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Alive & KickingFollowing the Sound Wave |
How well I remember the loudest, most annoying sounds I have ever heard in my life. I was down by the old raspberry patch when a ladybug saw fit to seek refuge inside my left outer ear canal. Barely inside, it encountered an elastic membrane barrier and—with all six legs—began to beat the drum.
I ran screaming toward the house, burst through the door toward an alarmed wife and gave that bug a swimming lesson under the kitchen faucet: a royal flush.
I had no mercy as it was escorted down the drain.
The ear drum that separates the outer ear from the middle ear responds to a great range of vibrations.
In fact, a healthy eardrum can handle anything from 20,000 vibrations per second, down to sound waves whose energy doesn’t amount to even one strum of a beetle’s leg.
But to me, it is a wonder the membrane and attached equipment could withstand such a barrage.
When she was young, daughter Cindy developed a severe ear ache. We took her to a hearing specialist for fear the bulging eardrum would burst. Twenty minutes later, I held her head as the doctor scratched a hole in the membrane to relieve the inner pressure and save the eardrum.
Cindy said she remembered a loud sound and an immediate reduction of her pain. Being a living membrane, the eardrum heals in time and remains functional even when “holed.” My dad was not so fortunate as a child, and was deaf in his left ear after his eardrum was stressed beyond its elastic limit.
If the pressure is unequal inside the middle ear with that of the great outdoors, the eardrum in tension and cannot vibrate freely. This is likely to occur when altitude is gained or lost in flight or when crossing a mountain pass by car.
The Eustachian tube, which leads from the middle ear to the throat, allows loss or gain of air to keep the eardrum in pressure equilibrium, so that it is able to vibrate freely when receiving sound energy waves.
Excess mucus production can partially plug the Eustachian tubes and alter sound reception or cause pain if pressure becomes great enough to critically load the eardrum. Small children have smaller air passageways, as many air travelers seated next to crying babies during descent have noted.
The middle ear chamber also houses an amazing mechanical amplification system, consisting of three small bones that together weigh about 55 milligrams (about the weight of one drop of water). The first of the three bones is connected to the middle of the eardrum and causes the second bone to articulate with the third bone, which is attached to an elastic membrane-covered opening, the oval window. The oval window, an amplification component, is smaller that the eardrum.
The three small bones produce a mechanical leverage system much like a pry bar by which one end moves further to produce a smaller travel at the other end, but with greater force.
Overall, in the middle ear the force of the sound is increased by a factor of ten.
It is at this point that sound energy enters the inner ear, which is a liquidfilled chamber shaped like a spiral shell. This is the cochlea, where the important auditory nerves are stimulated to carry the sound event to the brain for interpretation.
Consider this improbable sequence—going from air disturbance to an elastic membrane, to three small bones for mechanical amplification, then to a liquid medium (the inner ear), into a snail-like structure that can transfer the energy of a whisper to the energy of thunder.
Such incredible equipment! Next time: the transfer to electrical signals, nerve impulses. All this to feed the living mass— the brain.
Guest columnist John Cooke taught high school biology for 30 years and is pleased to share his insights with our readers.