The gas that makes your voice deeper is sulfur hexafluoride (SF6). This gas is about six times heavier than air, and because it is denser, sound waves travel through it more slowly. When you inhale sulfur hexafluoride and speak, your vocal cords vibrate at the same rate, but the sound waves move slower through the dense gas, making your voice sound much deeper

. In contrast, helium is a very light gas that makes your voice sound higher because sound travels faster through it. Sulfur hexafluoride has the opposite effect due to its high density, which lowers the frequency of the sound waves traveling through it, resulting in a deeper voice

. Other dense gases like perfluorobutane can also deepen your voice similarly, but sulfur hexafluoride is the most commonly known gas for this effect

. Summary:

• Gas that deepens voice: Sulfur hexafluoride (SF6)
• Reason: Denser than air, slows down sound waves, lowering pitch
• Opposite effect to helium, which raises pitch by speeding up sound waves through lighter gas