769NOISE
Noise and sound refer to audible pressure fluctuations in air.
Both are characterized by sound level in decibels and fre-
quency content in hertz. Although sound is vital for commu-
nication, noise is one of our greatest problems. Intentionally
generated acoustic signals including speech and music are
usually referred to as sound. Noise is a term used to identify
unwanted sound, including sound generated as a byproduct
of other activities such as transportation and industrial opera-
tions. Intrusive sound, including speech and music unwelcome
to the hearer, are also considered noise. Thus, the distinction
between noise and sound is subjective, and the two terms are
often used interchangeably.
When a body moves through a medium or vibrates, some
energy is transferred to that surrounding medium in the form
of sound waves. Sound is also produced by turbulence in air
and other fluids, and by fluids moving past stationary bodies.
In general, gases, solids and liquids transmit sound.
Well-documented effects of noise include hearing
damage, interference with communication, masking of
warning signals, sleep interruption, and annoyance. Noise
detracts from the quality of life and the environment; it con-
tributes to anger and frustration and has been implicated as a
contributor to psychological and physiological problems.
The National Institute for Occupational Safety and
Health (NIOSH) named hearing loss as a priority research
area, noting that noise-induced hearing loss is 100% prevent-
able, but once acquired, it is permanent and irreversible. The
Occupational Safety and Health Administration (OSHA)
noted that hearing loss can result in a serious disability, and
put employees at risk of being injured on the job. The World
Health Organization (WHO) notes that noise-induced hearing
impairment is the most prevalent irreversible occupational
hazard, and estimates that 120 million people worldwide
have disabling hearing difficulties. In developing countries,
not only occupational noise but also environmental noise is
an increasing risk factor for hearing impairment.
The European Union (EU) identified environmental
noise caused by traffic, industrial and recreational activities
as one of the main local environmental problems in Europe
and the source of an increasing number of complaints. It is
estimated that 20% of the EU population suffer from noise
levels that both scientists and health experts consider unac-
ceptable. An additional 43% of the population live in ‘gray
areas’ where noise levels cause serious daytime annoyance.
Estimates of the cost of noise to society range from 0.2% to
2% of gross domestic product.Noise control involves reduction of noise at the source,
control of noise transmission paths, and protection of the
receiver. Source control is preferred. For example, design of
transportation systems and machinery for lower noise output
may be the most effective means of noise control. But, after
trying all feasible noise source reduction, airborne noise and/
or solid-borne noise may still be objectionable. Interruption
of noise transmission paths by means of vibration isolation,
source enclosures, sound absorbing materials, or noise barri-
ers is then considered.
In some industrial situations, excessive noise is still pres-
ent after all attempts to control noise sources and transmission
paths. Administrative controls—the assignment of employees
so that noise exposure in reduced—should then be consid-
ered. As a last resort, employees may be required to use per-
sonal hearing protection devices (muff-type and insert-type
hearing protectors). Communities often resort to ordinances
that limit noise levels and restrict hours of operation of noise-
producing equipment and activities. Community noise con-
trol methods also include zoning designed to separate noise
sources from residential and other sensitive land uses.FREQUENCY, WAVELENGTH AND PROPAGATION
SPEEDFrequency. Audible sound consists of pressure waves with
frequencies ranging from about 20 hertz (Hz) to 20,000 Hz,
where 1 Hz 1 cycle per second. Sound consisting essen-
tially of a single-frequency sinusoidal pressure wave is called
a pure tone. In most cases, noise consists of sound waves
arriving simultaneously from a number of sources, and
having a wide range of frequencies. A sound wave which has
a frequency below the audible range is called infrasound
and sound of frequency above the audible range is called
ultrasound.
Propagation speed. The propagation speed of airborne
sound is temperature dependent. It is given by:c 20.04[T 273.16] 1/2 (1.1)where c propagation speed, i.e. the speed of sound, (m/s)
T air temperature (C).
At an air temperature of T 20 C (68F), the propaga-
tion speed is c 343 m/s (approx). Sound waves propagate
at a different speed in solids and liquids. The propagation
speed for axial waves in a steel rod is about 5140 m/s. Note thatC014_003_r03.indd 769C014_003_r03.indd 769 11/18/2005 10:46:06 AM11/18/2005 10:46:06 AM