… and is it the same in other animals as it is in humans? Is it comparable to any other instruments, or class of instruments?
Edit to add one more question: How does the voice create sound? My best guess is that the lungs breathe out air that then bounces around in our throats at different speeds and vibrates the walls of our throat/mouth (which creates a sound). Am I correct?
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Luckily for you I *just* took a class on the anatomy and physiology of speech. Lemme try to condense that term into an ELI5 comment. (Edited to add a line break here)
First you need air, which takes lots of different muscles to get in and out of your lungs, but most importantly the diaphragm, which pulls down a tendon to increase the volume of your lungs, making air rush in due to the pressure difference. As the diaphragm releases, and as other muscles contract, air flows back out.
The sound of the voice itself is created in the larynx, a structure that sits on top of your trachea (airway). The process of creating sound is called phonation. You have vocal folds (colloquially often called vocal cords) in your larynx, which are pieces of tissue that can be closed and opened by muscles. When the muscles adduct (bring together) your vocal folds and you start to breathe out, the pressure from your lungs blows your vocal folds apart. However, fast-moving air is lower-pressure than slow-moving air (incidentally, this is what lets planes fly). The air rushing through your vocal folds is fast, and so it is low pressure, which brings your vocal folds slamming back together. You can adjust the tension of your vocal folds to change how quickly they are blown apart and slammed together, but it is very fast. Just as a plucked string producing the note A4 is vibrating back and forth 440 times per second, a set of vocal folds that is singing middle A is slamming together 440 times per second.
After the sound is produced, it is shaped in the rest of your vocal tract by various articulators, such as the tongue, soft palate, and teeth, into recognizable speech sounds. For instance, the consonant /m/ is produced by closing your lips to prevent air escaping from your mouth, but lowering your soft palate so that the vibrating air can pass into your nasal cavity and out through your nose. The consonant /v/ is produced by taking this vibrating air and constricting it between your lips and teeth.
Also note that phonation gets rapidly turned on and off when you speak; speech is one of the most complex, difficult-to-coordinate behaviors humans do. Some sounds are produced without phonation. Take the sounds /s/ vs. /z/. The mouth is in identical places for both of these; the only difference is in whether or not your vocal folds are producing sound.
Many animals have a larynx and can produce sound this way. Many others don’t; birds have a completely different system in a different part of their trachea called a syrinx.
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