How the Sense of Hearing Works

The human ear is far more than an odd-shaped appendage that we hang our eyeglasses and earrings on. It's a complex system that changes sound waves into mechanical energy, then into fluid energy, and finally into electrical energy.

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The human ear is divided into three parts, which are known as the outer ear, the middle ear, and the inner ear. The outer ear is comprised of the auricle, which is the visible part, and the ear canal. Dividing the outer and middle ears is the tympanum, or tympanic membrane or eardrum. This is a delicate, tightly-stretched, pearly-gray membrane that vibrates when sound waves are delivered to it via the ear canal.

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The middle ear is the second section of the ear. It is a cavity that houses a chain of three tiny bones. These bones are called the malleus or "hammer," the incus or "anvil" and the stapes or "stirrup." These bones are named for objects that they resemble. The foot of the malleus rests on the eardrum, and it picks up the sound vibrations from the movements of the eardrum. The malleus is connected to the incus, which in turn is connected to the stapes. The foot of the stapes rests on a structure called the oval window. The stapes delivers sound to this window by moving in and out like a piston.

The third section of the human ear is the inner ear. It has parts designed for hearing and parts designed for balance. The cochlea, which is the part dedicated to hearing, is a structure that looks something like a snail's shell. The oval window mentioned above is situated in a wall of the cochlea. The cochlea is fluid-filled and contains a membrane that is attached to the hair-like nerve endings of the "hearing" nerve, or eighth cranial nerve.

But how does all this work? How do we hear, really?

The auricle and ear canal funnel sound waves toward the eardrum. The eardrum vibrates in response to the sounds, setting in motion the bones of the middle ear. As the foot of the third bone moves in and out of the oval window, the vibrations make waves in the fluid of the cochlea - something like dropping a stone in a still pond. The membrane inside the cochlea moves in response to these vibrations, stimulating the hair-like nerve endings. The nerve endings send the impulses to the brain for interpretation.

As you can see, the chain of systems for delivering sound information to the brain becomes more and more complex as it moves inward toward the brain.

There are many conditions that can reduce hearing. Some of these conditions prevent or disrupt sound waves from reaching the cochlea, causing what is called conductive hearing loss. Conditions that prevent or disrupt sound transmission in the inner ear and beyond it cause what are called neural or sensorineural hearing losses.

In future articles, learn how hearing aids and cochlear implants work and what results might be possible with these devices.

If you or someone you know has had a hearing loss, there are many resources available. For assistive equipment, technology devices, and deaf products, please click the link in my resource box.

How the Sense of Hearing Works

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