Hear Ye! Hear Ye! Read All About It!
by Fiona Bayrock
Sound is air molecules bumping together. Simple, huh? Every sound we hear—every piece of music, every noise—results from the patterns formed by these moving air molecules. Inside and out, the ear is designed to capture and process those patterns into electrical signals the brain can use to identify and interpret what we hear.
All sounds begin with something vibrating back and forth. Let's say you strike a cymbal. Every back-and-forth motion the cymbal makes as it vibrates pushes nearby air molecules, setting off a chain reaction: They bump into the molecules next to them, which bump into the next ones, which bump into the next ones, and so on. If you could see the air around the cymbal, you would see the same pattern repeating over and over again: circles of air molecules squished together, alternating with larger circles of air molecules spread out—in other words, waves. These sound waves travel away from the cymbal in the same way that motion travels through a Slinky.
Your pinnae, the outside ear flaps on either side of your head, are specially shaped to “catch” sound waves and funnel them into the ear canal, where they travel to the tympanic membrane (eardrum). The eardrum is a thin piece of skin about the size of your smallest fingernail that covers the opening to the middle ear. Sound waves hit the eardrum, which vibrates easily when nearby molecules give it a push. It doesn't take much to get the eardrum vibrating. For a very quiet sound, it can move as little as the width of a hydrogen atom.
The middle ear is an air-filled space containing the smallest bones in the body: the hammer, anvil, and stirrup (guess what they look like!). These delicate bones are arranged as a series of levers. Each bone pushes or pulls the next to pass the vibrations mechanically from the eardrum to the much smaller membrane at the entrance of the cochlea (inner ear). This membrane acts like a piston, setting the fluid of the cochlea moving in waves. The cochlea, about the size of a pea, is made up of three tubes about 4 cm long, stacked on top of one another and coiled to resemble the shape of a snail (cochlea is Latin for “snail”). A membrane embedded with tiny hair cells runs through the middle of the coil, like jelly in a jelly roll. Sound waves rippling through the fluid-filled chambers shake this membrane, causing the hair cells to bend and release electrical signals. These signals travel to the brain as nerve impulses, and there the sound is analyzed and identified.
Whew! Hearing is a delicate, complex process, isn't it? Your ears are amazing!
- What does an eardrum do when a sound wave hits it?
[anno: An eardrum vibrates when a sound wave hits it.]
- Why might a very loud sound be dangerous to the eardrum? What might happen to the eardrum?
[anno: A very loud sound might cause the eardrum to vibrate to the point at which it tears or ruptures.]
- Imagine that two people are standing side-by-side. Why might one person be able to hear a sound that the other person cannot hear?
[anno: Answers will vary but could include that one person might have a damaged ear and cannot hear as well as the other person.]
- Hold your hand up to your ear. Rub your thumb and your forefinger together. Now hold extend your arm. Rub your thumb and your forefinger together. How are the sounds different? How do the sounds affect your eardrum differently? Why? Write a few sentences to explain your answer.
[anno: Answers will vary but could include that the sound is much louder when the hand is held next to the ear. The sound is softer when the hand is held away from the ear. When the hand is held next to the ear, the eardrum vibrates more because the sound waves reaching the ear when the hand is held close to the ear are larger than the sound waves that reach the ear when the hand is held farther away.]