This is part of a series of articles by Garth Whitcombe, founder of TherapyMuse, that provides background information about the use of music in massage therapy settings.

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Let us follow a sound from its initiation as a pressure wave to its transformation into sensation in the human brain. Imagine, for instance, we play a single note on a piano, the A above middle C or A 440.

The hammer strikes the string and the vibrations create pressure waves of sound that radiate in all directions. Molecules of air vibrate 20 times per second for the lowest note that we can hear and 20,000 for the highest. The imaginary note we have just played is causing the air molecules to vibrate 440 times per second.

Even though we are striking one note, the resonance that results is still a complex overlay of sound due to the resounding of the harmonic overtones that delineate the timbre of the sound. So other frequencies of sound are also generated at a lower volume and these overtones will arrive along with the fundamental tone of A440.

Traveling at the speed of sound we perceive the note the instant we play it, but in n reality that note has completed a complex journey.

The beginning of its transformation from waveform to sound sensation is the outer ear. What we think of as ears, those sculpted masses of cartilage called pinnae (Latin for feather), are actually resonators of sound waves. If you lightly trace your ears with your fingers you will notice a maze of ridges and bulges that funnel sound into the ear canal. The pinnae mechanically alter our perception of sound waves by boosting high frequency sound, amplifying the range of 3,000 to 4,000hz which is roughly the top octave of a piano and the most important range of human speech.

As an experiment, next time you are in the kitchen or bathroom with a faucet running, try covering your ears lightly so the ear canal is still open and listen to the white noise of the running water. Even with your hand an inch or so away from your ears, you can alter your perception of the sound. Since our A note is 440hz, the higher overtones that sound along with the fundamental are imperceptible so it won’t be significantly altered by the pinnae.

Continuing its journey, our note, still a pressure wave, is funneled into the ear canal where it begins the first step in transformation into sensation. The pressure wave pushes against the ear drum and begins its passage into the air-filled middle ear. The push on the eardrum activates 3 small bones, called ossicles, that are attached by ligaments. The first ossicle, the malleus, or hammer, pulls on the second, the incus, or anvil, which in turn pushes the third, the stapes, or stirrup, into an opening in the fluid filled inner ear. The 3 bones vibrate in a pattern that encompasses every frequency inherent in our piano note, preserving the relation of the fundamental and the overtones that make up the timbre of the note.

The pressure wave that is our A note makes a transition from the medium of air to liquid as it enters the inner ear. If you have ever been swimming at a pool party and dived underwater, you will have noticed that the sound of music or conversation all but disappears. It requires substantially more force to move sound through the denser molecules of water and up to 99% of the sound is lost. This is where our 3 little bones, the ossicles, come in. They provide the leverage to amplify the pressure wave, concentrating the energy of our A note as it reaches the opening to the inner ear to 1/16 the surface area of the ear drum.

Loudness of sound is measure in decibels. Each 20 decibels corresponds to a 10X increase in sound level. The auditory threshold is set at 0db with the range of normal speech, 40-70db. This is 100-3,000 times greater than the threshold. The ossicles perform another vital function. They are essentially a braking system for dangerous levels of sound. Two miniscule muscles attach to the ossicles to hold them in place. When overwhelming levels of sound arrive, these muscles reflexively contract and stop more than 60% of the force from reaching the delicate mechanism of the inner ear. It takes about 1/100th of a second for the muscles to react, so they are unable to stop a sudden explosive sound such as the retort of a firearm or an explosion. The muscles also tire after long exposure to loud noise, accounting for the type of deafness that is found among machine operators and rock musicians.

The middle ear muscles also respond to our own vocalization. Since we hear our own voices not only through our ears but also through bone conduction, the noise of talking could be overwhelming and mask the ability to hear external sounds. Hence, the middle ear muscles keep perfect time with our voices by gating the inner levels. The ossicles also do a little sub-mixing of their own, boosting the midrange frequencies that are so critical in conversation. Having been amplified and equalized by the ossicles, our A note finally reaches the inner ear. The inner ear is 3 to 4 cm from the surface of the temporal bone. It contains the cochlea and the organs of balance in a maze of liquid filled cavities within the bone.

Up until this point in the journey of our A note, the components of the ear have been concerned with the transmission of a waveform. The cochlea is where the pressure wave is to be transformed into the electric potential that will stimulate the brain into recognizing it as a single note played on a piano. The cochlea, Latin for snail, is shaped like a seashell, and is essentially a tube coiled through 3 turns. Unraveled, it is stretches about 3.4cm, but the coiling allows it to be packed into a much smaller space. Although the coiling is thought to have no discernable acoustic property, the symbolism of the spiraling vortex is worth noting. It bears an uncommon resemblance to the arithmetic progression of the Fibonacci series or Golden Ratio, a geometric proportion that is found throughout nature, from the coiling chambered architecture of the nautilus to the radiant face of a sunflower. This symmetry is also embedded in the art of Da Vinci, the architecture of the Parthenon, and many of the great musical compositions from Bach to Debussy. The cochlea is one of the most beautiful and complex mechanisms in the human body.

Inside the cochlea, running the entire length of the chamber, lies the basilar membrane. Made of collagenous fibers, the basilar membrane is 0.1 - 0.5 mm wide and has a gradient of flexibility, becoming increasingly more flexible away from the inner ear. If you have ever looked inside a grand piano, you will have noticed the strings have different lengths and thicknesses, long and thick for bass notes and becoming progressively shorter and thinner through the mid range and treble. This is the same type of progression we find in the basilar membrane. The gradient of flexibility allows for the separation of sound frequencies causing each part of the membrane to resonate at a precise frequency, mapping the incoming sound.

The sound architecture of our A note having been transmitted and amplified by the stapes of the middle ear is now separated into its constituent frequencies pf fundamental tone and resonating overtones. Human hearing can discern differences in frequency of up to 3 parts in 1,000, or 1/10th of a musical tone. To achieve this fine distinction, the cochlea contains its own amplification device, the cochlea amplifier, that increases the localized motion of the basilar membrane. The amplifier creates a 40 db rise in the threshold of sound. This is comparable to some one raising their voice from a whisper to the range of normal conversation. Without the cochlea amplifier, our sound sensitivity is diminished 100 fold.

To put this amplification into biological perspective, the actual movement is microscopic. At 0db it is a movement of around 3 atom diameters. If the cochlea were stretched to 100 kilometers, it would be a movement of 1 mm or less than a 16th of inch over 80 miles.

Amplification implies an energy source and understanding cochlea amplification requires us to look at the cellular structure of an aspect of the cochlea called the organ of corti.

Mechanotransduction is the function of the organ of corti. Stimulated by shifts in the basilar membrane, stereo cilia, also known as hair cells, brush across a gelatinous membrane. The motion of the basilar membrane displaces the stereo cilia at the same frequency as the source sound. Conversion of mechanical motion into electrical signals requires both sensitivity on a molecular scale and a rapid enough reaction time to respond immediately to sound stimuli.

How this conversion from mechanical force to bio-electrical energy happens is still a hypothetical. The current reigning theory is that the stereo-cilia are held together by protein links and they have a specialized linkage protein called a tip link which is coupled to ion channels that open and close when the stereo-cilia are pushed and pulled, thus changing the electric potential of the cells.

The cochlea sorts sound into patterns and encodes 30,000 auditory nerve fibers, relaying information such as intensity and timing, mapping sound for the midbrain and higher centers. What we think of as sound does not exist without the transformative process that occurs in the cochlea.

Sound without perception is merely a pressure wave echoing back into silence. Only through the miracle of hearing does sound take on meaning.