Effect of helium on a human voice
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The effect of helium on a human voice
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The human voice is not like a string instrument, in which the a primarily vibrating object completely sets the pitch of the sound. Rather, in a human, the vocal folds act as a source of polytonic vibration, much like the reed(s) in woodwind musical instruments. As in a woodwind, the size of the resonant cavity plays a large part in picking out and amplifying a given fundamental or overtone frequency of vibration, during soundmaking. The voice of a person who has inhaled helium temporarily changes in timbre in a way that makes it sound high-pitched, because higher overtones are being amplified. The speed of sound in helium is nearly three times the speed of sound in air; because the fundamental frequency of a gas-filled cavity is proportional to the speed of sound in the gas, when helium is inhaled there is a corresponding increase in the pitch of the resonant frequencies of the vocal tract.[3][92] (The opposite effect, lowering frequencies, can be obtained by inhaling a dense gas such as sulfur hexafluoride.)
Inhaling helium can be dangerous if done to excess, since helium is a simple asphyxiant and so displaces oxygen needed for normal respiration.[3][93] Breathing pure helium continuously causes death by asphyxiation within minutes. Inhaling helium directly from pressurized cylinders is extremely dangerous, as the high flow rate can result in barotrauma, fatally rupturing lung tissue.[93][94] However, death caused by helium is quite rare, with only two fatalities reported between 2000 and 2004 in the United States.[94]
At high pressures (more than about 20 atm or two MPa), a mixture of helium and oxygen (heliox) can lead to high pressure nervous syndrome, a sort of reverse-anesthetic effect; adding a small amount of nitrogen to the mixture can alleviate the problem
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