The physics behind this little phenomenon, practiced in the back corners of parties everywhere, are actually fairly simple. What is going on in the voice tract however is not.
The mechanics of a person’s voice is an awesome biological phenomenon born from the need of women to gossip. It all starts with the air in the lungs. As air passes from your lungs to the outside of your mouth, it can be manipulated in several ways. Should you decide a vow of silence is appropriate and simply want to blow out candles your entire life, you can force air out and all that is heard by others is the sound of wind rushing from your mouth. Should you need to take a more Opera Winfrey approach, you can use your larynx, or voice box.
The air in your lungs gets pushed up by the relaxation of your diaphragm. It then goes through your trachea and out a small orifice that has two folds of skin (vocal cords) on either side of it, in the shape of a V. This is called your larynx or voice box. As the muscles that attach to your voice box tense and relax, they create a vibration of the cords. As these cords vibrate, they release pulses of air. The tension in these muscles creates the differences in frequency. The higher the tension, the higher the frequency and therefore the higher the pitch. This frequency is measured in hertz (how many times a second this repeats). For example, nearly all human speech sounds usually range from approx. 200 hertz-8,000 hertz. Meaning the sound waves vibrate at approx. 200-8,000 times per sec.
Once out of the voice box, the air travels into the area of your mouth that can be informally known as your vocal tract. As you manipulate your tongue, jaw and lips, you can change the resonant frequencies created by your vocal cords, allowing you to make the many different sounds of speech.
Together, the sound we hear created by air flowing at different frequencies and resonances, creates our voice. One other factor affecting pitch is the thickness of ones vocal cords. The thicker the folds of skin are, the deeper the voice. This is due to the amount of mass your chords have that the air has to manipulate. As you would expect then, men tend to have much thicker folds of skin then women, ah the soulful folds of Barry White!!
So now let’s talk about the air that’s coming out of your lungs. The number of molecules in a fixed volume of gas, like the volume of air in your lungs, does not change with the type of gas (assuming the pressures are reasonably low). As long as the temperature and the pressure are the same, it does not matter whether it is helium or air, the number of molecules is the same. The mass of those molecules is then measured by atomic weight. Atomic weight is a dimensionless physical quantity (which is why it works so well for a gas that does not necessarily have a given dimension). It is the ratio of the average mass of atoms of an element compared to 1/12 of the mass of an atom of carbon-12. All that basically means is the higher the number the heavier the gas.
Helium has an atomic weight of 4.002602. Air which is approximately 80% Nitrogen, has differing characteristics depending on the environment. (for instance if you live in Los Angeles its about 99.27% car exhaust). Because of this, its actual atomic weight is impossible to precisely define. However it is generally about seven times heavier then helium.
So why the perceived difference in your voice with helium? The answer lies in how sound waves travel through a given gas. The more dense, or heavier, the gas, the slower the sound wave will travel. Helium is much lighter than air. The speed of a sound wave through helium will then be much higher. So by inhaling helium and using it as the source of the perceived sound, you are simply increasing the speed or frequency of your voice. You are not changing the pitch, since your vocal chords are vibrating at the same speed as when you are using air. You are also not changing the configuration of your vocal tract. So while the base frequency of the chords remains the same, the frequency of the sound heard by others is increased due to the wave traveling through helium much faster than through air.
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