baby ear pointed by finger

How Does the Ear Hear Sound?

In this article, I will be teaching you how the ear works, how the ear is able to hear sounds, and how the ear transmits audio information to the brain.

Let’s first outline the series of steps that it takes for a sound wave to become audio information that the brain receives and interprets. First sound waves enter the ear, vibrating the eardrum, then vibrating 3 bones (auditory ossicles) touching the eardrum, which then pass the vibrations into the fluid-filled cochlea to again pass those mechanical movement to tiny hair cells inside the cochlea, and finally those hair cells send signals to the brain corresponding to their movements.

The Mechanism of Hearing

The Production of Sound Waves

First, the sound wave is produced by changes in air pressure. Examples of this include when a metal bar hits the concrete ground, when the birds chirps, when a person speaks his/her vocal cords, lungs, teeth, lips, and breath work together in a complex symphony to produce the speech. Another example are glass harps, which really can give you an idea of how air vibrations translate into sounds:

Für Elise on glass harp

If you think about it, our ears are just sensors for detecting and interpreting vibrations in the air. Our skin can serve a similar purpose because, for example, if the ground vibrates we are to “feel it”. Whereas if the air vibrates we are able to “hear it”. How we experience our senses is truly what the brain makes of it.

Even semantic meaning, a.k.a. language, that can be heard through sounds can also be felt through touch. Grail is a good example that shows this.

structures inside middle ear
Blausen.com staff (2014). “Medical gallery of Blausen Medical 2014”. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. (Own work) [CC BY 3.0], via Wikimedia Commons
Anyways, let me try to stay on topic. So the next thing that happens after the sound wave is produced is that it travels through the air. So just as a stone dropped into a pond produces ripples, the sound from its initial source travels through the air as a wave. Finally, the sound wave reaches the ear.

The Vibration of Eardrums

So once the sound wave reaches the outer ear, it then enters the ear through the ear canal and then finally the sound wave touches the ear drum. This causes the ear drum to vibrate. The vibrations of the ear drum is then transferred to 3 small bones further inside the ear, in a location called the “middle ear”. These 3 middle ear bones area called the malleus, incus, and the stapes.

The Vibration of the Middle Ear Bones

So once the sound vibrations reach the middle ear bones, those bones amplify the sound and then send it to the cochlea. The cochlea is further inside the ear, behind the middle ear bones, and the cochlea is located in an area called the “inner ear”. The cochlea is shaped like a snail shell and is filled with fluid.

Inside the cochlea, there is an elastic partition that runs form the beginning to the end of the cochlea that is split into upper and lower parts. This partition is called the basilar membrane. The basilar membrane is named as such due to its function as a base or grounding for hearing structures to sit on.

The Vibration of the Cochlea

green stereocilia on top of hair cell of ear
Here is a High-resolution micrograph of beautifully delicate, staircase-shaped structures inside of the inner ear, called stereocilia. Stereocilia are miniscule hair-like protrusions on the surface of sensory cells (also called hair cells) found deep within the cochlear and labyrinth structures of the inner ear. They serve as the key mechanosensors, responding to fluid motion for various functions, including hearing and balance. Emphasizing how sensitive these structures are, Kachar describes being able to hear a pin drop from across a room: the sound wave from the pin dropping produces an increase in pressure within the fluid contained in the inner ear, resulting in a shear force that presses the stereocilia against each other. The stereocilia then convert this mechanical movement into electrical signals, which are sent to the brain—all within a matter of milliseconds.

When the sound vibrations are transferred from the middle ear to the cochlea, the fluid inside the cochlea (perilymph) start to also vibrate. The sound uses the perilymph as the medium to travels as a wave through the cochlea. As the sound waves travel through the cochlea, the basilar membrane moves and the thousands of hair cells on top of the basilar membrane sense the mechanical wave movements and convert it into electrical signals. These electrical signals then travel along the auditory nerve to the brain for it to interpret.

The hair cells near the wide end or entrance of the snail-shell shaped cochlea detect higher-pitch sounds like the cry of an infant. The hair cells closer to the center of the cochlea detect lower-pitch sounds, such as the call of a whale.

Note that sitting on top of each hair cell is a structure called “stereocilia“. Stereocilia are mechanosensing (movement detecting) organelles of hair cells that responds to movements of the perilymph fluid that fills the cochlea. The stereocilia look like microscopic hair-like projections, and they bend when a sound wave move the hair cells. When the the stereocilia bends, pores at the tip of the stereocilia open up. These pores are “cation selective channels” that only allow cation ions to flow into the hair cell.

When cation ions enter the hair cells, it causes the production of electrical signal that travels from the hair cell, through neurons, and eventually to the brain to be interpreted as “sound”.

One final note that I would like to add is that the tallest stereocilia located near the outer portion of the cochlea may touch the membrane relatively above them, called the tectorial membrane, when moved by a sound wave.

Related Links

Books about Hearing

Sources

  1. How Do We Hear? [NIDCD]

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