A more likely explanation for the increased pitch caused by inhaling helium is due to the sound producing mechanism itself. Consider the voice box modeled as a simple vibrating membrane (or perhaps a speaker cone) whose vibration acts on the ambient gas around it causing the formation of sound waves. By controlling different characteristics of the membrane (tension, shape, etc.), qualities of the created sound waves (i.e. pitch) can be controlled. In this model, the ambient gas acts as a resistance to the motion of the membrane since energy from the vibrating membrane is imparted to the gas. When the molecules composing the gas are heavy, the resistance is greater than when the molecules in the gas are light: The heavier molecules have a greater inertia for the membrane to overcome. When humans breathe helium and speak, their vocal chords, which are 'tuned' for normal atmosphere conditions, do the same things that they always do with substantially less resistance from the lighter gas. The result is that the chords will vibrate at a higher frequency.


Answered by: Joe Larsen, Ph.D. Chemistry, Rockwell Science Center, Los Angeles, CA