Why does helium make your voice squeaky?
Dear Straight Dope:
Why does inhaling helium make your voice sound funny and high pitched?
This takes me back to the days when I would spend hour after hour at birthday parties and bar mitzvahs sucking helium out of dozens of balloons to the amusement of myself and my fellow junior high schoolers.
Cut me some slack, I hadn't discovered booze yet.
Anyway, Ian, let's talk acoustics. While no one will question that helium makes your voice sound funny, people who are knowledgeable about acoustics will insist that helium does not make your voice high-pitched. That bears repeating: helium does not change the pitch of your voice. Don't believe me? There's a web site that has sound files demonstrating this fact rather well. It's educational and you get to hear a man with an Australian accent sing a monotonous song with a vocal tract full of helium. Check out http://www. phys.unsw.edu.au/PHYSICS_!/SPEECH_HELIUM/speech.html. The fourth sound file is bound to be a Top 40 hit.
What helium does change is the timbre of your voice. Timbre is what allows you to distinguish between the same note played at the same volume on two different instruments. Timbre also allows you to distinguish different elements of speech (such as vowel sounds) from each other. For this last reason, timbre is very important to interpreting speech.
When you speak, air travels from your lungs up through the larynx on its way out of your mouth. In the larynx are a pair of vocal folds which meet to form a V-shaped slit. On its way through, the air hits the underside of your vocal folds causing them to vibrate. This vibration then excites air molecules in your vocal tract, setting up resonant frequencies that are interpreted by the ear. By manipulating your vocal tract (moving your tongue, lips, soft palate, etc.) you can create different resonant frequencies allowing you to make the many different sounds of coherent speech.
Everyday speech is a combination of the vibration of your vocal folds (which influences the pitch) and the vibration of the air in your vocal tract (which influences the timbre). When you say "ee," your tongue, lips, etc., are in a different configuration than when you say "ooh." Each speech element has a specific pattern of frequencies, like a fingerprint. The resonant frequencies that make the "ee" sound are distinct from those that produce "ooh". These distinct patterns allow us to recognize an "ee" sound regardless of pitch--a soprano singing "ee" produces the same resonant frequencies as a baritone singing "ee."
Helium is significantly less dense than air. As a result, the speed of sound is much higher in helium. By inhaling helium you are effectively increasing the speed of the sound of your voice. The configuration of your vocal tract does not change, however. If you increase the speed of a sound while keeping its wavelength constant, you increase the frequency of that sound. (Think of it as a sort of Doppler effect.) Since each element of speech is composed of a pattern of several frequencies, altering the speed of sound distorts that pattern. Result: you sound like Donald Duck.
Note that the pitch remains the same because your vocal folds are vibrating at the same frequency as when you were breathing air. Contrary to many explanations I have read, helium does not significantly affect the vibration of your vocal folds. To repeat yet again, helium drastically changes the timbre of your voice without having any significant effect on its pitch.
I have heard reports of wacky chemistry professors (how's that for redundant?) inhaling balloons full of xenon gas. Because pure xenon is much denser than air, the speed of sound through xenon is slower, and inhaling xenon lowers the resonant frequencies of your vocal tract. Some web sites recommend wearing a cowboy hat while speaking in a "xenon voice" for maximum comedic effect. Yee ha.
One final note: inhaling helium is not a terribly healthy thing to do. Every time you inhale pure helium, you are not inhaling oxygen. And for the love of Pierre-Jules-César Janssen (the first man to find evidence for the existence of helium), do not inhale helium from a pressurized tank! In addition to risking serious damage to your lungs, you could wind up with helium bubbles in the arteries that supply blood to your brain. This is known as a cerebral arterial gas embolism and can lead to stroke-like symptoms including, of course, death.
So unless you want to spend the next few days after your friend's bar mitzvah in a hyperbaric chamber, take it easy on the helium balloons, OK?