TWA - Time Weighted Average Noise Levels - and Noise Dose
The TWA shows a worker's daily exposure to occupational noise (normalised
to an 8 hour day), taking into account the average levels of noise and
the time spent in each area. This is the parameter that is used by the
OSHA Regulations and is essential in assessing a workers exposure
and what action should be taken.
Working Out the Noise Dose and TWA
Before working out the worker's TWA you have to measure the different
high noise levels that the worker is subjected throughout a normal
working day. The Time Weighted Average is calculated using these
noise levels together with the amount of time that the worker is
exposed to them. |
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TWA
Calculator |
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| First calculate the Noise Dose as: |
Dose = 100 x (C1/T1 + C2/T2 + C3/T3 + ... + Cn/Tn) |
where
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Cn = |
time spent at each noise level |
Tn = |
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(L is the measured sound level) |
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It is often easier to get Tn from a lookup table:
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Worked Example |
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A worker is exposed to 86 dB for
six hours and 92 dB for a three hours, giving a nine hour
working day.
Dose = 100 x (6/13.9 + 3/6.1) = 92.3% |
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L |
T |
L |
T |
L |
T |
L |
T |
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80 |
32.0 |
90 |
8.0 |
100 |
2.0 |
110 |
0.50 |
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81 |
27.9 |
91 |
7.0 |
101 |
1.7 |
111 |
0.44 |
|
82 |
24.3 |
92 |
6.1 |
102 |
1.5 |
112 |
0.38 |
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83 |
21.1 |
93 |
5.3 |
103 |
1.3 |
113 |
0.33 |
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84 |
18.4 |
94 |
4.6 |
104 |
1.1 |
114 |
0.29 |
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85 |
16.0 |
95 |
4.0 |
105 |
1.0 |
115 |
0.25 |
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86 |
13.9 |
96 |
3.5 |
106 |
0.87 |
116 |
0.22 |
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87 |
12.1 |
97 |
3.0 |
107 |
0.76 |
117 |
0.19 |
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88 |
10.6 |
98 |
2.6 |
108 |
0.66 |
118 |
0.16 |
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89 |
9.2 |
99 |
2.3 |
109 |
0.57 |
119 |
0.14 |
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Once you have the Dose% figure, you can calculate the TWA using the
following equation:
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TWA = 16.61 Log10 (D/100) + 90 |
where |
TWA |
is the 8-hour Time Weighted Average Sound Level |
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D |
is the Dose % as calculated above (or measured with a dosimeter) |
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Log10 |
is the Logarithm to base 10 |
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Worked Example continued... |
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From our example above
TWA = 16.61 x Log10 (92.3 / 100) + 90 = 89.4 dB |
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The OSHA action levels are based on either TWA or
Dose % (which are different representations of the same number). These
action levels are 85 dB (or 50% Dose) and 90 dB (or 100% Dose).
Too Complicated?
For workers who are moving between many different noisy locations it
is usually easier to use a Noise Dosimeter.
This device is attached to the worker at the start of the day and left
to monitor the actual noise exposure. The dosimeter will usually provide
you with the TWA and the Dose %, so there is no need to make any calculations. < OSHA Noise Regulations |
