QUESTION #36

What is the physics behind the voice change which occurs when one inhales Helium?

Sound is created when something compresses the air and the region of compression moves away from the source to soneone's ears.

An analogy can be made with a huge crowd of people packed closely together. If the people in the back of the crowd all decide to push foward rapidly and then back off, the region of compression will move out into the crowd. Each layer of people will be pushed from behind and in response they will move foward and push the people in front of them. When the push from behind stops, they will move back again. Everyone ends up in roughly the same place they started, but the compression wave moves. If the crowd is composed of big heavy people, the wave will move slowly, because big people are hard to move. If the crowd were instead made of balloons the wave would move much more rapidly.

It turns out that the number of molecules in a fixed volume of gas at a given temperature and pressure is the same, regardless of the gas used (provided the pressures are reasonably low). Helium atoms have a molecular weight of about 4 grams per mole (a mole is 602200000000000000000000 molecules). Air is about 80% Nitrogen which has a molecular weight of about 28 grams per mole; it is seven times havier than helium. This means that a compression wave will propagate faster through helium than it will through air.
When a series compression waves (your voice) leave the helium in your lungs and hit the denser air, the wave train is suddenly slowed down. The wave in front is slowed first while the wave behind it is still moving fast. Then the second wave is slowed while the third is still moving fast, and so on. It's like fast moving traffic on an open freeway suddenly coming to a slow point in the road, all the cars bunch up close to each other.
Your ear interprets closely bunched sound waves as a higher pitch than widely spaced waves. So when your voice originates in helium and then travels through the air to someone's ear, it sounds higher. If both you and the listener were in a room filled with helium, your voice would get to their ears faster than normal, but there would be no pitch change.
Another interesting experiment which is too dangerous to play with is to breath a gas that is heavier than air. When you do this, your voice sounds almost demonic.
In this case the sound waves spread out when they go from the dense gas to air and your voice sounds lower than normal.
This is dangerous because heavy gases settle in the bottom of your lungs and you can easily suffocate if you don't hang upside down immediatly after the experiment.
Answered by: Joe Larsen, Ph.D. Chemistry, Rockwell Science Center, Los Angeles, CA

Indeed, the majority of components in air are more massive than helium atoms. The end result is that across a helium/air interface, the wavelength of a sound wave will be changed. The frequency of this wave, however, will not be changed. The ear uses the frequency of a sound wave in order to determine pitch. Thus, the pitch generated by the voice box will be the pitch heard by the ear regardless of the fact that the sound wave has traveled through a helium/air interface.

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.

Notice that this model predicts an elevated pitch received by a listener even if the entire atmosphere were helium. This would be a good comparison of our respective models if we could figure out how to keep the experimental subjects from keeling over.
My model is a simplification of the real process and should not be taken as gospel. However, I believe it does hint at the underlying priciples involved.