Why Inhaling Helium Makes You Sound Like a Chipmunk

Here’s a novel idea from the King’s College Choir:

In the video, one of the choristers gets help hitting a high note by taking a breath from a large helium balloon. (Obligatory warning: it’s dangerous to try this at home.)  So… does this actually work?  Does inhaling helium really make your voice higher?

In an earlier post, we looked at how the pitch of your voice depends on the frequency at which your vocal cords vibrate, which in turn depends on how thick and how taut the vocal cords are.  Inhaling helium should not have any effect on this.

What helium does, though, is to change the formant frequencies of the voice.  We talked about formants in the last post — they are hills in the overall shape of the spectrum, and they play an important role in allowing us to distinguish different vowels and different speakers.  Formants exist because the vocal tract prefers certain frequencies, while eschewing others.  The preferred frequencies are called resonant frequencies.

It turns out that the resonant frequencies of a cavity (like the vocal tract) depend not just on its size and shape, but also on the speed of sound (more on this in a future post).  Because sound moves 2.7 times faster through helium gas than through air, the formants would all become 2.7 times higher when we inhale helium.  This means that the higher harmonics get a boost, so the voice becomes more piercing (see this post).  2.7 times is a very drastic increase, considering that the ratio between female and male formant frequencies is only about 1.1 — this explains why helium voice sounds so unnatural.

(In practice, the formant frequencies might not increase by quite this much, because breathing in helium from a balloon doesn’t displace all the air in the vocal tract — you’d get a mixture of air and helium.)

Let’s have a listen to the King’s College Choir performing the same piece, but sans helium this time.  Here’s the relevant passage:

The soloist certainly doesn’t have any trouble hitting the high note, and he hits it with a beautiful pure tone, unlike in the helium version.  It’s evident, though, that he is singing the same pitch in both videos.

Let’s take the highest note that he hits, and compare its spectrum between the two videos:

The spectra are a bit messy because there is a whole choir singing, but the soloist’s high note is pretty distinct.  We see that both spectra have strong peaks at about 1040 Hz, corresponding to the C6 note — so his fundamental frequency (or pitch) is the same in both videos.  Both spectra also show the harmonics — peaks at integer multiples of the fundamental frequency.  But the normal-voice spectrum only has 5 or 6 visible peaks, while the helium spectrum has 13.  His spectral envelope has been changed by the helium, strengthening the higher harmonics.  In other words, the timbre of his voice has changed, but not the pitch.

In conclusion: helium won’t help you to reach notes that are any higher than what you can already do, though the piercing tone it gives you may help you cut through the accompaniment and be heard more distinctly.  (It might also cause asphyxiation…)

Digitally Simulating the Helium Effect

People sound funny when they inhale helium because their formant frequencies become unnaturally high.  We associate this kind of sound with chipmunks, presumably because chipmunks have smaller vocal tracts, so we expect them to have higher formant frequencies than humans.  Let’s simulate this helium effect on a recording.  Here’s someone reciting a line from the Catalan poem La Vaca Cega:

Now let’s digitally double the formant frequencies of the recording, while keeping the pitch the same:

Alternatively, we can get a more extreme “chipmunk effect” by simply speeding up the original recording, as you may have experienced before.  If we play back the original recording 1.5 times as fast, it sounds like this:

As we saw in the previous post, changing the speed of playback also changes the frequencies of the sound.  This applies equally to both the pitch and the formant frequencies.  Whereas helium only increases formant frequencies, increasing the playback speed gives us faster speech, higher pitch, and higher formants.  All three of these factors contribute to the chipmunk sound.