Myth: Helium makes your voice high-pitched

by Stephanie Chasteen on March 26, 2009

A friend just pointed out an interesting misconception that I hadn’t thought about. When you inhale helium, your voice sounds higher. It turns out that your voice isn’t actually higher-pitched! At least, not in the way that we think it is.  The reasoning is a little convoluted…. read on.

Here’s the common misconception: The speed of sound is faster in helium because it’s lighter than air (thanks to the commenter for correcting me that it’s not the density but the molecular weight of gas that is important here… see Wikipedia on speed of sound). So, they say, since the speed is faster, that means that the frequency of your voice has to increase to compensate if the wavelength remains the same.  [remember, the speed of sound (meters/sec) = frequency (waves/sec) * wavelength (meters/wave)].

Or, another popular misconception is that frequency remains constant, but the wavelength goes up. When this long wavelength hits the air, it then gets converted into a higher frequency. (But if you do the math, you’ll find this logic is flawed, because it actually results in a lower frequency).

Here’s what actually happens, as far as I can figure.

You make sound with your vocal cords… or, more accurately, “vocal flaps” — they look like two fleshy lips slapping against each other rhythmically. See some video of vocal flaps here — look especially at the last one on the right. When you sing middle C, the vocal flaps vibrate together to make a mess of frequencies around middle C, and you change the shape of your mouth and throat to emphasize “middle C” out of the mess of different notes your vocal flaps are making.

In other words, the sound from our vocal flaps doesn’t make it directly to our ears. It resonates in our vocal tract, which picks out certain notes. This is sort of how if you yell into a big tunnel, the echo sounds rather low. That big tunnel “picks out” the low notes. This is why male and female voices sound different — our vocal flaps make the same jumble of notes when we sing or talk, but the vocal tract, or chamber, emphasizes the lower notes for men and the higher notes for women. You can see this if you get a man and a woman and have them sing the same note into a frequency analyzer — you’ll see the same spikes on the analyzer, but the woman will have stronger spikes in the higher frequencies and vice versa for the guy.

So, when your vocal tract is filled with helium, your vocal flaps make that same set of messy frequencies around middle C, but the vocal tract picks it up as a higher frequency. So, in this way, the pitch of your voice doesn’t change (it’s still middle C that you’re singing), but the timbre of your voice does — which frequencies are picked up. Faster speed of sound = higher frequency. (The wavelength is fixed by the size and shape of your mouth and throat).  So, it’s true that what you’re hearing is a higher frequency, but the difference is in what happens to air in the chamber of your mouth, after you’ve produced the sound.

Here’s what the March 1987 edition of Scientific American says in an article titled “Sopranos of the Skies”:

When a soprano sings a high C, her vocal cords actually produce a broad band of frequencies. . . . If [she] inhales helium, her voice seems to rise in pitch not because her vocal cords vibrate faster in the less dense atmosphere (they do, but only slightly); rather, because sound travels almost twice as fast through helium as it does through nitrogen, the acoustic properties of the vocal tract change so that it resonates with and amplifies higher-frequency tones.

For those of you who like math: When your lungs are filled with air, and you sing middle C, it has a frequency of 261 hertz and the speed of sound in air is 333 m/s (that’s 770 mph!), with a wavelength of about 1.27 meters (or about 4 feet, neato). When you inhale helium, the speed of sound is faster (972 meters/sec) because helium is lighter than air. Well, if the frequency created by your vocal flaps is the same, and the speed of sound goes up, then the wavelength must also go up. Using speed = frequency * wavelength again, you can calculate that the wavelength of the “middle C” that you try to sing on helium is actually about 3.7 meters long.

But here’s the real puzzler!  What happens when the sound leaves your helium-filled mouth and hits your ears? Why doesn’t that long wavelength just get downshifted to a shorter wavelength when it leaves your mouth and hits the air (so that you hear the regular “middle C” that the soprano was trying to make)? It’s because frequency has to stay the same, or “frequency is conserved”. If 300 pushes of air leave your mouth every second, then 300 pushes of air have to travel through the air outside your mouth as well or else you’ll get a traffic jam of air leaving your mouth (this is the same argument, roughly, as to why water has to leave a pipe at the same volume per second as it enters the pipe). Below is from the New Scientist:

Once sound leaves the mouth its frequency is fixed, so the sound arrives to you at the same pitch as it left the speaker. Imagine a roller coaster ride. The car speeds up and slows down as it goes around the track, but all cars follow exactly the same pattern. If one sets out every 30 seconds, they will reach the end at the same rate, whatever happens in between.

