Section 3.4 Hearing and Noise
Figure 3.27 Artist’s sketch illustrating the anatomy of a human ear
(from US NIOSH, 1973).
173
Define a sound pressure level:
⎛ P ⎞
L P = 20 log10 ⎜ ⎟
⎝ P0 ⎠
(3-17)
where the term P0 is a reference value,
P0 = 2 × 10−5
N
m2
(3-18)
Define a sound intensity level (LI),
⎛ I ⎞
L I = 10 log10 ⎜ ⎟
⎝ I0 ⎠
(3-19)
where the reference value I0 corresponds roughly to the reference pressure level (P0). At STP,
I0 =
P0 2
watt
≈ 1× 10−12 2
ρa
m
(3-20)
We also define a sound power level or acoustic power level (LW),
⎛ W⎞
L W = 10 log10 ⎜
⎟
⎝ W0 ⎠
(3-21)
W0 = 1× 10−12 watt
(3-22)
where
The unit used to express the sound pressure level, sound intensity level, and sound-power level is
called the decibel (dB).
Multiplying
each
side
by
10
and
recognizing
the
definitions
of
Error! Reference source not found. and Error! Reference source not found., one obtains
L I (r) = L P (r)
Eqs.
(3-23)
It is useful at this point to define a reference distance (r0) equal to 1 meter,
r0 = 1 m
(3-24)
⎛ r ⎞
L I (r) = L W + 10 log10 Q − 11.0 − 20 log10 ⎜ ⎟
⎝ r0 ⎠
(3-26)
⎛ r ⎞
L P (r) = L W + 10 log10 Q − 11.0 − 20 log10 ⎜ ⎟
⎝ r0 ⎠
(3-27)
Then,
or,
sound pressure level
(dB, relative to 2 x 10-5 N/m2)
sound power
(watts)
sound pressure
(N/m2)
examples
turbojet engine
104
160
2000
103
150
633
102
140
200
101
130
63.3
100
120
20
10-1
110
6.33
10-2
100
2
air compressor
pneumatic chipper at 5 ft
rock-n-roll band
textile loom
lawn mower at operator’s ear
10-3
90
0.633
10-4
80
0.2
10-5
70
0.0633
10-6
60
2 x 10-2
10-7
50
6.33 x 10-3
10-8
40
2 x 10-3
10-9
30
6.33 x 10-4
10-10
20
2 x 10-4
10-11
10
6.33 x 10-5
10-12
0
2 x 10-5
diesel truck at 40 mph at 50 ft
milling machine at 4 ft
vacuum cleaner
car at 50 mph at 50 ft
conversation at 3 ft
quiet room
Figure 3.29 Relationship between sound pressure, sound pressure level,
and sound power, and some common sources of noise
(adapted from US NIOSH, 1973).
Table 3.5 ACGIH and OSHA noise limit standards for the workplace (from
Internet websites and US Office of the Federal Register, 1988).
sound
intensity
(dBA)
80
82
85
88
90
91
92
94
95
97
100
102
105
110
115
ACGIH
exposure time
(hr)
24
16
8
4
2
1
0.5
0.25
-
OSHA
exposure time
(hr)
32
24.3
16
10.6
8
7
6
4.6
4
3
2
1.5
1
0.5
0.25 or less