What REALLY makes your voice higher when breathing helium?

October 16, 2008 5:35 AM

There is a current FPP from a Mythbusters episode that demonstrates this effect. There is something about the explanation for it that bothers me.

The reason given in the clip for the difference in sound frequency is that sound travels at different speeds in various gases. I thought sound travels faster in denser materials, which would make the Sulphur Hexafluoride voice higher, wouldn't it? I always thought the higher frequency from helium breathing was caused by the vocal cords vibrating faster because of less resistance pushing against less mass. What is the real reason for this effect?

posted by Enron Hubbard to science & nature (11 comments total)

That explanation kind of bugged me too. I think more accurately, your vocal cords might be able to vibrate faster in a less dense gas and slower in a more dense gas. I did fail grade 10 physics though.
posted by bonobothegreat at 5:51 AM on October 16, 2008

From Wikipedia:

The voice of a person who has inhaled helium temporarily changes in timbre in a way that makes it sound high-pitched. The speed of sound in helium is nearly three times the speed of sound in air; because the fundamental frequency of a gas-filled cavity is proportional to the speed of sound in the gas, when helium is inhaled there is a corresponding increase in the resonant frequencies of the vocal tract. (The opposite effect, lowering frequencies, can be obtained by inhaling a dense gas such as sulfur hexafluoride.)

Sound travels faster through Helium than it does through Air. The frequencies of the vocal cords do not increase, but the frequency of the sound waves created by the vocal cords traveling through Helium gas do increase. The increase in the frequency cause by a lower molecular weight gas corresponds to the increase in pitch.
posted by clearly at 5:54 AM on October 16, 2008

The best non-technical explanation I have seen is at the Straight Dope; most other detailed explanations involve a lot of differential equations (such as Wikipedia's entry). To try and simplify a little more, your vocal cords vibrate at a frequency that is determined by your laryngeal muscles and how taut they are as well as the size of the vocal cords themselves (smaller in women thus the higher voice) But you vocie is not a single, pure note; it has a lot of harmonic components that are determined by the resonance of the rest of your upper respiratory tract. This is why a stopped up noise alters your voice. Gases of significantly different density than air alter the frequency of this resonance favoring lower frequencies at higher density and lower frequencies at higher density. It is a complicated subject and I hope there is a physical chemist out there who can weigh in with a better explanation.
posted by TedW at 5:57 AM on October 16, 2008

Go here for a more scientific explanation.
posted by boeing82 at 5:57 AM on October 16, 2008

"I thought sound travels faster in denser materials"
Why did you think that? I can't find any evidence that supports that.
posted by browse at 5:58 AM on October 16, 2008

"I thought sound travels faster in denser materials"
Why did you think that? I can't find any evidence that supports that.

I think the original poster may be thinking of the fact that sound travels faster in liquids and even faster in solids than in gases, but that is because of the differen physical properties of those materials; the Wikipedia article I linked above has more details.
posted by TedW at 6:02 AM on October 16, 2008

Correction:

The speed of sound through a gas is inversely proportional to its density. Helium has a very low density compared to Air.

Density of Helium : ~0.18 g/L
Density of Air : 1.0 g/L

So, sound travels faster in Helium than it does in Air. To account for the increase in speed, the frequency must be increased, in this case, the vocal cords do resonate at a higher frequency (although at similar wavelengths), which is caused by their functioning against the lower molecular weight of Helium.

You are right about the lowered mass of the gas making the pitch higher although it corresponds to a decrease in density with the Helium.

Since Sulphur Hexafluoride has a higher density ~6.2 g/L sound travels slower through it, meaning a decrease in frequency, which is a lowered resonance of the vocal cords corresponding to the increase in molecular weight. It will make your voice much deeper.
posted by clearly at 6:15 AM on October 16, 2008

Boeing82's link is excellent and contains sound files. It's the final answer. Compare with some of the guesses above and you'll see a difference.

Essentially, the vocal folds don't resonate at a higher frequency or lower wavelength with helium. However, the timbre of the vocal "instrument" (your head and chest) is different with helium and the sound that comes out is mostly the higher harmonics of your voice. With air, the fundamental and lower harmonics dominate the sound produced. They are still there, but hardly heard, with helium.
posted by JimN2TAW at 7:13 AM on October 16, 2008

There was just a post on Metafilter that explained that sound does not travel faster through solids or liquids, but that the reason you hear the sound of a train approaching faster if you put a ear to the rail is because the rail acts as a a sort of funnel for the sound. Here's the relevant link.
posted by peacheater at 7:27 AM on October 16, 2008

peacheater: That's one of the answers that is more abut semantics than anything. If you reread the article, you'll see that the author explains that sound doesn't travel through solids "better" than gasses. There are also a few other gotchas (striking a solid object will always make a louder sound than what is heard through the air), but sound does, in fact, travel faster through solids.
posted by niles at 8:54 AM on October 16, 2008 [1 favorite]

JimN2TAW's point is almost right.

You've probably stood in a room and noticed that when your voice hit a certain note, it would reverberate in your ears. Well, if you filled the room with Helium, your voice would reverberate at a different frequency. This does NOT mean that the frequency of sound in the room is somehow different than the frequency of your vocal chords, as some above have claimed.

The frequency does not change. YOU set the frequency, not the gas. Frequency is determined by how many times per second something (i.e. your vocal cords) wiggles back and forth. That's it.

The wavelength of the sound waves will be different for a given frequency in Helium than in air due to the different speeds of sound in the two gases.

Your voice is not made up of just one frequency, but of many overlapping frequencies. What matters is the relative amplitudes of the various frequencies as they leave your throat. How well your "voice box" responds to any given frequency depends on the geometry of the voice box and the wavelength of the sound in your voice box. This is why the speed of sound in Helium matters.

As a simple toy model of your voice box, consider a resonance tube. The resonance frequencies will be different for different gases in the tube, because of the different speeds of sound. If you drive the gases with some pure frequency f, the amplitude of the response will be different depending on the gas, but the output will still be at that same frequency f.

The article that Boeing82 links to explains that the resonance frequencies for your voice box are higher with Helium than with air. So, these are the modes of your voice that get emphasized.

And yes, sound moves faster in Helium than in air. See this section of the previously linked Wikipedia article.
posted by dsword at 9:11 AM on October 16, 2008