Noise
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Frequency
The frequency of a sound wave is expressed in hertz (Hz), where one hertz is
equal to one cycle per second.The range of frequencies that give rise to
the sensation of pitch in humans extends roughly from 20Hz to 20,000 Hz,
with much individual variation. If the diaphragm of a loudspeaker were to
move back and forth at 262 Hz (262 oscillations per second), a listener would
perceive a tone having the pitch of middle - C.
The range of hearing (Stevens, 1951).
In practice sounds are seldom composed of only a single frequency. They are
instead a combination of many frequencies. The distribution of acoustical energy
as a function of frequency is generally referred to as a spectrum. The
spectrum of the sound from a modern orchestra, for instance, will show energy
concentrated from about 25 Hz to 5,000 Hz; the noise in a large computer room,
from 30 Hz to 10,000 Hz. Human listeners perceive the spectrum of a sound as
its quality or timbre. A violin playing middleC and a trumpet
playing middleC have different qualities because their spectra are different.
The Ear
The human ear is a marvel of natural engineering. Its structure is too complex
to discuss in detail here, but its most essential properties are the following.
Sound enters the ear canal of the outer ear and causes the eardrum to
vibrate in response. These vibrations are coupled through three tiny bones
in the middle ear to the main sensory organ in the inner ear,
or cochlea. Hair cells in the cochlea transmit nerve impulses along
the auditory nerve to the brain, where they are interpreted as sound. Although
there are several types of hearing loss, it is damage to the hair cells of
the cochlea that is usually associated with prolonged exposure to loud noises
(see, for instance Kryer, 1985).
The human ear does not respond to the pressure and frequency of a sound wave
in the same way that an electronic measuring instrument does. A sound level
meter might indicate the sound pressure level of a 60Hz tone to be 50 dB
and the level of a 1000Hz tone to be 25 dB, yet to a listener the tones would
be perceived as equally loud. This is because the sensitivity of the ear is
not uniform. It varies with frequency, as well as with level. At low sound
pressure levels, we tend to hear midfrequencies somewhat better than higher
frequencies, and much better than lower frequencies. At high sound pressure
levels, the variation is not as pronounced and we tend to hear low, mid, and
high frequencies about the same. (This is why, when listening to a musical
recording, we "lose the bass" when the volume is turned down low.)
Figure 58 illustrates the relative response of the ear at
the two loudness extremes, the threshold of
discomfort and the threshold of audibility (see, for instance, Fletcher & Munson,
1933; Stevens, 1951, and Robinson & Dadson, 1956).
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