Sound gets started when a vibrating source — imagine a guitar string — pushes air particles out of the way. Those air particles bump neighboring particles. The particles move back and forth as the sound source vibrates. The faster the movement of the air particles, the higher-pitched the sound.
Helium gas is lighter than the air around us. So sound waves moving through helium bounce around faster. The sound of your voice comes when slightly pressurized air from your lungs resonates in your vocal tract. If you inhale helium instead of air, the particles bump faster, so your voice has a higher pitch . . . much like the sound of my helium voice right now. Okay, we confess. I didn’t inhale any helium to create that sound.
Instead, we created the effect in the recording studio — because, although inhaling one helium balloon at a party probably won’t hurt you, there are risks. In large enough amounts — inhaled helium can force out the oxygen in your lungs and cause suffocation and other damage.
Helium is the second lightest gas — that’s why it’s so good for balloons. The one gas that’s lighter, hydrogen, is extremely flammable.
Graham’s Law of Effusion states that the average velocity(v) of a gas molecule is inversely proportional to the square root of its molecular weight (m). Therefore, the lighter the molecule the faster it boings around. The average kinetic energy ( = ½ mv2 ) of molecules is constant for a given temperature (and directly proportional to absolute temp) so they speed up as they are warmed.
“By kinetic theory the kinetic energy of a gas is proportional to the temperature of the gas. (1/2mv2 = 3/2kT). Since helium is lighter than air, it will have a greater velocity at the same temperature. The next point is that our voice can be considered to be produced by resonating sound waves in the mouth and throat regions. In a resonating cavity the velocity of the sound wave equals the frequency times the wavelength of the sound wave (v = freq * wavelength). The wavelengths are basically fixed by the dimensions of the sound cavity (and are the same for air and helium), therefore if the velocity of the gas is larger and the wavelength does not change, the frequency must increase to satisfy the condition that v = freq * wavelength. This results in the higher pitch sound.”