Helium is much lighter than air, so sound waves move much faster through the gas. This amplifies the higher frequencies in your voice. The gas sulfur hexafluoride works in the opposite way.

Helium is much lighter than air, so sound waves move much faster through the gas. This amplifies the higher frequencies in your voice. The gas sulfur hexafluoride works in the opposite way.

Therefore the frequency of the sound in helium is almost 2.7 times higher than the sound in dry air. This causes the octave change in your voice. For a fuller explanation see the article at ...