If you fill your lungs with helium and speak, you voice comes out at a very high

Question

If you fill your lungs with helium and speak, you voice comes out at a very high pitch even if your vocal cords keep its length and tension. From this observation which one of the following is correct regarding the speed of sound in helium?

(A) Speed of sound in helium is the same as the speed of sound in air.
(B) Speed of sound in helium is lower than the speed of sound in air.
(C) Speed of sound in helium is greater than the speed of sound in air.
(D) It is impossible to conclude about the speed of sound in helium from this observation.

Explanation / Answer

Pitch is a property that rates sounds on a frequency related scale.

For higher frequency sounds, pitch is higher.

So, for sound in Helium, frequency is higher than that in air.

Wavelength of sound depends on length and tension of vocal cords. Since they remain the same, wavelength remains the same in air and helium.

Velocity of sound = frequency * wavelength.

Since wavelength is the same, velocity is directly proportional to frequency.

So, velocity in Helium is greater than that in air.

(C) Speed of sound in helium is greater than the speed of sound in air.

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