Why exactly does helium make your voice sound so weird?

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     Franklin Institute chief bioscientist Jayatri Das  holds green balloons full of helium, and purple balloons full of sulfur hexaflouride. (Paige Pfleger/WHYY)

    Franklin Institute chief bioscientist Jayatri Das holds green balloons full of helium, and purple balloons full of sulfur hexaflouride. (Paige Pfleger/WHYY)

    We all know inhaling helium makes your voice sound high and squeaky, but do you know why? 

    If you’ve ever inhaled from a helium balloon at a birthday party or watched this scene from My Best Friend’s Wedding, you know helium does weird things to your voice.

    It makes it sound high and squeaky, like a rodent from Alvin and the Chipmunks. But, surprisingly, inhaling helium doesn’t actually change the pitch of your voice.

    Here’s what it does to make your voice sound weird:

    First, a refresher on how your voice works: 

    “When you talk, your vocal cords vibrate at a particular frequency or rate, and the movement of your vocal cords then pushes the air around it in your voice box,” said Franklin Institute chief bioscientist Jayatri Das.

    “That motion of air causes a sound wave that then gets picked up by the ears of your listener.”

    The rate of this vibration, which controls the frequency and pitch of your voice, doesn’t change when you suck in helium. What does change is the sound quality of your voice, known to musicians as timbre. Different timbres are the reason why we can distinguish between a piano and violin playing the same note. The sounds are the same pitch, but their tone (aka sound quality or timbre) are different.

    The human voice is made up of many different tones mixed together.

    “When your vocal cords vibrate, they don’t just vibrate at a single frequency, there’s a whole mix going on,” Das said. “It’s that mix that’s one of the most important factors of sound quality.”

    Inhaling helium makes the higher-pitched tones resonate more in the vocal tract, amplifying them so they are louder in the mix. At the same time, it makes the lower tones resonate less in the vocal tract. The two effects combine to create a Chipmunk-like, flat sound.

    “Essentially, the higher frequencies become stronger, they’re amplified over the lower frequencies,” Das said.

    If you want to nerd out a bit more on the science, here’s more background:

    Usually, the sound waves your vocal cords produce travel through air in your voice box. But when they go through the helium that you’ve inhaled, they travel about three times faster. That’s because helium is so much lighter than air.

    When sound waves speed up but their frequency stays the same, each wave stretches out.

    Depending on its unique shape, your voice box naturally resonates or vibrates when certain wavelengths hit it. When sound waves are stretched out because they are traveling through helium, lower-sounding wavelengths get so long that they don’t fit right in the voice box anymore, so your vocal tract doesn’t resonate and amplify those tones. The higher tones, meanwhile, are stretched out so they’re the perfect size to be amplified. Those get boosted, and it sounds like this:

    Sulfur hexafluoride has the opposite effect on the voice as helium.

    “Essentially, the phenomenon is the same, it just happens in the other direction,” Das said.

    It’s a gas that is much heavier than air, so when it is inhaled, it shortens sound waves so the lower tones in the voice are amplified and the higher ones fade out. That lesser-known but perhaps cooler party trick sounds like this:

    Inhaling a little helium or sulfur hexafluoride won’t hurt you in small amounts, but it’s best not to breathe in much: both gasses prevent oxygen from getting to the brain.

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