Labor & Industries (L&I), Washington State

Hearing Loss Prevention (Noise) Chapter 296-817, WAC

Effective Date: 08/01/03

Chapter 296-817 WAC Helpful Tool: Noise Computation Examples

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This helpful tool gives you examples of noise computations that should assist you with your own computations. Also found in this helpful tool are examples of employer actions based on the specific noise computation results. You’ll need to do your own noise computations and determine the specific actions needed based on the noise exposures in your workplace.

EXAMPLE 1

Assume an employee is exposed to 92 dBA for eight hours. Compute the employee’s noise exposure, the time-weighted average and what action, if any, would be required of the employer.

Exposure:

The exposure time is 8 hours. The reference duration for 92 dBA is 6 hours.

Time-weighted Average (TWA):

In Table HT-2, find the values for 130% and 135%. The difference in the time-weighted average values (92.2 - 91.6) equals 0.6. Since 133% is 3/5 of the way between 130 and 135, 133% equals 3/5 (0.6) + 91.6 = 92 dBA.

Employer Action:

Since the employee’s exposure is above the 90 dBA TWA8, the employer would be required to institute a full hearing loss prevention program, including:

• Controlling noise as feasible,
• Providing hearing protection and training for employees,
  AND
• Instituting an audiometric testing program.

EXAMPLE 2

Assume a continuous noise exposure for an employee of 90 dBA and a work shift of 8 a.m. to 4:30 p.m. with a 15-minute morning and afternoon break and a 30-minute lunch. Both breaks and lunch are in an area with less than 70 dBA exposure. (Although this exposure could be integrated into the employee’s total noise exposure, it is not significant and will not be considered in these calculations.) Calculate the worker’s exposure, TWA, and the employer’s responsibility.

Exposure:

Actual exposure (subtracting the lunch time and work breaks from the employee’s work shift) indicates a 90 dBA exposure for 7 ½ hours. D = 100 (C1/T1) = 100 (7.5/8) = 94 (94%)

Time-weighted Average (TWA):

From Table HT-2 a noise exposure of 94% converts to an equivalent 8-hour time-weighted average of 89.6 dBA.

Employer Action:

Since the employee’s time-weighted average is between 85 and 90 dBA TWA8, a hearing loss prevention program must be developed and maintained for the employee including hearing protection, training and audiometric testing. Engineering and/or administrative controls are not required, but may be beneficial, since the hearing loss prevention program would no longer be required if the employee’s exposure were reduced below 85 dBA TWA8.

EXAMPLE 3

Assume a technician works in a noise enclosure booth with a noise exposure of less than 70 dBA. The technician makes rounds to read gauges and instruments that are located in an area with a noise level of 105 dBA. The technician makes four trips a day, and each trip lasts 30 minutes. Calculate the employee’s noise exposure, TWA and employer’s responsibility.

Exposure:

With four trips a day and 30 minutes per trip, the employee is basically exposed to two hours of noise at 105 dBA with the remaining time spent inside the booth. From Table HT-1 of the rule the reference duration for exposure at 105 dBA is 1 hour.

Time-weighted Average:

The employee’s TWA from Table HT- 2 is 95 dBA.

Employer Action:

Since the employee’s exposure is above the 90 dBA TWA8, the employer would be required to institute a full hearing loss prevention program, including controlling noise as feasible, providing hearing protection and training for employees, and instituting an audiometric testing program.

EXAMPLE 4

Assume a timber trimsaw operator with a background noise level inside the operator’s booth of 85 Dba, cuts one timber every 10 seconds with a noise exposure during the cut of 105 Dba for three seconds. The employee works from 6 a.m. to 4:30 p.m. and has a 15-minute break in the morning and the afternoon and a 30-minute lunch break, all of which are below 70 Dba. Calculate the employee’s noise exposure and TWA.

Exposure:

First sound level - 105 dBA

The employee is exposed to this sound level for three seconds out of every ten or 30% of the time. Thus the time of exposure (C1) at this level is 0.3 x 9.5 or 2.85 hours. From Table HT- 1, the reference duration (T1) is one hour.

Second sound level - 85 dBA

The employee is exposed to this sound level for seven seconds out of every ten or 70% of the time. Thus the time of exposure (C2) at this level is 0.7 x 9.5 or 6.65 hours. From Table HT-1, the reference duration (T2) is sixteen hours.

Time-weighted Average (TWA):

From the conversion table we find a noise dose of 327% lies between 320 and 330 with values of 98.4 dBA and 98.6 dBA respectively.

