Editing Helium (section)

===Effects===
Neutral helium at standard conditions is non-toxic, plays no biological role and is found in trace amounts in human blood.

{{Listen|right|filename=Helium article read with helium.ogg|title=Effect of helium on a human voice|description=The effect of helium on a human voice|format=[[Ogg]]}}
The [[speed of sound]] in helium is nearly three times the speed of sound in air. Because the [[fundamental frequency|natural resonance frequency]] of a gas-filled cavity is proportional to the speed of sound in the gas, when helium is inhaled, a corresponding increase occurs in the [[resonant frequency|resonant frequencies]] of the [[vocal tract]], which is the amplifier of vocal sound.<ref name="nbb" /><ref>{{Cite journal |last1=Ackerman |first1=M. J. |last2=Maitland |first2=G. |title=Calculation of the relative speed of sound in a gas mixture |journal=Undersea Biomed Res |volume=2 |issue=4 |pages=305–10 |date=1975 |pmid=1226588 |url=http://archive.rubicon-foundation.org/2738 |access-date=2008-08-09 |archive-url=https://web.archive.org/web/20110127113335/http://archive.rubicon-foundation.org/2738 |archive-date=2011-01-27 |url-status=usurped }}</ref> This increase in the resonant frequency of the amplifier (the vocal tract) gives increased amplification to the high-frequency components of the sound wave produced by the direct vibration of the vocal folds, compared to the case when the voice box is filled with air. When a person speaks after inhaling helium gas, the muscles that control the voice box still move in the same way as when the voice box is filled with air; therefore the [[fundamental frequency]] (sometimes called [[Pitch (music)|pitch]]) produced by direct vibration of the vocal folds does not change.<ref>{{cite web|date=14 July 2000|title=Why does helium make your voice squeaky?|url=http://www.straightdope.com/columns/read/1803/why-does-helium-make-your-voice-squeaky|url-status=live|archive-url=https://web.archive.org/web/20130324072558/http://www.straightdope.com/columns/read/1803/why-does-helium-make-your-voice-squeaky|archive-date=24 March 2013|access-date=2013-06-08}}</ref> However, the high-frequency-preferred amplification causes a change in [[timbre]] of the amplified sound, resulting in a reedy, duck-like vocal quality. The opposite effect, lowering resonant frequencies, can be obtained by inhaling a dense gas such as [[sulfur hexafluoride]] or [[xenon]].