Geoff Blackwell, Queensland asked:
Is there a gas that can do the opposite of helium to the voice?
Yes indeed. Helium does strange things to your voice because it is much less dense than air. In your throat it is acting a bit like a musical instrument. You get sound waves vibrating backwards and forwards, up and down above your vocal cords. That gives rich timbre to your voice. It picks which frequencies of your voice to amplify. Helium is a much lower density gas than air and that means that sound travels much faster in it. Then your throat will vibrate at much higher frequencies. It will amplify the sounds at much higher frequencies.
Geoff Blackwell asked the Naked Scientists:
I farted once and a deep gruff voice boomed out, "Was that you?"
Xenon would probably be the best one to use but it's rather rare exoensive and difficult to get but it is being used in light bulbs and flash tubes so it must be available Soul Surfer, Sat, 21st Jun 2008
Yes, have a look at this http://www.youtube.com/watch?v=C40nWXv2Ksg Madidus_Scientia, Sun, 22nd Jun 2008
There is methane and some other heavy gases but you would never know the answer because they would kill you Alan McDougall, Wed, 25th Jun 2008
methane (RMM = 16) is lighter than air and hence would not lower voice timbre. It would anaesthetise you though.
Carbon monoxide is practically as dense as air and very toxic- a breath of that might kill you.
Xenon is an anaesthetic. The chemistry is poorly defined. Methane still isn't anaesthetic. Bored chemist, Mon, 28th Jul 2008
Methane can behave as an anaesthetic:
OK, look at what we both posted.
Nitrous Oxide N2O, when delivered to medical/dental patients causes a very distinct drop in the frequency of the voice. I always assumed it was because the vocal cords are vibrating in a 'heavier' gaseous medium. Snapoli, Thu, 5th Apr 2012
What about just CO2, it is denser than air, and cheap and easy to get.
The youtube link now leads to: "This video is no longer available due to a copyright claim by NBC Universal."
Even dilute CO2 is a bit irritant to the lungs.
You could always try oxygen in a hyperbaric chamber. Geezer, Fri, 6th Apr 2012
How about trying it with a liquid? Many years ago a mouse was shown on Tomorrow's World swiming about in a container filled with a liquid which carried oxygen much more efficiently than water - the mouse could breathe by inhaling this liquid instead of air. Now, if you put a person in such a liquid, they could breathe it in safely (I hope, unless the mouse was slowly being poisoned) and they could try speaking too. I wonder if that mouse squeaked deeply while it swam... David Cooper, Fri, 6th Apr 2012
Sulfer Hexaflouride. Savo, Fri, 10th Aug 2012