Noise in Industry - Fundamentals and Assessment Tools for Occupational Ergonomics - page 633

values are in dBA, the NRR can still be applied, albeit with some loss of accuracy. With dBA data, a 7-dB
“safety” correction is applied to the NRR to account for the largest typical differences between C- and A-
weighted measurements of industrial noise, and the equation is as follows:

Workplace TWA in dBA (NRR 7) ¼ protected TWA in dBA (31 : 19)

While these methods are promulgated by OSHA 19 for determining HPD adequacy for a given noise situ-
ation, a word of caution is needed. The data appearing on HPD packaging are obtained under optimal
laboratory conditions with properly fitted protectors and trained human subjects. In no way does the
“experimenter-fit” protocol and other aspects of the current test procedure (ANSI S3.19-1974) 26 rep-
resent the conditions under which HPDs are selected, fit, and used in the workplace. 23 Therefore, the
attenuation data used in the octave band or NRR formulae are highly inflated and cannot be assumed
as representative of the protection that will be achieved in the field. The results of a review of research
studies in which manufacturers' on-package NRRs were compared against NRRs computed from
actual subjects taken with their HPDs from field settings are shown in Figure 31.8. 27 Clearly, the differ-
ences between laboratory and field estimates of HPD attenuation are large and the hearing conservation-
ist must take this into account when selecting protectors. Efforts by ANSI Working Group S12 WG11

focused on the development of a new testing standard, ANSI S12.6-1997(R2002), 28 which utilizes
subject (not experimenter) fitting of the HPD and relatively naive (not trained) subjects to yield attenu-
ation data that are more representative of those achievable under workplace conditions wherein an HCP
is operated (described in Royster, Ref. 29). However, at the time of working this new standard had not
been adopted into law promulgating its use in producing the data to be utilized in labeling HPD per-
formance (although it is likely to happen in the future).
If the currently available HPD attenuation data are inaccurate, what steps should be taken to gain a
more accurate estimate of the NRR for use in determining protected exposure levels? The OSHA 30
Field Operations Manual of the Office of General Industry Compliance Assistance indicates: “Citations
for violations of 29CFR 1910.95(b)(1) shall be issued when engineering and

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or administrative controls
are feasible, both technically and economically; and (1) Employee exposure levels are so high that hearing

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30

Laboratory

25

20

15

10

Field

5

0

Plugs Muffs

FIGURE 31.8 Comparison of hearing protection device NRRs by device type: manufacturers' laboratory data
versus real-world “field” data. (Adapted from Berger, E. H., Franks, J. R., and Lindgren, F. (1996). International
review of field studies of hearing protector attenuation. In Axelsson, A., Borchgrevink, H., Hamernik, R. P.,
Hellstrom, P., Henderson, D., and Salvi, R. J. (Eds.), Scientific Basis of Noice-Induced Hearing Loss (pp. 361-377).
New York: Thiesse Medical Publishers, Inc. With permission.)