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Many bird song notes sound whistled to our ear. They are relatively pure tones that we can mimic best by whistling. But birds do not whistle those notes. They sing them. And it's a real puzzle to acoustics experts who design models of the avian syrinx and airway how they produce those notes. The program on Throat Singing talked about one possible explanation for those whistle-like tones.

But how do experts know that birds don't whistle these pure tones? To understand that, we need to understand the difference between singing and whistling.

Singing starts with a sound made by a vibrating membrane in a bird's syrinx or in our larynx. That sound has multiple overtones and is very rich. Whistling, on the other hand, starts with turbulent air that is forced to pulse through an opening at a single frequency. It is a relatively pure sound, poor in significant overtones. Because both singing and whistling depend on moving air, both change when you try to whistle or sing in helium instead. But because of their differences, they don't both change the same way.

Helium has smaller, lighter molecules than air. They move more easily. For that reason, sound travels about 3 times as fast in helium as it does in air. So, when you whistle the same tune in air and in helium, the helium version is shifted about an octave and a half higher. But helium doesn't change the vibration of the vocal fold in the syrinx or larynx. What it does change in singing is the resonances of the vocal tract. When you sing in helium, the resonant overtones that give a voice its character are higher, and the voice sounds squeaky and thin. These characteristics are so important to the way we hear voices that most people think that the entire voice is raised, as if it were a recording played at a different speed. But if you listen carefully, you can hear that the fundamental pitch of the voice doesn't change if you sing in helium. It sure sounds different though, doesn't it?

The black-capped chickadee's song can be found on the CD collection Bird Songs of California produced by the Cornell Laboratory of Ornithology.

The voices sung in air and in helium are from the physics in speech page of Joe Wolfe's wonderful music acoustics website at The University of New South Wales.