Hearing

The ability to perceive sound.

The ear, the receptive organ for hearing, has three major parts: the outer, middle, and inner ear. The pinna or outer ear—the part of the ear attached to the head, funnels sound waves through the outer ear. The sound waves pass down the auditory canal to the middle ear, where they strike the tympanic membrane, or eardrum, causing it to vibrate. These vibrations are picked up by three small bones (ossicles) in the middle ear named for their shapes: the malleus (hammer), incus (anvil), and stapes (stirrup). The stirrup is attached to a thin membrane called the oval window, which is much smaller than the eardrum and consequently receives more pressure.

As the oval window vibrates from the increased pressure, the fluid in the coiled, tubular cochlea (inner ear) begins to vibrate the membrane of the cochlea (basilar membrane) which, in turn, bends fine, hairlike cells on its surface. These auditory receptors generate miniature electrical forces which trigger nerve impulses that then travel via the auditory nerve, first to the thalamus and then to the primary auditory cortex in the temporal lobe of the brain. Here, transformed into auditory but meaningless sensations, the impulses are relayed to association areas of the brain which convert them into meaningful sounds by examining the activity patterns of the neurons, or nerve cells, to determine sound frequencies. Although the ear changes sound waves into neural impulses, it is the brain that actually "hears," or perceives the sound as meaningful.

The auditory system contains about 25,000 cochlear neurons that can process a wide range of sounds. The sounds we hear are determined by two characteristics of sound waves: their amplitude (the difference in air pressure between the peak and baseline of a wave) and their frequency (the number of waves that pass by a given point every second). Loudness of sound is influenced by a complex relationship between the wavelength and amplitude of the wave; the greater the amplitude, the faster the neurons fire impulses to the brain, and the louder the sound that is heard. Loudness of sound is usually expressed in decibels (dB). A whisper is about 30 dB, normal conversation is about 60 dB, and a subway train is about 90 dB. Sounds above 120 dB are generally painful

to the human ear. The loudest rock band on record was measured at 160 dB.

Pitch (how high or low a tone sounds) is a function of frequency. Sounds with high frequencies are heard as having a high pitch; those with low frequencies are heard as low-pitched. The normal frequency range of human hearing is 20 to 20,000 Hz. Frequencies of some commonly heard sounds include the human voice (120 to approximately 1,100 Hz), middle C on the piano (256 Hz), and the highest note on the piano (4,100 Hz). Differences in frequency are discerned, or coded, by the human ear in two ways, frequency matching and place. The lowest sound frequencies are coded by frequency matching, duplicating the frequency with the firing rate of auditory nerve fibers. Frequencies in the low to moderate range are coded both by frequency matching and by the place on the basilar membrane where the sound wave peaks. High frequencies are coded solely by the placement of the wave peak

Loss of hearing can result from conductive or sensorineural deafness or damage to auditory areas of the brain. In conductive hearing loss, the sound waves are unable to reach the inner ear due to disease or obstruction of the auditory conductive system (the external auditory canal; the eardrum, or tympanic membrane; or structures and spaces in the middle ear). Sensorineural hearing loss refers to two different but related types of impairment, both affecting the inner ear. Sensory hearing loss involves damage, degeneration, or developmental failure of the hair cells in the cochlea's organ of Corti, while neural loss involves the auditory nerve or other parts of the cochlea. Sensorineural hearing loss occurs as a result of disease, birth defects, aging, or continual exposure to loud sounds. Damage to the auditory areas of the brain through severe head injury, tumors, or strokes can also prevent either the perception or the interpretation of sound.