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Twentieth in a comprehensive series of technical monographs…
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Twentieth in a comprehensive series of technical monographs covering topics related to hearing and hearing protection.
The Naked Truth About NRRs
BY ELLIOTT H. BERGER
Senior Scientist, Auditory Research
Figure 1 - Comparison of NRRs published in North America (labeled values based upon
laboratory tests), to real-world "field" attenuation results derived from 20 separate studies.
Since hearing protection devices (HPDs)
are worn primarily for the purpose of
Noise Reduction Rating (dB)
noise reduction it is not surprising that 30 Laboratory
hearing conservationists place great sig-
nificance upon the sound attenuation
that such devices can provide. Not only 25
is attenuation, generally expressed in
terms of a Noise Reduction Rating 20
(NRR),9 an important and often pivotal
factor in making purchasing decisions,
15
but it is also frequently utilized for deter-
mining effective at-the-ear exposures in
particular noise environments. Empha- 10 Field
sis on noise reduction data as a purchas-
ing criterion, and reliance on such num-
5
bers for predicting protection, are both
unwarranted and potentially deleterious
to the effectiveness of a hearing conser- 0
vation program (HCP). This EARLog will
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Real -World Studies
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In the latter part of the 1970s hearing
conservationists began to investigate the The data, which for purposes of simplifi- which were examined in two of the more
amount of protection that HPD users cation are expressed in terms of the recent real-world studies whose data
were actually achieving in the work NRR, are summarized using dark bars appear in Fig. 1, were specifically se-
place, typically called field or real-world in Fig. 1. Similar findings are apparent lected because the authors believed
attenuation. Field attenuation is often when octave-band analyses are com- them to be exemplary. The findings from
measured by having noise exposed pared4 as is illustrated in Figs. 2 and 3. those two studies did tend to indicate
employees participate in real-ear attenu- better than average real-world attenua-
ation at threshold measurements. This In Fig. 1, the devices are grouped into tion, but unfortunately failed to surpass
can occur on-site by taking measure- two general categories, earplugs and all other existing data and thus did not
ment equipment to the work place, or one canal cap on the left, and earmuffs confirm the authors' optimistic expecta-
off-site, by having employees report with on the right. Devices and/or device types tions.
their own HPDs to an independent test were selected to assure a minimum
facility. For on-site measurements, hear- sample size of greater than 30 subjects Labeled vs. Field Data
ing protected employees are taken from (summed across studies) for each data For purposes of comparison to the field
their work stations, either with or without bar shown. For some categories the data, Fig. 1 also provides the associated
warning, and are then tested in a nearby sample size was very large, as in the labeled NRRs, shown by the lighter col-
measurement chamber. In the case of case of the E-A-RŪ foam earplug, for ored bars. The labeled values are based
earmuff users, an alternative is to directly which the data represent 575 subjects upon manufacturers' published North
measure on-the-job noise reduction by from 11 studies. American laboratory results.
fitting them with a pair of dosimeter-type
microphones mounted inside and out- The 80 industries that were studied in Laboratory testing of HPDs in North
side their earmuff cups. order to generate the results shown in America is conducted in conformance
Fig. 1 probably represent today's better with standards developed by the Ameri-
By 1992 there were at least 20 available HCPs. This presumption is based upon can National Standards Institute.1,2 The
studies providing measurements of real- the increased likelihood of finding procedures call for determining " opti-
world attenuation. Those studies span higher-quality HCPs among companies mum performance values which may not usu-
greater than 80 industries in seven coun- interested in and choosing to participate ally be obtained under field conditions."2 Optimum
tries, with a total of over 2600 subjects. in the complicated and time-consuming performance values, as opposed to esti-
For additional details and/or for a list of research of the type required for real- mated real-world values, have histori-
the available field studies see Berger.7 world evaluations. In fact, the HCPs cally been specified for laboratory test-
than is shown in Fig. 1. For example, if
an HPD with an NRR of 20 dB is not
Figure 2 - Premolded earplugs: labeled vs. field Figure 3 - Earmuffs: labeled vs. field results.
