085395e7   
   XPost: sci.physics.acoustics, sci.physics   
   From: dpierce@cartchunk.org   
      
   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.   
      
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