Hearing Life

How do I hear sound?

The linear movement of air is not sound, it’s wind. Sound waves are oscillations, the back and forth movement of molecules within a medium, like air or water. These vibrations are then passed on to nearby molecules which then spread out in all directions as sound waves. Since a vacuum has no air it can’t transmit sound because there aren’t any air molecules within it to vibrate.

Sound waves are air particles constantly being packed together in high concentration and spread apart in areas of low concentration, like an accordion or slinky. Depending on what caused the sound they have a certain amplitude, wavelength and frequency. The amplitude is how high or low the wave rises and this determines how high or low its volume or loudness. Think of the difference between the buzzing of a mosquito, normal speech and a jet engine. The wavelength and frequency determine its pitch or tone. The closer the sound waves are packed together the shorter the wavelength, the higher the frequency and the higher the pitch or tone. And the further apart the sound waves are from each other, the longer the wavelength, the lower the frequency and the lower the pitch or tone. Think of the difference between the lowest notes that come out of a tuba and the highest ones from a flute.

Sound waves travel through the air at about 330 m/sec. Since light travels at 300,000 Km/sec this means that light is literally about a million times faster than sound. The retina at the back of your eye has specialized cells called photoreceptors. They respond to the light focused on it through the cornea and the lens by sending out nerve messages to your brain so you can see. So, it would seem that for you to be able to hear sound waves you’d need something like that to convert them into nerve messages for your brain. That something is called the organ of Corti which is located in the cochlea inside your ear. 

Here’s how it all works.

Your ear is a very complex sensory organ. All its parts work together to produce and send waves of oscillating molecules to its cochlea. Although it’s the organ of Corti in the cochlea where the nerve impulses for hearing begin, the other parts of your ear play important roles that support this function. It can be divided into three main regions; the outer (external) ear, the middle ear, and the inner (internal) ear. 

The outer ear is made up of the pinna (ear flap), the ear canal, and the eardrum (tympanic membrane). The pinna acts like a satellite dish by collecting sound waves and funneling them down the ear canal to the eardrum. It helps you figure out the location of different sounds. The ear canal is the path for sound waves to get to the eardrum. It produces wax which provides lubrication while at the same time protecting the eardrum from dust, dirt and invading microbes and insects. Sound waves enter through an opening in the skull, move down the ear canal and strike the eardrum. The eardrum is a very thin cone-shaped membrane. It responds to sound waves by vibrating at the same amplitude, wave length and frequency. The eardrum represents the end of the outer ear and the beginning of the middle ear.

The middle ear is an enclosed air-filled chamber. The air pressure on either side of the eardrum must be equal to allow it to move easily when hit by sound waves. But, the air in the middle ear tends to be absorbed by the surrounding tissue which, if not corrected, can lead to a vacuum effect, reduced eardrum movement and impaired hearing. This is what you experience when the plane you’re flying in descends.

The auditory tube in the middle ear connects with the back of your nose and throat. When you swallow, yawn or chew this causes the auditory tube to open, allowing ambient air to enter the middle ear, replacing what has been absorbed. This makes the air pressure on both sides of the eardrum the same, the vacuum effect goes away and your hearing improves. That’s what happens when you feel the popping in your ears as you chew gum or candy as you come in for a landing.

The middle ear contains the three smallest bones in the body called the ossicles. They are the malleus (hammer), the incus (anvil), and the stapes (stirrup). Their job is to send the vibrations of the eardrum into the inner ear which houses the organ of Corti in the cochlea. The malleus is attached to the ear drum and the incus, the incus is attached to the malleus and the stapes, and the stapes is attached to the incus and the oval window of the cochlea.  Like a relay race, when the sound waves make the eardrum vibrate this makes the malleus vibrate which in turn makes the incus vibrate which in turn makes the stapes vibrate. The result is that the vibrations of the eardrum that were started by the sound waves coming down the ear canal are transferred to the oval window of the cochlea.

The cochlea consists of three fluid-filled inter-related coiled chambers which spiral together for about two and half turns resembling a snail shell. Within the cochlea is the organ of Corti, the sensory receptor that converts the mechanical waves into nerve impulses. The vibrations, started by sound waves striking the eardrum and transmitted by the ossicles in the middle ear to the oval window of the cochlea, now produce fluid waves within it.  

The organ of Corti contains about 20,000 hair cells (neurons) running the length of the spiraled cochlea. When stimulated by these fluid waves it makes them bend which sends nerve impulses through the auditory nerve to the brain. Higher frequencies cause more motion at one end while lower frequencies cause more motion at the other end. The specific neurons that service specific hair cells along the organ of Corti respond to specific frequencies of sound which when sent to the auditory cortex are processed, integrated, and then interpreted as hearing.  How your brain is able to perform this feat is as yet not fully understood.
        
Three Questions for Mr. Darwin

    1. How did my body anticipate the need for and where did the information come from to make all of the parts of the ear and assemble them properly so I can hear?

    2. How did my body anticipate the need for and where did the information come from to make my ears sensitive enough to the right amplitude and frequency so I can hear human speech?

    3. If nobody really knows how the brain takes the nerve messages from the auditory nerve and allows us to hear then how can anybody claim to know where hearing came from?   

Also see Dr. Glicksman's Series on

"Beyond Irreducible Complexity"

"Exercise Your Wonder"


Howard Glicksman M. D. graduated from the University of Toronto in 1978. He practiced primary care medicine for almost 25 yrs in Oakville, Ontario and Spring Hill, Florida. He now practices palliative medicine for a Hospice organization in his community. He has a special interest in how the ethos of our culture has been influenced by modern science’s understanding and promotion of what it means to be a human being.

Comments and questions are welcome.

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