Why does helium change your voice?
It is found that sound travels faster in helium gas than in the air. The sound waves are known to attain shorter wavelengths and a higher frequency when they pass from helium to air. This makes the sound higher. The other reason for hearing a change in your voice when Hhelium passes through is that the vocal folds vibrate with a higher speed in helium than in air, as helium is lighter than air. Also, helium does not restrict the movement of the folds much, while air causes more restriction.
It is explained that breathing in helium will not change the pitch or frequency of your voice. The change in the resonance of the voice tract will make the voice passing through helium change. The resonance vibrations are more, and they affect the timbre of the voice. So it is noticed that the timbre of the voice will be affected in helium, but not its pitch.
The above reason is elaborated like this. When the air passes through the vocal cord, the vocal folds vibrate. The vibrations of the vocal folds produce waves in the air, passed at a particular frequency. They not only generate waves at one frequency, but also at higher multiples of that frequency. A vocal tract will have its characteristic shape and size. This vocal tract can generate various frequencies of resonance at this size, shape and the speed of the waves. The frequencies that are nearer to the resonant vibrations are amplified, and the others are suppressed during the journey of the waves of sound through the tract. These vibrations of resonance can be changed by moving the tongue and lips. In this way you can make the different sounds of the letters.
As the speed of the sound waves in helium creates resonant frequencies, more wave frequencies are nearer to those resonant vibrations that are amplified. These amplified waves are higher in helium than in the air. Therefore, there is change in the timbre of the voice.