| a. |
State
and explain the differences between sound intensity and sound intensity
level. |
4
marks |
| |
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|
Sound intensity
at a point is the sound energy arriving at unit area per unit time. |
1 |
|
Mathematically, if the sound power passing
perpendicularly through a surface of area A is P, then |
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 |
0.5 |
|
The range of intensity is very large, from
the minimum detectable intensity of 10-12 Wm-2 to
the threshold of pain of 102 Wm-2. This is inconvenience
and the data are difficult to understand. |
1 |
|
Sound intensity level
uses a logarithmic scale to give manageable data. It is defined for a sound
compared to the minimum detectable intensity as |
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 |
1 |
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where Io is minimum detectable
intensity having a value of 10-12 Wm-2. |
0.5 |
| |
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| b. |
Give
brief accounts on the threshold of hearing and the threshold of pain, stating
the numerical values associated with them. State the approximate values
of decibel for the following sources of sound:
i)
normal conversation at 1 m |
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|
ii)
a MTR train approaching a station |
|
|
iii)
in a rock music concert. |
6
marks |
| |
|
|
|
Threshold of hearing refers to the minimum detectable sound
intensity. |
1 |
|
Its value for an average person is 10-12 Wm-2
or 0 dB. |
0.5 |
|
Threshold of pain refers to the sound intensity that is
so large to cause pain or even permanent loss of hearing. |
1 |
|
Its value for an average person is 1 Wm-2 to
100 Wm-2 or 120 dB to 140 dB. |
0.5 |
|
The approximate range for |
|
|
i) normal conversation is 20 dB to 60
dB |
1 |
|
ii) a MTR train approaching a station is 60
dB to 80 dB |
1 |
|
iii) a rock music concert is 80 dB to 120 dB |
1 |
| |
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| c. |
State
three major causes of noise pollution in the streets in Hong Kong. Suggest
ways to reduce the noise pollution, assuming that you are
i)
a mechanical engineer |
6
marks |
| |
|
|
|
The major causes of noise pollution are |
|
|
-
people talking/shouting to each other
-
engines of moving cars
-
construction sites
-
airplane
-
radio, TV, HiFi, speakers from shops
-
horns from cars
-
mahjong from home
|
any 3 |
|
Mechanical engineer |
|
|
Unwanted sound from a machine, like a car engine,
can be reduced by using fibreglass as the shell, which can efficiently
convert the sound energy into internal energy. |
1 |
|
In addition, the sound energy can be transferred
to a resonating air-cavity, where a high percentage of sound is trapped. |
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ii)
a civil engineer |
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| |
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Civil engineer |
|
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Noise due to traffic can be reduced by erecting barriers
alongside the road. |
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Double-glazing windows are good sound-proofing
equipment for schools and offices very close to highways. |
1 |
| |
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iii)
a lawyer. |
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| |
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Lawyer |
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Factory owners should be responsible for keeping
the noise level within some acceptable values. |
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Shop owners are not allowed to use loudspeakers
in promoting their services/products. |
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Car owners are required to check their cars
frequently for the noise produced by the engines. Alarm must be stopped
ringing after a certain period. |
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No construction sites are allowed to operate
at night or in public holidays. |
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Paths of airplane should avoid being too close
to living districts. |
1 |