Data averaged across 6 different products, 5 worn for as little as 30 minutes in an 8-hr
results. Data averaged across 5 different prod-
ucts, 9 studies, 556 subjects. studies, 169 subjects. work shift, its effective NRR is reduced
by 5 dB (see EARLog 53). Since the data
0 0 in Fig. 1 represent attenuation for a single
Standard Deviation (dB)
Standard Deviation (dB)
point in time when the HPD is actually
being worn, they say nothing about the
Real - Ear Attenuation (dB)
Real - Ear Attenuation (dB)
0 10 0 10
effects of such disuse. Accounting for this
10 20 10 20 factor can alter the relative field protec-
tion reported in Fig. 1, since wearers may
20 20
be prone to remove and replace some
HPDs more than others, depending
30
upon various factors such as comfort,
30
ease of donning and removal, and the
40
Labeled values Labeled values interference of the protector with audi-
Field values 40 Field values tory communications.
.125 .250 .500 1.0 2.0 4.0 8.0 .125 .250 .500 1.0 2.0 4.0 8.0
Frequency (kHz) Frequency (kHz) Importance of Motivation and Training
Current research has demonstrated that
ing because U.S. standards groups have values are arranged in ascending order a very good estimate of the real-world
felt that those values could be more con- from left to right within each category in attenuation achieved in the better pro-
sistently repeated, and were useful for Fig. 1, the same does not hold true for grams can be obtained by testing totally
rank-ordering HPDs. However, current the field data. Furthermore, the labeled naive HPD users in a laboratory protocol with abso-
data as described herein, and reported values suggest that earplug attenuation lutely no individual training by the experimenter.6
by Berger,6 suggest otherwise. Neverthe- is typically equivalent to or greater than When tested under those conditions, the
less, ANSI S3.19/S12.6-type data are the that of earmuffs, whereas the field data attenuation of HPDs still equals or ex-
only standardized values that regulators indicate otherwise. With the exception of
and manufacturers currently have avail- ceeds average real-world data of the
the foam earplug, only earmuffs can gen- type shown in Figs. 1-3.
able for labeling and informational pur- erally be expected to provide 10 dB or
poses. more of real-world protection for 84% of The fact that completely untrained test
the exposed population. subjects obtain more attenuation than
The labeled NRRs in Fig. 1 were com-
puted, as per the Environmental Protec- occupationally- exposed workers who
tion Agency,9 by subtracting a two-stan- Casual observers of Fig. 1 may find would have been expected to be trained
dard deviation (SD) correction from the single-digit field NRRs, with values drop- and motivated, and to have benefitted
mean attenuation values in order to es- ping even below 5 dB, to be unbeliev- from many months of practice in using
timate the minimum noise reduction theo- able. However, the magnitude of the real- their HPDs, is truly amazing! It suggests
retically achieved by 98% of the labora- world results in qualitatively supported that today's typical, or even above-aver-
tory subjects (NRR98). The field data were by analyses of audiometric data from age HCPs, are ineffective in fully moti-
computed in the same manner except existing HCPs and be real-world studies vating and training employees to con-
that only a 1-SD correction was included, of temporary threshold shift.7,16 sistently and properly wear their HPDs.
thus estimating the minimum attenuation
achieved by 84% of the actual wearers Additionally, one must recall that the field Better Estimates of Field Performance
(NRR84). A 1-SD correction simplifies ex- NRRs in Fig. 1 represent the minimum If the goal is to estimate the protection
amination of real-world data, since the attenuation achieved by at least 84% of achievable in the upper quartile of
2-SD correction used in the labeled the real-world subjects, not their average
NRRs (i.e. NRR98) would cause many today's industrial and military HCPs, then
attenuation. Since the NRR84 includes a the "truth" probably lies somewhere be-
field NRRs to become negative numbers. subtractive 1-SD correction, which usu-
Further justification for using a 1-SD cor- tween the labeled and field data in Fig. 1.
