why does helium make your voice higher?

Some people think that helium changes the pitch of your voice, but the vibration frequency of the vocal cords doesn't change along with the type of gas molecules that surround them. When your vocal tract is filled with helium, your vocal cords are vibrating at the same frequency as usual. It's actually the timbre...that changes, because those lighter-than-air helium molecules allow sound to travel faster and change the resonances of your vocal tract by making it more responsive to high-frequency sounds and less responsive to lower ones...

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