      
   David Nebenzahl wrote:   
   > 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.   
      
   It is not split, a significant portion of the windsheet   
   alternates between one side and the other, at a rate   
   determined by the primary resonant frequency of pipe   
   that's attached to it. When you first start blowing,   
   it may well be split, but there's a fair amount of   
   broad-band chaotic turbulence as well. The resonant   
   column quickly selects out the primary frequency and   
   then you get the regular in-out oscillation of the   
   windsheet on either side of the fipple.   
      
   Someone else mentioned the ocarina, and correctly pointed   
   out that it's not dependent upon a column length. Rather,   
   it's short shape makes it behave more like a Helmholtz   
   resonator, whose resonant frequency is determined by   
   the internal volume and the toal area of the open   
   holes. The latter determines the acoustic inertance   
   (the acoustic equivalent of mass), and the acoustic   
   inertance is related to the inverse of the area of the   
   holes (in a somewhat complex way): exposing more holes   
   makes the pitch higher.   
      
   But as in the recorder (or the flute or the organ pipe),   
   the sustained note is attained essentially because the   
   air that's being blown is flipping back and forth between   
   two states at a rate determined by an acoustical resonance.   
      
   Back when I though such experiments were cool (wait,   
   they still are), I did a little experiment: I took a   
   sufficiently large organ pipe (which is really just a   
   big version of a recorder) and blew it at the right   
   pressure (which was pretty low) from an air source   
   that had a smoke generator in it. Then I had a GR   
   Strobotac (a tunable strobe meter) which was synchronized   
   to the note the pipe was making by using a microphone   
   placed near the mouth of the pipe. There is a phase   
   adjustment on the stroboscope which allowed you to change   
   when during the cycle the strobe flashed. Because now   
   the flashing strobe light and the organ pipe were   
   perfectly synchronized, and thus the windsheet was   
   "frozen" in whatever position it was when the strobe   
   went off, I could, by twiddling the phase control,   
   watch the windsheet move back and forth between each   
   side of the mouth (or, if it was a recorder, the fipple).   
   Not the easiest demonstration to set up, it did, however   
   illustrate what was going on.   
      
   Lastly, if you are able to capture the waveform as you   
   play a note, you will clearly see that the note DOES NOT   
   start instantly. Rather, there's a bit of what looks like   
   pure noise at the very beginning of the ntoe, and it does   
   take a little bit for the note to finally settle down   
   and build up to it's steady-state condition. In some styles   
   of organ building, you can distinctly hear this as a   
   slight "chiff" at the beginning of each note, a faint   
   but audible percussive effect which can help in the   
   articulation of the instrument when playing complex   
   polyphonic contrepuntal music (for which, largely, the   
   organ was developed).   
      
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