Wikipedia...
As with all gases, the density of SF6 affects the resonance frequencies of the vocal tract, thus changing drastically the vocal sound qualities, or timbre, of those who inhale it. On the other hand, it does not affect the vibrations of the vocal folds. The density of sulfur hexafluoride is relatively high at room temperature and pressure due to the gas's large molar mass. Unlike helium, which has a molar mass of about 4 grams/mol and gives the voice a childish and "Donald Duck" quality, SF6 has a molar mass of about 146 g/mol, and the velocity of sound through the gas is 0.44 times the speed of sound in air due to the large inertia of a SF6 molecule. For comparison, the molar mass of air, which is about 80% nitrogen and 20% oxygen, is approximately 30 g/mol. Inhalation of SF6 causes a lowering of the timbre, or frequency of the formants, of the vocal tract, by contrast with inhalation of helium, which raises it.
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And, here is a helium demonstration...
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