9. Noise characteristics
9.1. Sound composition :
The physical amount of sound is expressed in a logarithm and the sound level (dB) is calculated by simple
addition or subtraction.
Lc, the sum of n sounds L1, L2, ....Ln (dB) is calculated by the following equation.
Lc = 10 log (10
L1
/10 + 10
L2
/10 + ..... 10
Ln
/10)
Generally, the noise level by composing two sounds of L1 (dB) and L2 (dB) is found in the following figure.
Example) When composing 60dB and 57dB sounds, the level of
complex sound is calculated as follows.
L1-L2=60-57=3
n the left figure,
∆
L
¡
1.7
Complex sound level : L3 = L1+
∆
L
= 60+1.7=61.7dB
L3 = L1+10log(1+10
-(L1-L2)
/10)
= L1+
∆
L(L1
≥
L2)
The equation is applied to the sound decomposition method.
When complex sound (measured sound) is L3(dB) and
background noise is L2(dB), the object sound L1 (dB) is
calculated as follows.
L1= L3 + 10 log (1-10
-(L3-L2)
/10)
= L3 -
∆
L'(L3
≥
L2)
Example) While machine sound is measured 65dB, the noise at
machine stopping is 63dB.
Accordingly the noise level of machine itself is
L3-L2 = 65-63 = 2
In the right figure,
∆
L'
¡
4.33
Noise level of machine is
L1 = L3-
∆
L' = 65-4.33
¡
60.7
(Figure: Calculation chart of dB composition)
(Figure: Calculation chart of dB decomposition)
3.0
2.8
2.4
2.0
1.5
1.2
0.8
0.4
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
L
1
L
2
(dB)
∆
L
6.87
4.33
3.00
2.19
1.65
1.25
1
0
1
2
3
4
5
6
7
2
3
4
5
6
7
8
9
10
L
3
L
2
11 12
0.96
0.36
0.46
0.58
0.75
0.26
(dB)
∆
L'
TCL Air Conditioner Service Manual
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