Our sense of hearing
What is sound, and why can we hear it?
Sound is a series of pressure waves. Sound waves are made by objects that are vibrating. The vibrating object pushes on the air around it. Those air molecules bump into more-distant air molecules, setting up a chain reaction that carries the wave outward in all directions. Our ears are filled with intricately moving parts that convert these mechanical waves into nerve signals that travel to the brain.
Three qualities of sound
Sound has three main qualities that our ears and brains can discern: volume, pitch, and timbre (TAM-ber). Each of these properties comes from a different characteristic of a sound wave. Sound waves travel through the air at a constant speed—about 343 meters per second—but the waves can have different shapes. The shape has to do with the characteristics of the vibrating object that makes it.
Volume is how loud or quiet a sound is. Volume is the height, or amplitude, of the waves: taller waves are louder, and shorter waves are quieter. Volume is measured in units called decibels.
Pitch, or frequency, is how high or low a sound is. Pitch, has to do with the distance between the waves (also called wavelength). Waves that are closer together have a high pitch, and waves that are farther apart have a low pitch. High-pitch sound waves are made when an object vibrates faster, or more frequently. Low-pitch sound waves are made when an object vibrates more slowly, or less frequently. Pitch is measured in units called Hertz, which are the number of waves per second that pass a point. Though there's a lot of individual variation, humans can hear sounds that range from about 12 to 28,000 Hertz.
Timbre is what we often perceive as the texture or character of a sound. It's how we can tell the difference between two musical instruments, say a violin and a piano, playing the exact same note at the exact same volume. Things like instruments and voices may have one dominant pitch or note, but they actually vibrate at multiple frequencies at the same time, producing layers of waves with more complex shapes. You can think of the overall shape as the timbre of a sound.
How the ear detects sounds
Our ears convert vibrations in the air to waves in liquid, and then into nerve signals that travel to the brain. Many parts work together inside the ear to make this possible.
Hair cells: the cells of hearing
Hair cells are the sensory cells that allow us to hear. They are a type of mechanoreceptor — cells that sense movement. Hair cells get their name from a bundle of hair-like stereocilia (stehr-ee-oh-SIL-ee-yuh) that sticks up from the top of the cell. Tiny fibers called tip links connect the top of each 'hair' to its neighbor. The tip links are connected to stretch-sensitive ion channels. You can think of the channels as tiny trap doors that when open let ions (charged molecules dissolved in the surrounding fluid) flow into the cell. As sound waves make the basilar membrane bend and wave, the stereocilia bundle gets pushed back and forth. This movement opens and closes the trap doors, causing the hair cells to release a neurotransmitter (a signaling molecule) onto nearby nerve cells.
Hair cell damage causes deafness
Very loud sounds cause waves in the inner ear that are so violent, they can damage hair cells' stereocilia. Severe damage will even kill hair cells. Once we lose a hair cell, it's gone forever - we can't grow them back. There are just a few hair cells at each position along the length of the cochlea. When these are damaged, we are no longer able to hear sound of a certain frequency or pitch. The hair cells that detect high-frequency sound are the must susceptible to damage. As we get older, it's normal to gradually lose hearing in the high range.