Jonathan Gill, of Kings High School, asks :-
When sucking helium into your mouth, why does your voice go squeaky when talking?
Marlyn Jakub, a physicist at the University of Otago, responded.
To answer this question we must look at the way humans generate sounds. Human voice production can be simply described as a source of slightly pressurised air (from the lungs) that causes air to flow over vibrating membranes (the vocal cords) and these vibrations excite the air in a 17-centimetre-long resonance chamber (the trachea plus the mouth and nasal chambers). By inhaling some helium gas, some of the air in this resonance chamber is replaced by helium, which increases the propagation speed for sounds traveling inside the chamber. Ultimately, this process raises the frequency of resonance sounds emanating from the mouth, as described below.
Because helium is such a light gas it has a sound speed three times faster than sound in air, so if half the air in the chamber is helium, the sound speed will be increased by roughly a factor of 1.5 times the speed in air. The resonance frequencies of the chamber will correspondingly be increased by the same 1.5 factor, since the size of the resonance chamber is unchanged and a resonance frequency is proportional to the sound speed. So it should not be surprising that the higher resonance frequencies make the human voice sound `squeaky', or, sound like the cartoon character, Donald Duck. A listener usually finds that such higher-pitched 'helium speech' is much harder to understand than 'air speech'.
There is an important application involving helium-affected speech sounds, as the deep-sea divers regularly use 80 (or even higher) percent helium gas in the gas mixture that they breathe during long times in the water. It is important for the divers to be able to converse with other divers and with people on the surface ships. Some efforts have focused on prior speech training for the divers. In some applications, electro-mechanical devices have been constructed so that the diver's communication signal is altered to one that can be more easily understood by listeners.