attributed to navy sonar, the vibrations from which literally cause
heads to explode. In the remote backcountry of Yosemite National
Park, aircraft are audible 70 percent of the time, raising ambient noise
levels by about 5 decibels. That’s enough to reduce the distance at
which prey species can hear a predator approaching by 45 percent.
Lab experiments show that when female gray tree frogs hear traffic
noise, it takes them longer to find males who are calling to mate, if
they can find them at all. No backseat romance for them.
Sound is designed to be processed swiftly by the brain. Sound
waves travel through the air and collide with our eardrums, which
wiggle back and forth in response to volume and amplitude. Nerve
cells pick up these perturbations and send signals to our auditory
cortex, the brain stem and the cerebellum, which together process
fear, arousal and motion. As to the perennial question of whether a
tree falling in the forest makes a sound if no one is there to hear it
(first posed by Irish philosopher George Berkeley), the answer is
technically no. There is no sound apart from a sentient brain’s
interpretation of molecules vibrating through air or water. The brain
turns those molecules hitting the eardrums and pinnae into a mental
idea of sound. Birds will hear the toppling tree, and fish will hear it
too. But there is no thing called sound unless the vibrating molecules
are processed into pitch.
Hearing evolved well before vocalization, and eventually became
useful for communication. It’s difficult to know which came first in
evolution: the ability to hear or the ability to see, but fish are thought
to have developed vibration-sensitive hairs hundreds of millions of
years ago, before they could see. The fancy three-boned middle ear of
mammals is—along with mammary glands—our defining trait. In the
womb, we can hear before we can see. By birth, hearing is our most
fully developed sense. Because sound waves vibrate through bones
and the brain (the frequency of a violin note, for example, will cause
neurons in the auditory cortex to fire at exactly that frequency) it is a