John Ewbank, professor of chemistry and biochemistry, replies:
Many of us have experienced the effects of helium on our speech. Our voices are sometimes said to “squeak” or to sound like Donald Duck. The basis of this effect is very simple, but the details are quite complicated.
The main effect of a gas on sound is to control its speed. In air, which is mostly nitrogen, sound travels at about 350 meters per second. This is why a clap of thunder follows the lightning flash by about 5 seconds for each mile of distance. However, in a “lighter” — or lower density — gas like helium, the speed is nearly three times faster; likewise, a heavier gas slows the sound.
During speech, the vocal chords generate a fundamental frequency that is accompanied by a set of harmonics — higher frequency multiples of the fundamental. The spacing of these harmonics is determined by the shape and size of the vocal tract, which constantly changes during the formation of words. At any given time, some of these harmonics will match the vocal tract configuration and be reinforced, or resonate, while others will be dampened.
When helium is present, the speed of sound increases, shifting the resonance to higher frequencies for a given arrangement of the vocal tract. Thus, the so-called “timbre” of the voice is now dominated by sounds that occur at higher frequencies, leading to the squeakiness or the “Donald Duck” effect.
For a detailed explanation see the University of New South Wales’ Web site.