Environmental Engineering Reference
In-Depth Information
discomfort. When the sound is judged by the brain to be unwanted, discordant, or
disagreeable, then we commonly describe it as noise.
The two major properties of sound waves, independent of whether the sound is
deemed wanted or unwanted, are the speed at which the sound vibrates and the
intensity of the vibrations. The frequency of vibration defines the pitch of the sound
and is measured in hertz (Hz). The human ear responds to sound between 20 and
20,000 Hz but the body may respond to sound below 20 Hz. It is essentially the
intensity of each vibration that gives us the sense of loudness. Loudness of sound is
measured on a decibel (dB) scale, which is based on the physical measurement of
sound pressure. The dB scale is not linear but rather logarithmic (based on powers
of ten) so that an increase in 10 dB represents a sound that is perceived as being
twice as loud as the preceding sound. The ear is not equally sensitive to sounds
at all frequencies. Thus, a special frequency-dependent scale (designated dBA) has
been devised to compensate for this variable sensitivity. The dB scale ranges from
zero, the lowest point, to over 170 dB. Whispers can be measured at 20 dBA, a quiet
home between 30 and 40 dBA, normal conversation at 60 dBA, and some household
appliances can range between 60 and 85dBA. New York City subway stations can
reach over 90 dBA, rock concerts over 120 dBA, jet take-offs over 140 dBA, and
military weapons may exceed 170 dBA.
Noise has been generally been understood to be unwanted, uncontrollable, and
unpredictable sound that is interpreted by the listener at the cognitive level as intru-
sive. A sound need not be loud to be intrusive. The dripping faucet is noise while
you are trying to fall asleep. Your upstairs neighbor's footsteps produce noise that
awakens you. The backup beeps of the garbage truck below your window in the
morning are also intrusive. Certain sounds judged to be intrusive by some may be
pleasant to others. For example, the music emanating from a parked or passing vehi-
cle that interrupts your sleep or study may be highly enjoyable to the occupants of
the vehicle. Some have thought it difficult to assess the influence of sound on the
physiological well-being of humans because individuals differ in their interpretation
of - and response to - the sound that surrounds them. To the contrary, for those lis-
teners who define the incoming sounds as noise, one can still measure the adverse
effects just as we ascertain the effects of allergens on those who are susceptible.
4.3 Effects of Noise on Hearing
Sound, whether deemed to be wanted or unwanted, can indeed harm the listener. It
is generally accepted that a loud sound can damage the inner ear, which, in turn, can
lead to a loss of hearing. There is general agreement that exposure to sound less than
70 dBA does not lead to hearing loss. When levels of sound exceed 85 dBA, espe-
cially when exposure lasts for more than eight hours, there is the potential of hearing
loss [6-11]. However, a single exposure to a very loud sound can impair hearing.
It has been said that former President Ronald Reagan lost some of his hearing on a
movie set when a loud gun rang out next to his ear.
