However, because sound can set bodies vibrating, it can pack an incredible wallop. It can do work, it can produce heat, it can be reflected, bent, and absorbed.
Given the proper combination of acoustic energy and frequency, sound can destroy rock formations, mix paints, crack plaster, break windows, and wash dishes. In one classic experiment, exposure to intense sound lit a pipe and brewed a cup of coffee in seven minutes.
We live in a sea of sound waves, the vibrations of which may be as slow as three per second or as rapid as millions per second. The most familiar response to these vibrations is the sensation of hearing. Though the entire body "senses" the vibrations, the human sense of hearing responds only to the ones that fall within the range between 20 cycles per second to somewhere in the region of 15,000 cycles per second. (Remember, this is different from the decibel scale, which measures sound intensity, not frequency.) Below 20 cps is infrasound, sound of such low vibration it is inaudible though sensed as a vibration. Sound above 15,000 cps, called ultrasound, is inaudible to most people in industrialized countries.
We are able to "hear" because among the human senses is the ability to detect the very small and rapid fluctuations in the pressure of the air called sound waves. The detection apparatus is called the ear. It is this organ that first bears the brunt of acoustic abuse.
The basic hearing mechanism of the ear involves: the outer ear (external ear or pinna plus the external auditory canal), the middle ear, about one-third of an inch long (containing the familiar hammer, anvil, and stirrup bones), and the inner ear, a system of cavities lying within and protected by dense bone and containing the all-important cochlea. This apparatus must transmit to the brain an accurate pattern of all sound vibrations received from the environment. The human outer ear, from pinna to eardrum, is approximately four centimeters long. (The elephant's ear canal is eight inches long.) The middle ear has been described as so small it can be filled with five or six drops of water. The inner ear is no bigger than the tip of a little finger. All three parts together are approximately one and one-half inches long, an example of natural miniaturization. The outer ear serves as more than the collecting point for the sound waves. The ear canal can amplify the intensity of certain pitches by means of sympathetic resonance. This is one explanation for the amazing sensitivity of the ear to the sound vibrations. It's almost as if the ear had a hunger for sound, so great is its sensitivity.
Sound vibrations are condensed, and conducted by the ear canal to the eardrum, a membrane about one-forth of an inch in diameter. The resulting pulsations of the eardrum activate the three tiniest bones in the body, the malleus, incus, and stapes (more familiarly, the hammer, anvil, and stirrup). The three bones of the middle ear serve as a bridge between the eardrum and a membrane at the entrance to the inner ear (the oval window). The stapes, last link of the bony bridge, is attached to the oval window and causes it to vibrate. The vibrations of the oval window set up vibrations in the fluid in the two canals of the cochlea which surround the organ of Corti. Thus the three parts of the ear convert the mechanical waves of airborne sound energy into waves in liquid, and finally into electrical impulses.
It is in the cochlea of the inner ear that, via the all-important organ of Corti, conversion from mechanical to electrical energy takes place. Imbedded in the organ of Corti are some twenty to thirty thousand sensory cells, each of which is capped with fine hair, or cilia. Each hair cell of the inner ear responds only to a specific frequency. The cilia sensitive to high frequencies are at the beginning of the snail-like cochlea, and the low-frequency sensors are at the apex, or far end of the spiral.
Each hair is joined together with the others to become the auditory nerve. By some mechanism, not yet fully understood, the wave-like motion of the hair cells sets up a "coded" electrical signal that is transmitted to the auditory center of the brain.