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Old Apr 1st 2014, 03:25 PM   #1
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Helium and resonance frequencies

Hello humans,

I want to understand what it is that happens when it seems like a person gets a high-pitched voice after inhaling helium.

So far, I've dug up that it isn't actually an effect on the vocal folds themselves but rather an effect on the speed of sound travelling through the vocal tract (since helium is less dense than air, sound travels faster - actually why is this? Anyway that's a question I'll work out later on my own).

I'm assuming you wonderful humans are familiar with the idea of "resonant frequencies" but just in case: When a sound is produced by air rushing past and vibrating the vocal folds, it then travels through the vocal tract (your throat and out your mouth). Certain wavelengths of sound pass more readily through different shaped (length or cross-sectional area) tubes. When a tube is thinner, higher frequencies are emphasized; likewise, if a tube is shorter, higher frequencies are emphasized.

What is happening when the air moves faster through the vocal tract when you inhale helium? Why do higher frequencies get emphasized? Because that is what happens: the resonance frequencies shift upward when you talk with helium as opposed to with normal air (N and O)? Another way to say with would be that helium changes the "timbre" of your voice- not the actual pitch. (We change the timbre of our voice all the time when we speak- it's the secret to vowels actually).

PLEASE: I cannot find a nice physical explanation. Why do lower frequencies get selectively tuned out when a person speaks after inhaling helium as opposed to when a person speaks after inhaling air?

What exactly is going on in that vocal tract?
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Old Apr 2nd 2014, 10:06 AM   #2
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The resonance depends on the relationship between the distance and the speed.

If the wavelength exactly matches the length there is a resonance.

Note: speed = wavelength * frequency

so changing the speed changes the resonant frequency
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Old Apr 2nd 2014, 11:51 AM   #3
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Just to add - the reason why the speed of sound is higher in He than in air is because sound waves travel at:





where c = speed of sound, K = bulk modulus (effectively the "stiffeness" of the gas) and rho = density. If you decrease density the value for c goes up. And as MBW noted, if the speed goes up the for a given wavelength the frequency goes up.
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Old Oct 20th 2014, 03:37 AM   #4
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The formula that you are considering (the speed of sound in a medium is proportional to the square root of the temperature) applies to sound waves in a medium where the density is inversely proportional to temperature - that is, if considering a gas it can't be in an enclosed tube. Further, once the temperature reaches the point where the molecules of air are torn apart into ionized atoms the Ideal Gas Law no longer applies, and hence the relationship between temperature and density breaks down. Finally, consider that the "signal" of a sound wave is conveyed by the electromagnetic interaction between adjacent molecules - the absolute speed limit for such interaction is the speed of light, so its impossible for any mechanical signal to reach or exceed that limit - in fact since the molecules have mass, their inertia slows down the propogation process to well below the speed of light no matter how dense the medium.
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Old Oct 20th 2014, 03:20 PM   #5
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Nice, but this has nothing to do with the problem at hand.

-Dan
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