HEARING
Sound is a series of vibrations moving as waves through air or other gases, liquids, or solids. A ringing bell, for example, sets off vibrations in the air. Detection of these vibrations, or sound waves, is called hearing. The detection of vibrations passing through the ground or water is also called hearing. Some animals can detect only vibrations passing through the ground, and others can hear only vibrations passing through water.
Humans, however, can hear vibrations passing through gases, solids, and liquids. Sometimes sound waves are transmitted to the inner ear by a method of hearing called bone conduction. For example, people hear their own voice partly by bone conduction. The voice causes the bones of the skull to vibrate, and these vibrations directly stimulate the sound-sensitive cells of the inner ear. Only a relatively small part of a normal person’s hearing depends on bone conduction. If you crunch a biscuit in your mouth, for instance, with your mouth tightly closed, you can hear the noise made by your chewing act through bone conduction.
Humans hear primarily by detecting airborne (i.e., carried by air molecules) sound waves, which are collected by the auricles. The auricles also help locate the direction of sound. Although some people have auricular muscles so well developed that they can wiggle their ears, human auricles, when compared to those of other mammals, have little importance. Many mammals, especially those with large ears, such as rabbits, can move their auricles in many directions so that sound can be picked up more easily.
After being collected by the auricles, sound waves pass through the outer auditory canal to the eardrum, causing it to vibrate. The vibrations of the eardrum are then transmitted through the ossicles, the chain of bones in the middle ear. As the vibrations pass from the relatively large area of the eardrum through the chain of bones, which have a smaller area, their force is concentrated. This concentration amplifies, or increases, the sound.
When the sound vibrations reach the stirrup, the stirrup pushes in and out of the oval window. This movement sets the fluids in the vestibular and tympanic canals in motion. To relieve the pressure of the moving fluid, the membrane of the oval window bulges out and in. The alternating changes of pressure in the fluid of the canals cause the basilar membrane to move. The organ of Corti, which is part of the basilar membrane, also moves, bending its hairlike projections. The bent projections stimulate the sensory cells to transmit impulses along the auditory nerve to the brain.
Human ears are capable of perceiving an extraordinarily wide range of changes in loudness, the tiniest audible sound being about one trillion times less intense than a sound loud enough to cause the ear pain. The loudness or intensity of a noise is measured in a unit called the decibel. The softest audible sound to humans is 0 decibels, while painful sounds are those that rise above 140 decibels. The decibel is a unit of measure (abbreviated dB) originally used to compare sound intensities and subsequently electrical or electronic power outputs. Today, it is also used to compare voltages. An increase of 10 dB is equivalent to a 10-fold increase in intensity or power, and a 20-fold increase in voltage. A whisper has an intensity of 20 dB. A jet aircraft taking off nearby (140 dB) is the threshold of pain.
Besides loudness, the human ear can detect a sound’s pitch, which is related to a sound’s vibration frequency, or the number of sound waves passing into the ear in a given period. The greater the frequency, the higher the pitch. The maximum range of human hearing includes sound frequencies from about 15 to about 18,000 waves, or cycles, per second (i.e., 15 Hz to 18000 Hz). Because the human ear cannot hear very low frequencies, the sound of one’s own heartbeat is inaudible. At the other end of the scale, a highly pitched whistle producing 30,000 cycles per second is not audible to the human ear, but a dog can hear it.
The third characteristic of sound detected by the human ear is tone. The ability to recognize tone enables humans to distinguish a violin from a clarinet when both instruments are playing the same note. The least noticeable change in tone that can be picked up by the ear varies with pitch and loudness.
Another sonic phenomenon, known as masking, occurs because lower-pitched sounds tend to deafen the ear to higher-pitched sounds. To overcome the effects of masking in noisy places, people are forced to raise their voices.
The ear, oddly enough, not only receives sounds but can generate its own noises, called spontaneous otoacoustic emissions. About 30 percent of all people experience them. Some hear their ears' sounds, typically a faint buzzing, if they are in a completely quiet room, but most people do not notice them at all. In particularly bad cases, people have walked into doctors' offices complaining of a ringing in their ears, and the doctors could hear the noises while standing next to the patients. Such sounds, while certainly annoying, have never been thought to be more than that. But psychologists at the State University of New York at Buffalo have found that otoacoustic emissions may actually produce an unusual type of hearing loss, one that occurs without physical damage to any parts of the ear.
The researchers' evidence comes not from humans but from chinchillas. These small rodents are prized by some people for their long, silky fur, but to a sensory psychologist they have a different appeal: they have a hearing range similar to that of humans. Psychologist Salvi and his colleagues were experimenting with a chinchilla when, with a small microphone, they discovered a sound coming from the animal's ear. The rodent's ears were producing otoacoustic emissions at a frequency of 4,000 hertz (vibrations per second).
To see what effect these sounds might have on the chinchilla's hearing, the researchers placed microelectrodes in the animal's auditory nerve (an experiment they couldn't have done with humans) to test it for sensitivity to specific frequencies. They found that the chinchilla was insensitive to external sounds at frequencies around 4,000 hertz. The constant buzz from its own ears was apparently causing its brain to ignore that frequency.
Otoacoustic emissions are thought to be produced by delicate microscopic hair cells in the outer ear. Why they are produced by chinchillas and people, and why they are produced by some people and not by others, remains largely a mystery. But Salvi now thinks the sounds create a sort of busy signal in the auditory nerve. “The ear is the cause of its own hearing loss,” he says. “The hearing loss is the result of the masking effect only, and there is no cellular or long-term damage to the ear.” Salvi next hopes to find out whether humans experience similar types of hearing loss.
