In order to understand how helium has this effect on a voice, it is helpful to first consider how sound waves form and travel, as well as some basic properties of gases.
Sound waves are formed by the vibration of something (a drum-skin or your vocal chords, for instance) in a medium such as air. In the case of a drum, as one strikes its skin, it vibrates up and down. As it moves up, it pushes against the gas molecules of the air, forcing them upward against other molecules. The gas molecules are compressed together and this ripple of compressed molecules moves up away from the drum. Meanwhile, the drum skin moves down and back up again, resulting in another compression. This moving series of compressions is a sound wave, and the distance between them is known as the wavelength.
All gas samples have the same number of molecules per unit volume at a given pressure and temperature, whether the gas is helium or nitrogen (the primary constituent of air). But not all gas molecules have the same mass. Nitrogen (and thus air) has a mass roughly seven times greater than that of helium. Nitrogen is thus denser than helium and sound waves travel through it more slowly than they do in helium. At 20 degrees Celsius, for example, sound travels at 927 meters a second through helium, but only at 344 meters a second through air.
Like the vibration of a drum or a violin string, the vibration frequency of the vocal cords is independent of the type of gas that surrounds them. Whereas the velocity of the sound waves is faster in helium (and the wavelength greater), the frequency remains unchanged because it is determined by the vibrating vocal cords. Rather the timbre, or quality, of the sound changes in helium: listen closely next time and you will notice that a voice doesnt become squeaky but instead sounds more like Donald Duck. It is the lesser density of the helium--which serves as the medium for the sound waves--flowing through the larynx that produces this differing quality in the voice.
Answer originally posted on June 14, 2004.