The two major factors involved here are how our throats produce sound using air and what happens when we replace that air with the helium. Before we study what happens, let us understand their functions individually.
The lungs give up the air that travels through the voice box called larynx. When this air hits the vocal cords on the underside of the vocal folds, they vibrate. The result is that the air molecules in your vocal tract are simulated and create resonant frequencies. The difference of these resonant frequencies will allow you to form speech through movements of your tongue and lips. The oscillations of pressure departing your mouth in the form of waves will be received as your voice.
It is important to note how the medium is also a factor here to know that how your voice will be perceived.
The properties of your speech like pitch and timbre, the frequency of a sound and the quality that differentiates among types of sound can vary. The pitch varies with the vibration of vocal folds and the timbre varies with variations in the vibration of air in the vocal tract.
The partner in crime is Helium that will be the next to be put under the microscope. Helium is the second most abundant element in the universe, first being hydrogen. It has an atomic number 2 and an atomic weight of 4.002602. Of all the elements, Helium has the lowest melting and boiling points at -458.0°F and -452.1°F respectively.
So what happens when the above two meet? Helium has a mass seven times lighter than the mass of Nitrogen. The medium that our sound waves travel through is made up of 78.08 % nitrogen. Hence, the sound waves which travel at 344 meters per second through the air will travel at 927 meters a second through Helium. The inhalation of Helium changes the air in our vocal tract and consquently, the speed of sound waves of our voice increases resulting in a squeaky and funny tone.
The effect can be inverted by using denser gasses such as sulfur hexafluoride and xenon to slow the speed of sound, creating resonant frequencies of the lower spectrum and making your voice deeper and heavier. No need to catch a cold like Pheobe to sound sultry. Likewise, there is a reason whyMorgan Freeman’s voice is used in documentaries, and we are far from complaining, but here is what he sounds like on helium and it’s awesome.