Hearing and How the Ear Processes Sound

“Hey, you freakin’ SOB!” someone yelled, shocking me from my core, though he used a nastier f-word I’d rather not repeat.  I looked up to find a crotchety-looking older guy on a bike, screaming past runners, walkers and moms with baby strollers and speeding my way.  I’m running on a beachside path one fine Sunday morning.  “Get the hell out of my way!” he screamed.

Holy moly.  I’m already far off to the right side, nearly scraping against a low-lying brick wall.  Do I dive over it into the sand?

Ah, the ear: a receiver of sounds, both pleasant and unpleasant.  Unfortunately, it doesn’t have an automatic filter to block unwanted noise coming from unruly people and cantankerous dudes on bikes.

So let’s talk about sound.  If you skipped the delightful primer on the physics of sound mechanics, feel free to explore the previous post, Hearing and The Physics of Sound Mechanics, for an exhilarating look into this fun-filled topic.

Those sounds spewed forth from bike-guy were generated by his crotchety larynx and mouth.  As air travels up from his crotchety lungs into his crotchety trachea, it passes through his crotchety vocal cords causing vibrations or oscillations to the column of air molecules speeding past.  This is the way sound is generated, by the creation of vibrating pressure waves caused by the movement of air molecules in wavelike fashion.  As these vibrating waves enter his crotchety dirty mouth, the sound is further shaped to form coherent speech in the form of words.  The sound then travels at 767 MPH to finally strike its target, which unluckily was my ear and the ears of anyone within ear-shot. 

Those obnoxious, crotchety pressure waves of air molecules strike the auricle, the part of the ear that is visible on the side of your head, you know, the appendage you might’ve pierced a bunch of times to adorn fancy jewelry.  The auricle funnels the sound to the small hole in the ear, known as the external auditory canal (EAC for short). The moving molecules strike your eardrum (tympanic membrane, abbreviated TM).  The forces of those air molecules vibrates the TM, causing it to move in and out at a rapid frequency.  This in turn vibrates the three tiny bones of hearing known collectively as the ossicles.  In case you’re wondering, the names of the ossicles (laymen’s terms in parenthesis, which were coined from the physical appearance of each ossicle) are the: malleus (“hammer”), incus (“anvil”) and stapes (“stirrup”).

The TM and ossicles collectively act as a transformer, converting sound energy from one form to another.  In this case sound from the medium of air is converted to mechanical energy along solid bone, which afterwards is transformed into pressure waves in fluid found in the cochlea.  We’ll touch more on this shortly. 

The ossicles are joined to one another by tiny, bendable joints.  This allows movement of the ossicles, passing the vibratory input ultimately onto the stapes which sits atop the oval window of the cochlea.  This form of mechanical energy is then converted to fluid energy within the cochlea.

Now the fluid within the cochlea is similar to water and far denser than air, and thus the reason you need the transformer of the middle ear (the TM and ossicles) to effectively deliver sound energy within the cochlea.  Try this next time you’re in a pool or a bathtub: have someone yell at you while your head is submerged in water; the loudness is diminished as well as the intelligibility.  You can see how ineffective the cochlea would convert sound energy from one medium to another without the help of the middle ear transformer.

The cochlea is a snail-shaped organ with 2 ½ turns and encased in dense bone.  Before some of you smart alecks strike a comment, the illustration I drew contains fewer turns for the sake of simplicity.  So quell those technical wise-cracks. 

The cochlea has three different channels of fluid: the scala vestibuli, scala media and scala tympani, really cool words meant to impress your friends.  The stapes vibrates onto the oval window which is the entrance into a fluid chamber called the scala vestibuli.  The fluid within this chamber is called perilymph.  A pressure wave is then created in the perilymph.

This pressure wave travels further along the coil of the scala vestibuli.  The farther it travels into the perilymph, the weaker the pressure becomes, but the pressure wave does not abate entirely.  It must eventually exit somewhere, for as you can imagine, pressure buildup within a closed structure is bound to cause some unpleasantries, such as over-expansion and explosion of the structure.  I’ll assume nobody wants a cochlea exploding in their head.  That would be bad, very bad.  I imagine it would hurt a great deal.

So after the pressure wave reaches the apex of the coil, it goes the opposite direction in the part of the cochlea called the scala tympani.  It travels along and eventually reaches the round window, a thin membrane located below the oval window.  Unlike the oval window, the round window has no bone covering it.  The pressure wave of perilymph fluid is then converted back to a pressure wave in air, though much weaker than the initial pressure wave hitting the TM, and it dissipates within the middle ear space.  Since there is no transformer effect over the round window, the resultant sound and air pressure is markedly reduced and thus does not interfere with one’s hearing.

Within the middle of the cochlea is another coiled tube, a membranous organ found within the bony tube of the cochlea called the scala media.  This organ contains a liquid called endolymph, which has the consistency of water but different types and concentrations of stuff dissolved in it.  The fluid pressure wave also causes the endolymph to vibrate, stimulating individual cells within the scala media called hair cells.

The hair cells actually don’t have hair such as the stuff found on your head or other unmentionable parts of your body.  The hair in this instance is more correctly termed sterocilia, thin microscopic projections on the cell’s surface.  When the vibrations of the endolymph bend the sterocilia, an electrical current is generated by that cell, which passes this along to a neuron attached to it.  A neuron is a single nerve cell that can be quite long.  This neuron extends from the hair cell, joins other neurons from other hair cells to ultimately form the auditory nerve (Cranial Nerve VIII, in other medical terms) which then travel to your brain.

Interestingly, along the length of the scala media the hair cells are frequency-specific.  This means each cell becomes stimulated only with a particular frequency.  Hair cells closer to the oval window (closer to the basal part of the cochlea) are more sensitive to higher frequencies, whereas the father you travel along towards the apex of the cochlea (farther away from the oval window) the frequencies where the hair cells are sensitive gradually decrease.  So the highest frequencies of sound stimulate the hair cells closest to the oval window and this gradually progresses from high frequency to low frequency as one travels towards the apex of the cochlea. 

This is how the ear organizes the many different sound frequencies before delivering it down the neurons to ultimately reach your brain, that marvelous organ (for most of you I’ll assume) which process sound stimuli to something meaningful to the organism receiving it.  So when the brain finally receives the sound input from the crotchety biker, it needs to act rapidly to process, interpret and act upon “Get the hell out of my way!”

Undoubtedly this is more information than some of you might care, and I admit I might’ve gotten a bit carried away.  But as they say with get-togethers such as barbecues or dinner parties, it’s always better to have more than enough rather than vice versa.  Bon appetit!  And don’t let those mean and crotchety old-dudes ruin your day.

©Randall S. Fong, M.D.