How we hear
Our ears connect us with the world around us, giving us constant clues as to what and who is near and far. But how do we actually hear?
Our ears connect us with the world around us, giving us constant clues as to what and who is near and far. But how do our ears work to localize sound? Why do we need two? How do they know the approximate distance and location of the sound source? How do our ears differentiate the buzzing of a fly from the buzzing of a bee? If you’re allergic to bees this could be a life saving environmental awareness tool!
To start, let’s not give our ears all of the credit. Our Auditory System can be broken down into two categories: our ears & our brain. Our ears work to convert sound energy into neural signals. Our brain’s task is to receive and process what those signals contain and create meaning of the sounds we hear.
Let’s take a look at the anatomy of our ears to better understand how our ears and brain work together to recognize and localize sounds and then interpret them into something we can understand.
Our ears can be broken down into three main parts: The Outer (External) Ear, The Middle Ear, and the Inner Ear.
The ear is the organ of hearing and balance.
Let’s examine the three parts by what they consist of…
The External/Outer Ear*:
● Pinna/Auricles – The outermost part of the ear
● The External Auditory Canal/Ear Canal – The tube that connects the outer ear to the middle ear.
● The Eardrum/Tympanic Membrane – The membrane that divides the external ear from the middle ear
Middle Ear = Air Filled:
● Ossicles – The three of the tiniest bones in the human body which are connected and transmit the sound waves to the Inner Ear.
Inner Ear = Fluid Filled:
● Semicircular canals
● Basilar Membrane
● Auditory Nerve
(There are many parts of the ear, listed below are the key parts related to this article*)
So… HOW do our ears convert sound energy into neural signals and HOW does our brain receive and processes this information? And if our middle ear cavity is air filled, and our Cochlea (located in our inner ear) is fluid filled, how are sound waves converted into waves in the fluid?
Let’s follow sound through our auditory system to see how this happens…
The starting point is when the source of sound creates vibrations which travel as waves of pressure through particles of air, liquids, and solids which is collected by our outer ear and funneled into our ear canal.
Through the ear canal, the sound waves hit our Eardrum (Tympanic Membrane) which divides our Outer Ear from our Middle Ear. Our Middle ear contains the three tiniest bones (ossicles) in the human body. When the sound waves hit our Eardrum it creates vibrations like drum that set our ossicular chain into motion. The ossicles known as the Malleus (hammer), Incus (anvil), and Stapes (stirrup): in a healthy, functional auditory ossicular chain, the hammer strikes the anvil, which hits the stirrup. These motions amplify sound vibrations and send the sound into the Inner Ear.
In the Cochlea, the sound vibrations convert into the vibrations of fluid into a wave from one end of the cochlea to the other by the motions created (like ripples) by each bone pushing the fluid within the chambers. The Basilar Membrane runs the length of the cochlea, lined with over 10,000 hair cells containing stereocilia. Sterocilia move with the vibrations of the cochlear fluid and basilar membrane.
This movement of the hair cells and fluid triggers a signal that travels through the hair cells into the Auditory Nerve through the brain. There, the signals are interpreted.
Sound makes vibrations in the basilar membrane but not each hair cell moves, the moving hair cell all depends on the frequency of the sound which determine the movement. Think of the Basilar Membrane as a frequency spectrum analyzer. At the base, closest to the oval window, the membrane is stiff and only vibrates in response to short wavelength, high frequency sounds (like the buzz of fly).The membrane becomes more flexible toward the apex, there it only vibrates in the presence of longer wavelength, low frequency sound stimulation (like the humming of a bee).
Because high and low frequency sounds vibrate in different locations of the Basilar Membrane (like keys on the piano of the brain) the brain is able to complete an important task of localization to identify the source of a sound in space and interpret speech.
But we could not do this with one ear and a brain alone. Localization is made possible by having two functional ears which is one reason why binaural hearing (hearing with both ears) is important.
Each ear compares the sounds coming into the ears to locate the sound in space by analyzing the time and intensity of the sound. This information is sent to a part of the brainstem to analyze the time and intensity difference between the two ears.
The results and quickly processed and sent up the brainstem to the Auditory Cortex for analysis.
The brain reviews the patterns of action to identify what the sound is and where it is located in space. Thus giving us meaning to what a sound source is, it’s location, and allows us to use the information and if you’re allergic to bees, possibly save your life!
Has anyone ever complained that you listen to the TV too loud?
Do you notice yourself having to ask speakers to repeat themselves?
Do you have ringing in your ears?
Do you hear better out of one ear than the other?