In Pursuit of Silence Read Online Free PDF Page A

Heffner’s work. When she spoke about it, her voice took on a bitterness otherwise reserved for deer whistles and fleacollars. “The pinnas,” she said, using the technical name for the external ear, “act as independent sound shadowers. They alter the degree of the sound shadow cast by the skull. This is part of their work as directional amplifiers. Animals point their ears at something and then they can hear it better.” But the extent of the pinna’s impact as a frontline amplification system continues to defy researchers. “The head is basically a lumpy sphere with two big funnels on it,” Heffner said. “Those funnels intensify a sound as it drops down toward the eardrums. But we’ve never tried to measure pinna dimensions because—what do you measure? Ideally, you’d get some of these animals and take several of their pinnas and measure the physical properties of what the pinnas do to sound coming into the sound canal, but it’s not remotely practical. Pinnas are very complicated shapes. Some are kind of flat. Some have big openings. There are all kinds of folds. And while animals with big heads generally have big pinnas, that’s not always the case. It’s known that the external folds help to augment sound and create a difference between what’s heard in each ear. So if you have a sound off to the right quadrant somewhere, and you’re a little bat with big ears full of fancy convoluted folds, sound coming in is going to have very different features than it does for a cow.”
    The mysteries of the inner ear are still more pronounced. Jim Hudspeth, who works at Rockefeller University studying the molecular and biological basis of hearing, has shown that the motion of the hair cells not only converts the mechanical wave into an electrical signal that can be read by the auditory nerve in the brain; the various reactions set in motion by the oscillation of the hair cells also serve to magnify the sound. A huge “power gain” takes place, he says, within the inner ear itself. How exactly this happens is still not understood.
    Regardless, we now know that all three parts of the ear play a dynamic role in boosting sound. If our auditory mechanism is working normally, Hudspeth told me, by the time we realize we’ve heard a sound it’s a hundred times louder than it was before it began bouncing around inside our ears. When you consider how little energy is released by a pin falling onto the floor, the amplification power of our ears is clear. Indeed, since so much of what the ear accomplishes involves making noise louder, it’s unsurprising that a majority of hearing problems represent an inability not to perceive sound but to properly amplify it.
    People like to distinguish between the ears and the eyes by saying that the latter have lids. But, in fact, the amplification function of the middle ear is complemented by a series of equivalent mechanisms that mitigate the effects of a loud noise. Our middle-ear bones have small muscles attached to them that are part of a reflex to reduce the vibration of the bones under the impact of a loud sound. One of them jerks on the eardrum itself so that it tightens and vibrates less violently. Another yanks the stirrup back from the oval window. The eustachian tube performs a complicated maneuver to equalize air pressure. But why amplify to begin with if you’re only going to end up deadening the noise?
    Because in nature, there aren’t very many loud sounds.
    “Most animals don’t announce their presence if they can help it,” Heffner told me. “Even the famous roar of the lion is an exceptional event to threaten an intruder.” For the most part, animals move through space as quietly as possible. Today peoplemake noise to reassert their importance, but for our predecessors silence was almost always the secret to survival. “That’s why kids today are at such risk,” Heffner added. “I can guarantee you they’re going to have hearing loss, because when