In stringed instruments, the pitch depends on the length, thickness and tension of the string, so the instrument is unaffected by the composition of the air. Releasing helium in the middle of an orchestra would therefore create havoc. The wind and brass would rise in pitch, while the pitch of the strings and percussion would remain more or less the same

You can see some other writings on this at the Straight Dope and the New Scientist.

Note that you can have all sorts of crazy fun by breathing in sulfur hexafluoride. Well, actually, don’t do it, but instead watch it in the below YouTube video.Sulfur hexafluoride is heavier than air, so it has the opposite effect. A balloon filled with sulfur hexafluoride feels heavy, like it’s filled with water or foam. And that’s why you shouldn’t do this at home — since it’s heavier than air, it sits in your lungs and you can’t expell it. You have to turn yourself upside down or over a chair for a few moments to get it out of your lungs.

I’ve inhaled sulfur hexafluoride and I have to say, it was one of the strangest sensations! It felt like I was trying to talk through mud.

Here’s another video that shows a neat demo about how heavy sulfur hexafluoride is.  At the end they “float” a little aluminum boat on a “sea” of sulfur hexafluoride (which sits, invisibly, in a container. It’s a gas but doesn’t float away because it’s heavier than air). They then scoop some of the gas into the aluminum boat and you can see it slowly sink as it’s filled with the invisible gas….

{ 7 comments }

Paul March 26, 2009 at 4:29 pm

Is density really the issue, or is it molecular weight? I don’t think density per se matters much to speed of sound calculations in gases.

sciencegeekgirl March 26, 2009 at 5:11 pm

Is density really the issue, or is it molecular weight? I don’t think density per se matters much to speed of sound calculations in gases.

Thank you Paul, sloppy me! I said it right in one place and wrong in another. I fixed the post. See http://en.wikipedia.org/wiki/Speed_of_sound for a nice discussion.

David Samuels March 26, 2009 at 9:13 pm

It might be worth mentioning the reason that frequency stays constant as the sound wave travels from helium to air – like in light waves, in sound waves energy is proportional to frequency, and since energy is conserved, so is frequency. It’s analogous, in this case, to light traveling from air into glass. If we observe the light in the glass, such as by creating a diffraction pattern, we observe that its wavelength shortens (a red photon would shift towards the blue). Since the energy of a photon is proportional to its frequency, and energy must stay the same as the photon enters the glass, frequency is conserved, while speed decreases, thus wavelength must decrease as well.

We can’t ever actually see this phenomenon with our eyes, of course, because our rods and cones, for instance, analyze the wavelength of light that gets to them after passing through the aqueous humors of our eye, and even if we put our eye right up next to the glass, it still has to travel through the humors and shift wavelengths again before being analyzed – ditto for any other analyzer we might build – that is why you have to analyze the wavelength in situ. Which raises a creepy, but interesting question – if we could change the index of refraction of the stuff in our eye, or remove it altogether, would we see the world in different colors? I think the answer is no, since the eye works through photopigments which are sensitive to a particular energy (frequency) of photon through their electronic chemical valence structure, hence “wavelength”, per se, is not the operative measure, even though it’s always the one we talk about. But I’m not 100% sure of that…

gab April 8, 2009 at 12:46 am

hi im doing a science fair progect on why helium makes your voice high piched thanks for your help hope you respond soon its dew in may

sciencegeekgirl April 8, 2009 at 1:27 am

hi im doing a science fair progect on why helium makes your voice high piched

Then I hope that this post helps out, though it’s a little high level for you.

bushra September 26, 2009 at 2:30 pm

hey can u please tell me why soun diffracts when it leaves your mouth? thanks!

sciencegeekgirl September 26, 2009 at 9:09 pm

hey can u please tell me why soun diffracts when it leaves your mouth? thanks!

Hi Bushra,

Diffraction is the spreading of waves when they pass through a small opening. Think of water waves headed towards a wall with a small gap. The ones that do make it through the little gap will spread out in a semicircle. I’m not sure if that answers your question?

Stephanie

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