Googling yields contradictory results, so here my question:

When I sing, my vocal chords vibrate, but my whole body is the resonance body, right? So I would say that when I think about standing waves etc. I must think of a resonantor that's closed (i.e. hard boundary conditions) at both ends, because my bottom end is certainly closed, and the top end has the vocal chords which are vibrating similar to woodwind instruments, so it's closed on that end as well.

If I am mistaken, could someone point out the physics behind this?

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    $\begingroup$ but my whole body is the resonance body, right? ... nope, only the air carrying cavities in your upper body - larynx, pharynx, lungs etc. en.wikipedia.org/wiki/Vocal_resonation $\endgroup$ – user346 Mar 9 '11 at 16:52
  • $\begingroup$ Sorry, yes. I was very unspecific. I'd still imagine that this is then a closed-closed resonator, with the lungs and the diaphragm being the bottom end and the vibrating vocal chords being the top end. $\endgroup$ – Lagerbaer Mar 9 '11 at 17:36
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    $\begingroup$ "" with the lungs and the diaphragm being the bottom end and the vibrating vocal chords being the top end"" of course not, the lungs are a kind of extremely fine but not liǵht sponge. (Go to a butcher and have look at some piece) This end is severely damped. The speaking and singing is done by the cords and the room to the mouth. And that end is not closed, but it isn't really open at the same time, as many air instruments. $\endgroup$ – Georg Mar 9 '11 at 18:15

Both "perfectly open" (zero acoustic impedance) and "perfectly closed" (infinite acoustic impedance) boundary conditions are only idealizations that never occur in practice. For the case of the human vocal tract, they aren't even very good approximations.

The "bottom end" of the resonating cavity is not, in fact, the lungs, but the vocal folds (as Georg pointed out). This end has some acoustic impedance that's neither extremely low nor extremely high. I'm sure the impedance also changes somewhat with the pitch and volume of the phonation.

The "top end" of the cavity is of course the mouth, and its impedance changes with the vowel sound you're pronouncing. For aptly-named "open" vowels such as "ah", "oh", and so on, the impedance is actually low enough that it might be a good approximation to say it's perfectly open. But for closed vowels ("ee", "oo", ...) and especially for humming with closed lips, the acoustic impedance is much higher (but still far from infinite).

  • $\begingroup$ Thanks. My friend is TAing a lab on accoustics and even her Prof wasn't sure what the correct model was. So, as always, the answer is that it's complicated :) $\endgroup$ – Lagerbaer Mar 9 '11 at 21:52

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