Whenever we breathe in helium, why does it always happen that our voice becomes unnaturally high sounding?

Question

today we put our mouths to helium balloons and intook the air and after that i sounded so high pitched i couldnt believe it!

Answer 1

Sound is basically a vibration of the air. When you speak, you make the air vibrate with your vocal folds (or vocal cords).

Helium is much more lighter than the air we usually breathe (21% O2, 79% CO2).

Thus when you breathe helium and you speak just afterwards, the vibrations you will produce will be quicker, which results into a higher pitched sound.

Answer 2

First of all, while this is very entertaining, one risks suffering from a gas embolism with exposure to helium unless inhalation occurs just briefly.  

To answer your question, helium is lighter than air and does not cause the same vibration of our vocal cords that room air produces.  So the cords vibrate faster which causes a higher pitch, and people sound like little kids.

Answer 3

The speed of sound varies with the medium (solid, liquid or gases of different densities).  Speaking requires the vocal folds to vibrate and the column of 'air' to resonate within the air column above and below the vocal folds and including the throat and oral cavity.  Helium has a very low density relative to air and supports the higher harmonics of the vocal vibrations with nil support for the lower fundamental resonations which are lost.