Just to throw in one more wrinkle, we could consider the voice as an instrument
and compare it to brass instruments.
**For both, vibrations are created by flaps (vocal cords or lips) which
alternately open an close as pressure builds up.
**For both, the frequency of these vibrations can be adjusted by changing the
tension.
**For both, the direct output is a series of puffs which contain many harmonics
and which sound anything but musical.
**For both, resonances in an enclosure (the throat/mouth or brass tubing)
enhances certain harmonics at the expense of others.
The big difference is the amount of feedback. In a brass instrument, the
smooth, hard sides allow strong standing waves to build up. This results in a
strong pressure antinode at the closed (lip) end. Each time the pressure builds
up, the lips tend to open, reinforcing the pressure pulse. This strong positive
feedback and the frequency is mostly controlled by the frequency of allowed
standing waves. So if you try to blow a "C" but the tubing length is set for
"D", it is very difficult. The pressure pulse will return to your lips a
fraction of a second earlier than you want to create a puff, most likely causing
your lips to involuntarily speed up.
In the voice, with very soft "tubing" , the standing wave is strongly damped.
It this case the feedback is weak, and the frequency is primarily controlled by
the tension. The shape of the tubing can be changed, changing the impedence
matching of various harmonics, resulting in the different vowel sounds.
(Playing the "lips" sans instrument or mouthpiece would be analogous to singing
- yes, a variety of pitchs can be produced, but it takes some talent to control
the pitch.)
Tim Folkerts
The versitility of the voice is the adjustable shape of the tube.