In a popular and amusing science demonstration, a person inhales helium and then his voice becomes high and squeaky. Why does this happen? (Warning: Inhaling too much helium can cause

The person inhales helium and the voice becomes high and squeaky.

Consider the formula for the wavelength.

$\mathit{\lambda }\mathbf{=}\frac{{\mathbf{v}}_{\mathbf{s}}}{\mathbf{f}}$

Here, the frequency is f and the speed of the wave is $v_s$

Note that the helium is light in weight compared to the air. The light weight causes its speed to become more that of the air. In case the person inhales the helium, the speed of the sound will increase in the voice of the person. Though, the shape of the vocal tract of the person remains intact but the reason for the voice becoming squeaky is that with the increase in the speed of the sound the wavelength of the sound is also increased.

Therefore, the change in wavelength of the sound and the speed results in the high and squeaky voice of the person.