Explainer: How the ears work
Ears can be floppy and leathery like an elephant’s, pointed and fluffy like a cat’s, or flat, round disks like a frog’s. But no matter their shape or size, vertebrates use their ears to magnify incoming waves of sound and transform them into signals the brain can interpret. The result allows us to hear the elephant’s trumpet, the cat’s purr and the frog’s croak. Also, of course, our favorite songs.
Sound travels through the air in waves that compress, stretch and then repeat. The compression exerts a push on objects, such as ear tissue. As a wave stretches back out, it pulls on the tissue. These aspects of the wave cause whatever a sound hits to vibrate.
Sound waves first hit the outer ear. That’s a part often visible on the head. It’s also known as the pinna or auricle. The outer ear’s shape helps to collect sound and direct it inside the head toward the middle and inner ears. Along the way, the shape of the ear helps to amplify the sound — or increase its volume — and determine where it’s coming from.
From the outer ear, sound waves travel through a tube called the ear canal. In people, this tiny tube is about 2.5 centimeters (1 inch) long. Not every animal has an outer ear and ear canal. Many frogs, for example, just have a flat spot behind their eyes. This is their ear drum.
In animals with an outer ear and ear canal, the ear drum — or tympanum — is inside the head. This tight membrane stretches across the end of the ear canal. As sound waves slam into this ear drum, they vibrate its membrane. This triggers pressure waves that swell into the middle ear.
Inside the middle ear is a small cavity with three tiny bones. Those bones are the malleus (which means “hammer” in Latin), the incus (which means “anvil” in Latin) and the stapes (which means “stirrup” in Latin). In people, these three bones are known as ossicles. They are the smallest bones in the body. The stapes (STAY-pees), for instance, is only 3 millimeters (0.1 inch) long! These three bones work together to receive sound waves and transmit them on to the inner ear.
Not all animals, however, have those ossicles. Snakes, for instance, lack both the outer ear and the middle ear. In them, the jaw transmits sound vibrations directly to the inner ear.
Inside this inner ear is a fluid-filled, snail-shaped structure. It’s called the cochlea (KOAK-lee-uh). Inside it stand ranks of microscopic “hair” cells. They contain bundles of tiny, hair-like strands embedded in a gel-like membrane. When sound vibrations enter the cochlea, they make the membrane — and its hair cells — sway to and fro. Their movements send messages to the brain that register the sound as any of many distinct pitches.
Hair cells are fragile. When one dies, it's gone forever. So over time, as these disappear, people begin to lose the ability to detect certain sounds. Hair cells that respond to high-pitched sounds tend to die off first. For example, a teen may be able to hear a sound with a very high frequency of 17,400 hertz, while someone with older ears may not. Want proof? You can test it yourself below.
amplify To increase in number, volume or other measure of responsiveness.
auricle The visible part of an ear. In humans, it’s shaped like a funnel, but in other animals may be pointed, rounded or a large flap. It’s also called the pinna.
cavity (in biology) An open region pocketlike structure surrounded by tissues.
cell The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Most organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.
cochlea (plural cochleae) A spiral-shaped structure in the inner ear of humans and other mammals. The natural battery in the mammalian inner ear provides power to drive signals from the ear to the brain. Those signals travel along the auditory nerve.
compression Pressing on one or more sides of something in order to reduce its volume.
frequency The number of times a specified periodic phenomenon occurs within a specified time interval. (In physics) The number of wavelengths that occurs over a particular interval of time.
gel A gooey or viscous material that can flow like a thick liquid.
hair cells These are the sensory receptors inside the ears of vertebrates that allow them to hear. These actually resemble stubby hairs.
hertz The frequency with which something (such as a wavelength) occurs, measured in the number of times the cycle repeats during each second of time.
incus One of the bones of the middle ear. The word means “anvil” in Latin. It transfers sound vibrations from another bone, called the malleus, to a third, known as the stapes.
malleus One of the bones of the middle ear. The word means “hammer” in Latin. It transmits the vibrations of the eardrum to another bone called the incus.
membrane A barrier which blocks the passage (or flow through) of some materials depending on their size or other features. Membranes are an integral part of filtration systems. Many serve that same function as the outer covering of cells or organs of a body.
microscopic An adjective for things too small to be seen by the unaided eye.
ossicles The three tiny bones of the middle ear, including the malleus, the incus and the stapes. These bones amplify and transmit sound, and are the tiniest bones in the human body.
pinna The visible part of the ear. In humans, it’s shaped like a funnel. In other animals it may be pointed, rounded or a large flap. It’s also called the auricle.
pressure Force applied uniformly over a surface, measured as force per unit of area.
sound wave A wave that transmits sound. Sound waves have alternating swaths of high and low pressure.
stapes One of the bones of the middle ear. The word means “stirrup” in Latin. It transfers sound vibrations from another bone, called the incus, to the inner ear.
tissue Made of cells, any of the distinct types of materials that make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms. Different organs of the human body, for instance, often are made from many different types of tissues.
transmit (n. transmission) To send or pass along.
tympanum Also called the ear drum, it’s a membrane that vibrates in response to sound. In mammals and people with visible, outer ears, the tympanum is located out of sight. In species such as frogs, however, the tympanum is sometimes visible as a round spot behind the animal’s eye.
vibrate To rhythmically shake or to move continuously and rapidly back and forth.
wave A disturbance or variation that travels through space and matter in a regular, oscillating fashion.