I learned this today. Our voices change when we inhale helium because sound travels through it faster than it does through air.
There are two things we need to look at: Why sound travels at different speeds through different mediums and how our voices work.
Sound is a wave of vibrations that travel through a medium. In air, the vibrations travel at 343m/s. If I hit a nail with a hammer, the percussion shock makes the air molecules around the hammer vibrate. These vibrating molecules make the molecules next to them vibrate, and so on. This is the sound wave. It is a transfer of kinetic energy from one molecule to another. As the vibrations get farther away from the original source of the sound, they lose energy and vibrate less and less until they stop.
There is no sound in space because there are no molecules to vibrate and no way of passing the kinetic energy on. It stands to reason that the properties of the molecules will affect the speed with which the energy is transferred.
Sound travels at different speeds through different mediums. There are two things that affect the speed of sound. The first is the number of molecules there are in a certain space and the second is how much energy those molecules have. Sound travels faster through steel than it does through air because steel is denser and has far more molecules that are closer together. It is easier for one molecule to pass its kinetic energy to the next. The speed of sound in steel is 5,100 m/s. Sound travels more slowly in air than in steel because the molecules are much farther apart.
Sound travels faster through hot air than it does through cold air because the molecules in hot air have far more energy. More energy means they move a lot more, which increases the chance they will collide with other molecules, which makes it much easier for them to transfer energy. The speed of sound at 0℃ is 332 m/s and at 20℃ it is 343.2 m/s.
Helium molecules are much smaller and lighter than the oxygen and nitrogen molecules in air. This means that at the same temperature, helium molecules move a lot more than oxygen and nitrogen atoms do, which gives them a far higher likelihood of colliding and a far greater energy transfer speed. The speed of sound in helium at 20℃ is 1007 m/s, which is 2.9 times faster than in air.
How do our voices work? First, we push air up our throats from our lungs. This air travels through our larynx, through the voice box. The larynx serves a number of purposes, but one of them is speech. About halfway down the larynx are folds called ventricular folds. On the edges of these folds are the vocal cords. When we breathe out, the air makes the vocal cords vibrate, resulting in sound. The sound is amplified by our voice boxes, goes up our throats and out of our mouths. We can change the position of the folds, the vocal cords, and widen and narrow the gap between them, all of which makes them vibrate in different ways. We can also use the position of our tongue and the shape of our mouth and lips to change the sound.
High pitched sounds have short wavelengths and low-pitched sounds have longer wavelengths. Our voice boxes naturally amplify the wavelengths in the middle. Short wavelengths and long wavelengths are naturally quiet. That is why if you try to speak in a very deep voice or very high voice it is difficult to get volume.
The air we breathe out makes the vocal cords vibrate at a certain frequency and it has evolved to work with air, oxygen and nitrogen. When we inhale helium, it doesn’t change the frequency that our vocal cords vibrate at, it changes the pitch of the sound. Sound travels 2.9 times faster through helium than air, but the vibration of the vocal cords doesn’t change. This means that the sound waves stretch out. The short high-pitched sound waves become longer until they are the same length as the middle tones we usually make. The sound waves of the regular tones we make get much longer. Our voice box is designed to amplify the middle length wavelengths and it still does this. Now, though, the middle length wavelengths are the high-pitched tones. The normal wavelengths are now too long to be amplified, so your voice box amplifies the high tones and your voice sounds higher. The regular tone is still there, but it is too quiet to hear.
Sulfur hexafluoride has the exact opposite effect. Air travels more slowly through it, so the wavelengths are shortened. The wavelengths of the deeper tones are shortened and become the range that the voice box amplifies, making your voice much deeper.
So, helium makes our voices higher because sound travels through it faster than in air, which changes the wavelength of the sounds. Our voice boxes have evolved to amplify medium length sounds, which are now the high-pitched sounds. And this is what I learned today.
Photo by Natalie: https://www.pexels.com/photo/woman-holding-white-and-red-balloons-3371095/
Sources:
https://www.tutorialspoint.com/physics_part1/physics_speed_of_sound_in_different_media.htm
https://byjus.com/physics/speed-of-sound-propagation/
https://www.theguardian.com/notesandqueries/query/0,5753,-9439,00.html
https://www.sciencefocus.com/the-human-body/why-does-helium-change-your-voice/
https://wonderopolis.org/wonder/why-does-helium-change-the-sound-of-your-voice