Deep-sea divers breather a mixture of helium, nitrogen, and oxygen to prevent nitrogen narcosis. Under these conditions their speech is similar to "duck talk". Why? Compare the resonant frequencies in the (average) L=17 cm long human vocal tract m S. Note that m filled with air and helium. The velocities of sound in air and helium are the glottis closes the vocal tract at the lower end. Vair = 353- and Ve=970- S 1. 4002