The sound-production anatomy of a person includes various resonating cavities, such as the throat.
The relatively fixed geometry of these cavities will determine the relatively fixed wavelengths of
sound that a person can produce. Those wavelengths will have associated frequencies given by
f v . The speed of sound is determined by the gas that is filling the resonant cavities. If the
person has inhaled helium, then the speed of sound will be much higher than normal, since the speed
of sound waves in helium is about 3 times that in air. Thus the person’s frequencies will go up about
a factor of 3. This is about a 1.5 octave shift, and so the person sounds very high pitched.