HEARING (AUDITION)

Despite the claim that 90 per cent of our information input is visual, hearing is a close second to vision in terms of knowledge about the world. The reason is that the kind of information gained through hearing is extremely important, since it provides us with the cues on which social contact and communication are based, the signals that warn us of danger, and the sound of music which gives us so much enjoyment. Centuries ago Aristotle claimed that while vision is important for survival, hearing is more important for intellectual development. This opinion was shared by Helen Keller.The physical stimulus for hearing consists of waves of molecules generated by the vibration of a physical object. A wave of positive pressure is transmitted through the air at the rate of about 1100 feet per second as molecules push against neighboring molecules. But since vibrating objects move back and forth, this positive pressure is periodically followed by negative pressure—that is, a vacuum. The alternation of positive and negative pressure constitutes a sound wave.The physical properties of sound waves are frequency, intensity, and complexity. Their psychological counterparts are, roughly speaking, pitch, loudness, and timbre. Frequency is stated in cycles per second, the number of times a wave alternates between positive and negative pressure in a single second. Middle C, for example, has a frequency of 261.6 cps. When frequency is varied we experience changes in the highness or lowness of notes, or pitch. The relationship, however, is not a simple one. A tone of 400 cycles per second is experienced as a little more than twice the pitch of one 100 cps, and one of 20,000 cps is only 50 per cent higher in pitch than one of 4000 cps. It has been found that pitch depends on intensity as well as frequency, since a change in loudness can vary pitch without changing frequency at all. A train whistle sounds much higher as it passes us than at a distance.The human range for hearing loud tones is approximately 20 to 20,000 cps. The higher frequencies become progressively inaudible in the later years of life. Dogs, cats, rats, and bats have a higher range than man. Bats emit sounds at 30,000-80,000 cps and are guided by their echoes; porpoises hear sounds up to 80,000 cps, four times higher than man. Most of our speech occurs between 300 and 7000 cps, and in this range man’s sensitivity to sound is as good as any animal’s.Noise as well as tone (except for the occasional pure tone) is a mixture of sound waves; but in tones the basic vibrations are regular and periodic, while in noise they are aperiodic. Some noise, however, is usually produced along with tones; a whistle, piano, or any other instrument always emits noise of some kind. Noise has little or no tonal quality, and since it lacks a fundamental tone, it has no clearly defined pitch.The intensity of sound is determined by the height or “amplitude” of the sound-wave curve. This characteristic represents the magnitude of the pressure changes and is measured in decibels on a log scale because the intensity range of the human ear is so great. The most intense sound we can respond to is five million times as loud as the softest. Normal conversation is about 60 decibels above threshold; loud thunder is 120 db. Sounds in the 120- 140 db. range are usually painful and can cause “exposure” deafness. Airplanes, a speeding subway train, or very heavy traffic, produce sound greater than 80 db if the hearer is relatively near the source. These sounds can also be painful if the individual is exhausted or under emotional tension.Intensity is a physical measure; the corresponding psychological dimension is loudness. This characteristic is particularly important in music and the expression of emotion. Gentleness is expressed by sounds of low intensity, as in whispers or lullabies; fear and anger by screams and shouts. Variations in HEARING loudness are important in public speaking, but equally important are variations in pitch and rate of speech, the modulations and inflections the speaker uses to convey meaning, emphasis and emotional expression. Together, these qualities constitute the speaker’s “dynamic range.” Musicians as well as speakers should also remember that very low and very high tones have to be sounded with much more intensity than tones in the middle range to make them audible.Practically all the sounds we hear are made up of complex waves; we rarely encounter pure tones. In any complex sound, the lowest frequency wave is called the fundamental, and waves which are multiples of this frequency are called harmonics. The psychological counterpart of tonal complexity is timbre, or tonal quality. This dimension enables us to distinguish musical instruments from each other and accounts for the infinite diversity of human voices. Sounds of the same pitch and loudness often have marked timbre differences: the same note sounds quite different when sung by two different singers or when played on a clarinet and a French horn. The reason is that the fundamental tone is accompanied by different harmonics and overtones generated by the structure of the instruments or the singers’ vocal apparatus.The human ear (Fig. 28) evolved from structures closely related to the static or equilibrium mechanisms in lower animals. The outer ear, or “pinna,” is actually the vestige of an organ that originally helped to collect sounds. It is of little use in man and is important only in animals that can move their ears toward the sound source without turning the head. TheFig. 28. A cross-section of the human ear. Note that the auditory nerve has two branches, one from the cochlea, which contains cells that respond to auditory vibrations transmitted through the eardrum and ossicles; the other from the semicircular canals, which serve the sense of equilibrium. sound wave moves through the auditory canal to a thin membrane, the eardrum, which is so sensitive that it responds to wave pressures that move it less than one billionth of an inch. Next, the impulse is conducted from the drum through the middle ear via the ossicles, which consist of three tiny connected bones, the hammer (malleus), anvil (incus), and stirrup (stapes). A membrane called the oval window separates the middle and inner ear, and the impulse is conveyed through it to the cochlea, a structure shaped like a snail.The cochlea is filled with a fluid which responds to the movement of the ossicles, producing pressure changes that in turn stimulate some of the 24,000 hair cells of the organ of Corti. These cells vary in length like the strings of a harp; some respond to higher, others to lower frequencies. The impulses they generate are then carried to the auditory cortex of the brain, which apparently sorts them out and registers and interprets the different sounds. There is, however, considerable difference of opinion as to whether this process can fully account for the full range of hearing