What happens to your voice when you inhale a gas like Helium or Sulfur Hexafluoride? Experimental setup: Cell phone tone generator suspended in 6” diameter acrylic tube that can be filled with SF6 from bottom. Microphone hanging at top of tube and output monitored by spectrum program. Working with 440Hz tone, I measured the spectrum in different cases: tone source and mic outside tube (on a table), tone source and mic in tube filled with air, and tone source and mic in tube filled with SF6. Here are results: Here the tone source is on a table next to the microphone. There is a primary 440Hz peak and some harmonics. Note there is lower frequency room noise as well. Here the tone source is in the tube with the microphone. Tube is filled with air. Note same primary 440Hz peak but there are several lower frequency bands as well. Here the tone source is in the tube with microphone. Tube is filled with SF6. Primary 440Hz peak is still there but now there are several stronger peaks relative to the primary peak. Note: autoscaling has happened on plot so baseline noise only looks larger. So, if fundamental remains the same what is the decreasing pitch we hear when a tube is being filled with SF6 (i.e. sound made tapping tube filled with air vs filled with SF6 has measurably different frequencies). Consider the process of filling the tube with SF6 while playing a tone at 440Hz as above. Start is at bottom of graph. Gas is turned on to fill tube then gas turned off. Note 440Hz tone remains the same frequency throughout but rushing gas leads to audible sound that decreases in frequency as tube fills. For reference, we can compare this to a natural resonance of the air filled tube (found by sweeping through various frequencies): ~140Hz. Note this pitch is one of the lines where the filling tube starts. So, in summary: The drive frequency 440Hz is not affected by gas but frequencies of natural resonances of the tube do change as expected based on decrease in sound speed and fixed geometry of tube. Changing the gas acts by changing gain as a function of frequency: any harmonic that now falls near the tube’s natural resonances are enhanced and those that move away from resonance are diminished. For our voice then, the vocal folds continue to vibrate at the same rate (or changes are small). These vibrations are not described by a single frequency. The change in voice quality we hear is the change in the relative ratios of the harmonics. For SF6, a tube has lower frequency resonances than when filled with air. Hence, the lower harmonics of the vibrations of the vocal folds are enhanced (relatively louder): voice sounds lower. Can we see this on power spectrum: yes Here I sing “eeee” , first air in lungs, then SF6 then back to air. The “eeee” is apparent in the higher harmonics; these bands disappear if I change to singing “ooooo”. Note, primary frequency remains the same throughout. Here is the same measurement using Helium (upper bands).