How do we Hear?

By Scott Mattison (@FoolsPizza)

Hearing is one of the five traditional senses, but by the time individuals reach 60 years old, one in three will develop significant hearing loss in one or both ears. To treat this hearing loss, we must first have a basic understanding of how sound and our hearing work.

Sounds travel by mechanical vibrations of particles that contact one another. Air, water, and solids all allow for these vibrations to occur; however, in space, no one can hear you scream because outer space is a vacuum with no small particles to transfer sound (sorry Star Wars fans).

The ear is divided into three regions: the outer ear, the middle ear, and the inner ear. The outer ear is what is most familiar to people, it consists of what we traditionally call ears and a canal that leads to the eardrum. The role of the outer ear is to collect sound and direct it to the middle ear through the ear drum. The unique shape of the ear enables directed sensing of sounds which gives people the ability to determine where sounds come from.

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A nice illustration of the middle ear bones (ossicles). Courtesy of Wikipedia.

The middle ear is an open, air filled space in the skull that separated from the outer ear by the ear drum. The active portion of the middle ear consists of three bones known as the malleus, incus, and stapes. The malleus is attached to the eardrum, and when the eardrum vibrates in response to sounds, those vibrations are transferred to the malleus. Movements of the malleus move the incus, which in turn move the stapes. The stapes then vibrates against what is called the oval window of the inner ear. The bones of the middle ear form a lever arm that provides an amplification of sound by around 130%. Additional amplification occurs due to the size difference between the eardrum and the oval window of the inner ear. Overall, sound vibrations entering the inner ear are around 18 times larger than sound vibrations against the eardrum. The middle ear also has special muscles that when activated will reduce the intensity of loud sounds. This is called the acoustic reflex and may reduce sound intensity by around a factor of 10. The acoustic reflex protects the delicate structures of the inner ear.

The inner ear is where all the magic of hearing happens. The inner ear takes the mechanical vibrations of sounds and converts them into electrical impulses within the brain. How this actually works is both a lot cooler and a lot more complicated than it might seem.

At their best, humans can hear a range of sound frequencies (think notes on the piano, each note is a different frequency) between 20 Hz and 20,000 Hz. 20 Hz is a really deep bass sound and 20,000 Hz (If you can still hear it) is an extremely high pitched whine. As we age, sensory cells in our inner ear begin to die and we lose the ability to hear higher pitched frequencies. By age 20, most males have lost the ability to hear 20,000 Hz, females typically begin to lose higher frequency hearing later into their twenties. Fortunately most of human communication is limited to the 100s of Hz frequency range. The fact that we can differentiate between sounds of different frequencies means that our inner ear has to respond differently to different sound waves.

Nervous cells have a recovery period of 1 – 5 ms after each time they fire, meaning the fastest a nerve could respond is 1000 Hz. If this is true, how can our ears generate nervous signals in response to sounds vibrating up to 20 times faster? The answer to this question is a fun word: Biomechanics. The inner ear, known as the cochlea, consists of 3 fluid-filled tubes that spiral up to a blind end. Typically this is drawn as its own snail-shaped solid, but in reality the cochlea is a fluid filled cavity with the temporal bone of the skull. Sounds enter the cochlea through the oval window into the bottom of the 3 tubes. Sound waves travel to the top of the spiral of the cochlea then back down through the top tube and vibrate against the oval window. This top tube and bottom tube mainly play the role of sound conductor; the nervous tissue is all sitting within the middle tube.

Inner Ear
A rough sketch of the cochlea with the spiral stretched out into a strait line. Sound waves enter through the oval window.  Higher frequency sounds are propagated through the basilar membrane toward the start of the cochlea whereas lower frequency sounds pass through the basilar membrane toward the end.

Between the bottom tube and the middle tube is what is called the basilar membrane. This membrane has changing mechanical properties all the way down its length so that at the entrance of the cochlea it vibrates strongly in response to higher frequency sounds and at the top of the spiraled cochlea the basilar membrane vibrates best in response to lower frequency sounds. Nervous cells in the inner ear respond to the movement of the fluid within that middle tube so when the basilar membrane vibrates more strongly due to higher frequency sounds at the base of the cochlea, the nervous cells in that region know to tell the brain that you just heard a high pitched sound. And when nervous cells respond to movement in the upper part of the cochlea, the brain knows you just heard a lower pitched sound.

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An illustration of what is known as the organ of Corti. This rests on top of the basilar membrane and plays a large role in our hearing. Image source: Wikipedia.

The basilar membrane is a really cool example of biomechanics in of the body solving complex challenges, and Georg von Békésy even won the Nobel Prize in Physiology or Medicine in 1961 for his work in in demonstrating its function. However, movement of the basilar membrane alone does not completely account for how well the hearing of humans and other mammals can tell the difference between two frequencies. This is the role of the organ of Corti, which I will introduce in a later blog post. Obviously, there is a bit more to the inner ear than I have explained here, and this is an exciting question for which scientists are still exploring answers!