320% = 98.4 dBA
330% = 98.6 dBA
327% = (7/10)*(0.2) + 98.4 = 98.5

EXAMPLE 5

Assume a security guard works an eight-hour shift and makes eight rounds a night. In making a round of the facility the guard will spend 20 minutes in Building A, 30 minutes in Building B and 10 minutes in the yard. In Building A the noise levels are less than 70 dBA. Noise level in the yard is 85 dBA. In Building B there is a cyclic machine operation where the noise levels are:

100 dBA for 3 seconds (30%),
95 dBA for 3 seconds (30%) and
90 dBA for 4 seconds (40%);

Calculate the employee’s noise exposure and time-weighted average (TWA).

Since the employee’s noise exposure in Building A is less than 70 dBA, this exposure is not significant and will not enter into the computation (the theoretical dose would be less than 2%). In Building B we find three noise exposures, 100, 95, and 90 dBA respectively. The yard also has an exposure (85 dBA), which will enter into the total computation.

Calculating the partial exposures at each noise level we find:

At 100 dBA

At 95 dBA

At 90 dBA

The yard at 85 dBA

The employee’s total noise exposure can be calculated from the noise exposure formula using the following values.

Location	Sound Level	Time of Exposure	Reference Duration
Building B	100 dBA	C1 = 1.2 hours	T1 = 2 hours
Building B	95 dBA	C2 = 1.2 hours	T2 = 4 hours
Building B	90 dBA	C3 = 1.6 hours	T3 = 8 hours
Yard	85 dBA	C4 = 1.33 hours	T4 = 16 hours

The employee’s total noise exposure (D) is computed as follows:

Table HT-1 Reference Durations, in Hours, for given Noise Levels
Noise Level, L	Reference Duration, T		Noise Level, L	Reference Duration, T
80	32.0		106	0.87
81	27.9		107	0.76
82	24.3		108	0.66
83	21.1		109	0.57
84	18.4		110	0.50
85	16.0		111	0.44
86	13.9		112	0.38
87	12.1		113	0.33
88	10.6		114	0.29
89	9.2		115	0.25
90	8.0		116	0.22
91	7.0		117	0.19
92	6.1		118	0.16
93	5.3		119	0.14
94	4.6		120	0.13
95	4.0		121	0.11
96	3.5		122	0.095
97	3.0		123	0.082
98	2.6		124	0.072
99	2.3		125	0.063
100	2.0		126	0.054
101	1.7		127	0.047
102	1.5		128	0.041
103	1.3		129	0.036
104	1.1		130	0.031
105	1.0		131	0.027

Table HT-2 Dose to Equivalent TWA8 for Given Dose
Dose	TWA8		Dose	TWA8		Dose	TWA8
10	<=70		350	99.0		670	103.7
20	78.4		360	99.2		680	103.8
30	81.3		370	99.4		690	103.9
40	83.4		380	99.6		700	104.0
50	85.0		390	99.8		710	104.1
60	86.3		400	100.0		720	104.2
70	87.4		410	100.2		730	104.3
80	88.4		420	100.4		740	104.4
90	89.2		430	100.5		750	104.5
100	90.0		440	100.7		760	104.6
110	90.7		450	100.8		770	104.7
120	91.3		460	101.0		780	104.8
130	91.9		470	101.2		790	104.9
140	92.4		480	101.3		800	105.0
150	92.9		490	101.5		810	105.1
160	93.4		500	101.6		820	105.2
170	93.8		510	101.8		830	105.3
180	94.2		520	101.9		840	105.4
190	94.6		530	102.0		850	105.4
200	95.0		540	102.2		860	105.5
210	95.4		550	102.3		870	105.6
220	95.7		560	102.4		880	105.7
230	96.0		570	102.6		890	105.8
240	96.3		580	102.7		900	105.8
250	96.6		590	102.8		910	105.9
260	96.9		600	102.9		920	106.0
270	97.2		610	103.0		930	106.1
280	97.4		620	103.2		940	106.2
290	97.7		630	103.3		950	106.2
300	97.9		640	103.4		960	106.3
310	98.2		650	103.5		970	106.4
320	98.4		660	103.6		980	106.5
330	98.6		670	103.7		990	106.5
340	98.8		680	103.8		1000	106.6

SUMMARY

As you can see, the more variable the noise sources or exposure times, the more involved the computations become. Noise dosimeters overcome this problem by electronically accumulating and integrating the noise signals into the employee’s noise dose. Having one person observe several noise dosimeters can save additional time. However, a simultaneous survey using a sound level meter must be conducted to support the dosimeter results.

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