The voice of a person, who has inhaled helium, has a remarkably high pitch. Explain on the basis of resonant vibration of vocal cord filled with air and with helium.

A frequency generator with fixed frequency of 343 Hz is allowed to vibrate above a 1.0 m high tube . A pump is switched on to fill the water slowly in the tube . In order to get resonance , what must be the minimum height of the water ? (Speed of sound in air is 343 ms^(-1) ) .