How we hear. (Extracts: Harvard Health Publication - www.health.harvard.edu)

Because hearing seems automatic, we take it for granted until it is impaired. But the process of hearing is truly awe-inspiring. The ear is a precision instrument with an astonishingly intricate mechanism, and the journey of sound through the ear is the stuff of adventure, involving navigation through air and water, and even, metaphorically speaking, moving boulders.

The journey of sound.

The tale begins when a person speaks, a musician strikes a chord, or some other noise occurs. The sound, in the form of sound waves, travels through the air, then follows the swirling channel of the outer ear into the ear canal. The ear canal is a dark, slippery passageway, a mere inch long. It’s slippery because it’s lined with earwax, a material secreted by glands in the ear canal that helps protect the ear by keeping out unwanted substances like bacteria and dirt. The ear canal acts like an amplifier, boosting the sound’s volume as it funnels sound to the eardrum—a slender skin-like structure about a half-inch across and the width of a strand of hair. Although small, the ear- drum, also known as the tympanic membrane, forms a tight barrier that separates the outer ear from the middle ear.

The outer, middle, and inner ear.

The outer ear consists of the parts you can see: the fleshy outer part (called the auricle), the ear canal, and the eardrum.

The middle ear is an air-filled cavity containing the ossicles, three small bones (malleus, incus, and stapes) that transmit vibrations to the inner ear.

The inner ear is a complex system of membranous canals protected by a bony casing. Inside, the spiral- shaped cochlea contains the hair cells that transmit sound to the auditory nerve, which conducts sound to the brain.

The vestibular system, which regulates balance, is also part of the inner ear and includes the three semi- circular canals known as the labyrinth.

Sounding the drum.

Like a drummer beating a drum, sound waves strike the eardrum and make it vibrate. The frequency of the vibrations determines the pitch of the sound. For example, a sound wave that vibrates at 256 cycles per second, like the middle C on the piano, is said to have a frequency of 256 hertz, the unit in which frequencies are measured. The higher the frequency of the sound wave, the higher the pitch.

As the eardrum vibrates, it transfers sound waves to the ossicles, three bones that form a bridge across the middle ear, which is actually an air-filled chamber about the size of a peanut. These three bones have Latin names that describe their shapes: the malleus (hammer), incus (anvil), and stapes (stirrup). The tiny sound waves must move these bony structures, causing them to vibrate—the part of the adventure akin to moving boulders. The purpose of the eardrum and the ossicles is to adjust the loudness of incoming sound so that you can hear it comfortably. how we hear

Natural volume control.

In the middle ear, the volume of the sound can be either increased or decreased. In most cases, the ossicles vibrate rapidly to boost the volume. They are aligned in such a way that this happens automatically. If the noise is too loud, however, two middle-ear muscles attached to the ossicles lower the volume by contracting. As these muscles contract, they pull on the three bones, reducing their ability to vibrate and pre- venting extremely loud noises from hurting your ears.

But this is not the end of the journey. The vibrating ossicles transfer the sound-wave vibrations to the “oval window” that separates the middle ear from the inner ear. This window consists of the stapes footplate and a fibrous membrane that holds the footplate in place and seals the chamber.

The inner chamber.

It’s the inner ear that houses the cochlea, a snail- shaped structure consisting of bone on the outside and fluid-filled membranes on the inside. As sound waves undulate through the liquid passageways, they send a ripple across rows of sensory cells, called hair cells, lining the cochlea. There are 10,000 to 15,000 hair cells in each ear, and sound waves of different frequencies stimulate the hair cells in different sections of the cochlea. As the hairlike prongs on the hair cells bend, a chemical signal triggers the auditory nerve to send a signal to the brain, encoding a particular sound’s frequency and loudness.

Although sound waves must pass through the different structures of the ear, the entire process—from the moment a sound is made to the moment it reaches the brain—is practically instantaneous.

How the brain hears.

Your brain does more than tell you what sound you just heard—which musical note was played, which word was spoken, and so on. It also sorts out the incoming sounds by their relative importance. The purpose is to tune out unimportant sounds, such as the flush of a toilet, the hum of the refrigerator, or the din in a restaurant, so that the sounds you really want to hear, like human voices, come through clearly. It’s not that you don’t hear the unimportant sounds; rather, your brain makes sure you don’t notice them as much as the more significant sounds.

What is sound?

Sound is a vibration of molecules that moves in the form of waves. Sound waves travel quickly, at about 770 miles per hour in air. There are two measurable qualities that influence how we perceive sound. One is frequency (pitch). The other is intensity (loudness).

A sound’s pitch is determined by its frequency, or the number of cycles a sound wave makes in one second. The number of cycles is measured in hertz (abbreviated Hz). The higher the frequency of a sound, the higher the pitch. High-pitched sounds, such as a siren, have frequencies of thousands of hertz. Low-pitched sounds, such as thunder, have frequencies of only a few dozen hertz. People with normal hearing can hear frequencies as low as 20 and as high as 20,000 Hz. However, humans are most sensitive to sounds in the frequency range characteristic of human speech, 500 to 8,000 Hz.

A sound’s intensity is measured in decibels (dB). Decibels are not uniform units of measurement, like feet or yards, but rather a logarithmic progression. Therefore, an increase of 10 dB does not indicate the addition of 10 units, but rather a multiplication to 10 times the original level. The softest sound that an adult with normal hearing can hear is 0 dB, and the loudest sound, the sound of a rocket taking off, is more than 180 dB.

A third quality of sound, which is not measured by any specific unit, is timbre, or tone. It is timbre that distinguishes between different types of sounds, such as voices and musical instruments, even when they have the same frequency and intensity. For example, musical instruments produce more than just the “dominant frequency” that determines the pitch you hear; they also produce overtones, or secondary sound waves at different frequencies that give each instrument its distinct tone. In addition, instruments vary in the attack, sustain, and decay of the

With age, however, your brain becomes less skilled at helping you ignore unwanted background noises. This is not hearing loss per se, but a cognitive decline. Your brain simply doesn’t process as efficiently as it once did, so its ability to diminish unwanted sound lessens. Here’s one way to understand the difference between the brain of a teenager and that of a 60-year- old: the teenager can do his homework while the TV is on and his sister is talking loudly on the phone in the next room. An older person might have trouble concentrating on a television show if another person in the room is talking at a normal volume.

A balancing act.

Hearing isn’t the only thing the ear does. The inner ear also contains the body’s balance mechanism, which is why problems with balance and hearing often accompany one another. The balance mechanism, called the vestibular system, is housed in an inner ear structure called the labyrinth. The labyrinth is made up of three semicircular tubes arranged in a cloverleaf shape. The tubes of the labyrinth are similar to the cochlea in that they consist of bone-encased membranes filled with fluid and lined with hair cells. As you move, the fluid in the canals shifts and bends the hair cells, stimulating them to fire a message to the brain telling it how much you moved and in which direction. If you spin around very fast, the fluid can’t move fast enough to tell your brain your exact position, so you feel dizzy. Dizziness can also be a signal that something is wrong in the inner ear, like an infection or an injury. 

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