The Hearing Journey
The Hearing Journey
Have you ever just taken a minute to listen to all the amazing sounds around us? The leaves bustling with the wind, the perfect guitar solo in your favourite song, children laughing, a bug buzzing around your head. We often don’t stop to think about just how many sounds, both simple and complex, are present in our daily lives or how unbelievably accurate our auditory system is at instantaneously taking in all of the information and making sense of what we’re hearing. So let’s break down the hearing process into steps to highlight just how amazingly complex our auditory systems are.
The Sound, A Vibration.
Every sound that you hear is simply the movement (propagation) of air particles through space. When an object moves, it disturbs the surrounding air particles which then bump into each other and create a propagating wave of particles moving away in all directions from the source. This movement of air particles can be thought of as vibration of the air, better known as a sound wave. It is when those vibrations reach the ear and then further the brain that we actually HEAR the sound. So, the classic question “If a tree falls in the forest and no one is around to hear it, does it make a sound?” The answer is yes it does make a sound. It creates the vibrations in the air (sound wave) regardless of whether an ear is present to hear it. We can think of the vibration of air as acoustic energy.
The Outer Ear – Filtering the Acoustic Energy
Let’s imagine we are listening to our favourite guitar solo on the radio. The vibrating speakers will create a sound wave that vibrates the surrounding air particles creating the travelling wave of acoustic energy discussed above. These acoustic vibrations then get funneled into the ear canal by the pinna (the part of the ear that we can see) towards the eardrum. The outer ear consists of the pinna and the ear canal and its main function is to pick up environmental sounds and direct them towards the middle ear. The eardrum is a membrane that separates the outer ear and the middle ear. It acts like the skin of a drum and transforms the acoustic energy into movement, called mechanical energy.
The Middle Ear – Transforming Acoustic Energy into Mechanical Energy
The eardrum is linked to the three smallest bones in the body known as the ossicles. As the eardrum begins to vibrate in response to the acoustic energy entering the ear, the ossicles also being to move. These three tiny bones form a chain that amplify the movement of the eardrum and send these vibrations to the inner ear, specifically the cochlea. The cochlea is a small snail-shaped organ that contains thousands of tiny hair cells and is full of fluid. The vibrations from the ossicles creates a travelling wave the ripples through the fluid-filled cochlea. And this is where the magic happens!
The Inner Ear – The Electrical Messenger
As the travelling wave moves through the fluid-filled cochlea the tiny hair cells bend in response to the fluid movement. The bending of the hair cells opens a channel on the cell surface allowing chemicals to rush into the cell and this creates an electrical signal. This electrical signal is then sent along the auditory nerve up the brain to be processed for meaning. The role of the inner ear is to convert the mechanical vibrations from the middle ear into a language that the brain can make sense of.
The hair cells are organized in a very specific way inside the cochlea, and different hair cells are responsible for encoding different frequencies. If you were to unravel the snail-like cochlea completely, it would resemble a piano with low frequency hair cells on one end moving up to high frequency hair cells on the other. The point along the cochlea in which the travelling wave has the most energy (is the “biggest”) will correspond to the frequency of the sound source. Most complex sounds that we hear are not a single frequency but are made up of many frequencies all pooled together. Even so, the cochlea is able to tease apart these complex sounds into simple electrical signals for the brain to process and assign meaning to so that we are able to recognize the sound as your favourite guitar solo.
Hearing is a very fascinating sense that most of us tend not to pay too much attention to until there is a problem. When we break down the pathway of sound into the different parts it truly is amazing how many tiny parts are working together in order for us to not only hear but to comprehend the sounds of our incredibly complex world. To learn more about how the different parts of the system can malfunction and cause a hearing loss click here.