Download Noise Pollution

Concise Environmental
Engineering
Noise
Pollution
Noise Pollution
Noise is defined as unwanted sound in environmental engineering. This is in contrast to noise
definitions in other fields where noise usually means unwanted information or data that is not
relevant to the hypothesis or theory being investigated. For example, in telecommunication, noise is
the unwanted random addition to a signal. In relation to sound, noise is not necessarily random (e.g.,
loud music from neighbours). Noise’s effects include moderate annoyance to permanent hearing loss
and may be rated differently by different observers. Nowadays, noise is pervasive and is almost
impossible to escape from it. It is important for environmental engineers to understand noise so that
its effect could be mitigated at design and operation of various buildings and machines.
Sources of Noise
Noise can be from a point source (e.g., a loudspeaker), a line source (e.g., power line under
strong wind) or an area source (e.g., wind noise from a forest). They include sources from road traffic,
aircraft, industrial plant, construction activities, sport and crowd activities, loud music from neighbours,
etc. Poor urban planning may give rise to noise pollution, since side-by-side industrial and residential
buildings can result in noise pollution in the residential area. Inadequate sound proof of buildings also
contributes to noise problems.
In time, noise sounds can be classified as 1) continuous: an uninterrupted sound during the period
of observation; 2) intermittent: a continuous sound with interruptive gaps, such as dentist drill; 3)
impulsive: a sound of short duration, usually less than a second, such as gunfire.
In contrast to water and air pollutions, noise pollution has some unique characteristics: 1) the
unwanted sound can be subjective so it may mean the wrong sound in the wrong place at the wrong
time. Therefore, any sound could be noise; 2) noise pollution is usually local. Sound intensity follows
an inverse square law with distance from the source; doubling the distance from a noise source
reduces its intensity by a factor of four; and 3) there is no residual pollution after the noise source is
removed.
Physical Properties of Noise
Concise Environmental
Noise
Engineering
Pollution
The ‘noise’ sound is a wave described by: wavelength (l), frequency (f) and speed (c) which are related by
c=f
(10.1)
The speed of sound in air at sea level at 20oC is about 340m/s. It travels faster in water (about 1500m/s)
and solids (e.g., 5000m/s in iron). The human hearing frequency range is from 20 to 20,000 Hz.
The formula for the sum of the sound pressure levels of several incoherent radiating sources is
 P 2 + P 2 + ...P 2 
L = 10 log

.
10


1
2
Po

(dB)
10
n

(10.
4)
Concise Environmental
Engineering
Noise
Pollution
The typical sound pressure levels are listed in the following Table
Examples of sound pressure levels
Sound in air
Sound pressure level (dB)
Auditory threshold at 1kHz
0
Calm breathing
10
Very calm room
20-30
Normal conversation at 1m
40-60
TV at 1m
60
EPA noise limit
70
Passenger car at 10m
60-80
Threshold of pain
130