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CAOHC

CAOHC Newsletter: UPDATE

A Different Look at Noise Exposure and Hearing Loss

Emily S. Drott, Purdue University and CSTI Acoustics and Robert D. Bruce, CSTI Acoustics

Noise exposure and the hearing loss it causeshave been a concern since at least the 1940's, yet even as the years (and legislation) have passed, it is still not understood by the majority of people who are affected by it. Part of the confusion lies in the use of the decibel scale; it is hard to perform the necessary calculations unless you have either years of experience or a calculator. Therefore, a different way of looking at noise exposure is being proposed that uses units of Pascals-squared-seconds, or PASQUES (PASS ks), as the limits for Sound Exposure (SE). Pascals are the metric unit for pressure; 14.7 pounds per sq. in. equals 101,353 Pascals. This use of SE as the metric has been suggested for day-night sound exposure near airports (Ref. 1), but extra benefits come from applying it to industrial noise control. ANSI Standard S3.44-1996 (Ref. 2) defines SE in Pascals-squared-seconds, or PASQUES. The squared pressure can be calculated (if you are so inclined) from the sound pressure level through equation 1

Pexp2 = 10(Lp/10) * Pref2    (1)

Where:

Pexp2 is the square of the exposure sound pressure, in Pa2
Pref2 is the square of the reference sound pressure 20 µPa, in Pa2 and
Lp is the sound pressure level, dB

The SE is then calculated by equation 2.
SE = (Pexp2) x (length of exposure in seconds)    (2)

With this approach, it is simple to calculate the noise dose a person has received. The following table presents the 1-second sound exposure for sound levels from 80 to 100 dB.

Sound Exposures for One Second
Sound Level, dB SE, PASQUES
80 0.04
81 0.05
82 0.06
83 0.08
84 0.10
85 0.13
86 0.16
87 0.20
88 0.25
89 0.32
90 0.40
91 0.50
92 0.63
93 0.80
94 1.00
95 1.26
96 1.59
97 2.00
98 2.52
99 3.18
100 4.00

Using the table, the math becomes easy. The SE for 80 dB for 1 second is 0.04 PASQUES, so for 100 seconds of exposure it would be 4 PASQUES. For 8 hours (8hrs x 60 minutes/hr x 60 seconds/minute), the sound exposure would be 1152 PASQUES. If the level were 10 dB higher, the exposure in PASQUES would be 10 times as much.

There are many advantages to using this method for determining sound exposure. First, expressing the sound exposure in a unique unit, as opposed to in dB per 8 hours, provides flexibility in the calculations. Instead of interpolating the sound exposure from information in OSHA tables, the exact exposure of a worker can be determined. In addition, irregular noises can be included in calculations. Occupational Hearing Conservationists (OHCs) and/or workers can calculate exposures using simple arithmetic. For example, the following table identifies the sound level to which the worker is exposed in the left column. The next column identifies the length of time the noise exposure lasts. The third column converts the length of exposure into seconds. The fourth column presents the 1-second SE for that sound level. The fifth column gives the SE for that length of time. These individual SEs can be summed to give SE for the worker on that day.

Sound Exposure for Day
Sound Level Length of Time Seconds 1-Second Sound Exposure Sound Exposure at this Level (Time in seconds multiplied by 1-Second Sound Exposure)
85 7.5 hours 27000 0.13 3415
95 29 minutes 1740 1.26 2201
100 60 seconds 60 4.00 240
Total Exposure for Day 5856

Workers or OHCs could track the amount of noise a worker receives over his lifetime by adding the sound exposure linearly, much simpler and more intuitive than working in decibels. In the same way that dieters count their calories until they have reached their quota for a day, concerned workers could count their sound exposure over their working life time, until they had reached a limit past which hearing loss is probable.

The question arises with the new units of PASQUES, "What should be the limit for lifetime noise exposure?" The first step in this is to look at previous standards and recommendations. NIOSH recommends 85 dBA as an 8-hr TWA for a 40-year working lifetime to reduce the risk of noise-induced hearing loss (Ref. 3). The OSHA limit is 90 dBA for an 8-hour duration per day (Ref. 4). Finally, the EPA recommends a level of 75 dBA over 8 hours to prevent all hearing loss (Ref. 5), but a level of 80 dBA to ensure that only 5% of the population has a 5 dB threshold shift (Ref. 6). Over an average working lifetime of 40 years, these levels, in PASQUES, are as follows:

Sound Exposure
Regulating Organization Sound Level, dBA Daily (8 hrs) Yearly (250 days) Lifetime (40 years) % Increased Risk
OSHA 90 11,520 2,880,000 115,200,000 22%
NIOSH 85 3,643 910,736 36,429,439 12%
EPA - 1978 80 1,152 288,000 11,520,000 5%
EPA - 1974 75 364 91,074 3,642,944 0%

Bruce, et. al proposed defining one Lifetime Occupational Noise Exposure (1 LONE) as 11.5 million PASQUES. The following table presents the yearly exposure that it would take to reach 1 LONE in 40 years at different sound levels (Ref. 7).

A-weighted Sound Level Hours of Exposure/Year to Reach 1 LONE in 40 yrs
80 2000 hrs
85 632 hrs
90 200 hrs
95 63 hrs
100 20 hrs
105 6 hrs
110 2 hrs
115 0.6 hrs (38 minutes)

OHCs and workers are concerned about hearing loss induced by noise. Looking at SE in terms of PASQUES can help to reinforce the importance of proper hearing protection, annual audiometric evaluations, noise surveys of facilities, and proper record keeping. Overall, this is intended to lead to a better understanding of and further protection from hearing loss.

REFERENCES
  1. Eldred, Kenneth McK "Sound Exposure Without Decibels" Proceedings of Inter-noise. 1986.
  2. ANSI S3.44-1996. "Determination of Occupational Noise Exposure and Estimation of Noise-Induced Hearing Impairment." American National Standards Institute. 1996.
  3. "Criteria for a Recommended Standard: Occupational Exposure to Noise, Revised Criteria 1998." National Institute for Occupational Safety and Health (NIOSH) in the Department of Health, Education, and Welfare. 1998.
  4. 29 CFR 1910.95. "Occupational Noise Exposure." Occupational Health and Safety Administration.
  5. "Information on Levels of Environmental Noise Requisite to Protect Public Health and Welfare with an Adequate Margin of Safety." Environmental Protection Agency. 1974.
  6. "Protective Noise Levels, Condensed Version of EPA Levels Document," Environmental Protection Agency. Report 550/7-70-100. 1978.
  7. Bruce, R. D., Bommer, A. S., Lefkowitz, K. A., & Hart, N. W. "Safe Lifetime Occupational Noise Exposure - 1 LONE." Proceedings of NOISE-CON. 2010.

Emily Drott is a senior at Purdue University; she is expected to receive her BS and MS from the College of Engineering in 2011. Emily has been working in acoustical related jobs since graduating from high school as co-valedictorian in May of 2006. During her summers, she has worked at CSTI acoustics, the Office of Naval Research at the Naval Research Laboratory in Washington, DC, and with Southwest Research Institute in San Antonio. After graduation she plans to work as an acoustical consultant.

Bob Bruce has worked in industrial noise control for over 40 years. He has shared his knowledge on noise measurements and control through chapters in 12 books and over 50 technical papers. Bob is one of the INCE representatives serving CAOHC; he is the Secretary Treasurer of the Council.