Does subglottal resonance have a significant influence on register transition when singing falsetto? To find out, investigators at the University of Iowa decided on an innovative approach – involving helium. Or, more accurately, Helox (a.k.a. Heliox) a mixture of helium and oxygen [see safety note below].
Because helium is considerably less dense than normal air mixtures, the vocal resonant frequencies of those who breathe it tend to be higher – suggesting a possible application in falsetto research.
Reference: see this video of Lionel Ritchie singing ‘Hello’ (with helium).
Four volunteers, who had taken a substantial lungfull of the gas, attempted to sing a normal-to-falsetto transition, and their resulting vocal output was recorded for analysis. Which yielded clearcut results.
“… in this experiment subglottal resonance was not found to have a significant influence on register transition as originally hypothesized.”
The paper – An Investigation of a Modal-Falsetto Register Transition Hypothesis Using Helox Gas was published in the Journal of Voice, Volume 15, Issue 1, Pages 15-24 (March 2001).
[Safety note] Although helium – commonly used as ‘balloon-gas’ – is extremely inert and with very low toxicity, breathing it in large quantities from balloons can be dangerous, simply due to the exclusion of oxygen in the lungs. Hence the invention of Heliox.
BONUS video  Vin Diesel (with helium)
BONUS video  The Bee Gees (without helium)