ally amounts to 8 dB or more for ear-
rection with real-world NRRs is based plugs, the attenuation achieved by half
upon practical, psychophysical, and sta- One approach to reducing lab/field dis-
tistical considerations.7 of the wearers (NRR50) which is computed crepancies would be to increase field
with a 0- SD correction, is about 8-dB performance. In fact, all agree that in-
Observations Regarding the Field Data larger. For example, a real-world NRR84 dustrial hearing conservation practice
The most obvious feature of Fig. 1 is the of 4 dB is typically equivalent to an NRR50 needs to improve if better real-world HPD
very poor correspondence between the of 12 dB (see Fig. 4). And, in this example, performance is to be achieved. Improve-
magnitude of lab and field NRRs. Mea- if one asks what is the protection ments would be valuable in the areas of
sured as a percentage of the laboratory- achieved by the top few percent of the fitting and training of HPD users, educa-
rated attenuation, the field NRRs for ear- wearers, approximately another 2 SDS tion and motivation of the work force, en-
plugs yield only 6 to 52% of the labeled (16 dB) must be added, yielding an NRR forcement of proper HPD utilization, pro-
values (averaging about 25%), and for of 28 dB. gram management, and the develop-
earmuffs, from 33 to 74% (averaging ment of easier-to-use and more comfort-
about 60%). WearingTime able hearing protectors.
Wearing time is an important parameter
Not only do the absolute values disagree, but so too since it can decrease the effective pro- At the same time, however, laboratory
do the relative rankings. Although the labeled tection provided by an HPD, to even less tests of hearing protector attenuation that
yield data which more closely correlate ation, or even to properly rank order HPD
with existing, or even potential field per- performance. Of equal importance is the The 50% derating cited above has no
formance must be identified. Those meth- inherent within-laboratory and between- relationship to the well-known 7-dB cor-
ods must more realistically model train- laboratory variability in test data.8 Con- rection specified in Appendix B of the
ing and fitting procedures that are sidering these facts, one can state that Amendment. The 50% derating adjusts
achievable in practical applications. For differences in the NRR of less than 3 dB labeled values to better reflect real- world
example, because of their test practices, have no practical importance, and even performance, whereas the 7-dB correc-
laboratory data from some Australian 4- to 5-dB changes are of questionable tion accounts for use of the NRR with A
and European facilities already indicate significance unless closely controlled instead of C-weighted sound levels (see
lower laboratory-rated attenuation, es- data are being compared (cf. ref. 10). EARLog 1213). When using both the der-
pecially for earplugs.5 In the U.S. an ANSI ating and the correction together, 7 dB is
OSHA's 50% Derating subtracted prior to derating by 50%. Be-
working group which is cognizant of such
In Appendix B of the Hearing Conserva- cause of the potential to use either or
findings (S12/WG11), has reviewed re- tion Amendment13 OSHA explicitly states
cent research and conducted some of both of these adjustments, the situation
that the adequacy of an HPD for a given becomes even more bewildering (see
its own in order to develop a procedure noise exposure (i.e. whether it can at- Table I).
to provide more accurate estimates of tenuate the exposure to either 90 or 85
real-world performance6 (see E-A-RLog #21). dBA, as required) shall be calculated The Danger of High Labeled NRRs
using the manufacturers' labeled NRRs The average NRR on devices sold in
Is the NRR the Problem? or octave-band data. Subsequent to pro- North America today is over 22 dB. This
Some have argued that the method of mulgation of the Amendment, OSHA is- number clearly overstates the protection
computing the NRR from the octave- sued administrative guidelines to field afforded to most occupationally noise-
band data has inherent inaccuracies,11 inspectors on how to compare the pro- exposed workers. Such NRRs are dan-
and have proposed more accurate (but tectiveness of HPDs and hearing con- gerous because they mislead the buy-
also more complex) rating methods such servation programs to hat afforded by ers and users of HPDs. Taking an NRR
as the HML three-number system em- engineering and/or administrative noise of 22 at face value, a hearing conserva-
bodied in a current draft international controls.14,15 The intention of this assess- tionist is led to presume that his or her
standard.10 Yet, the principal portion of ment of relative performance was to clarify work force will be protected for time-
the lab/field discrepancies are due to the when, in spite of the presence of a hear- weighted average exposures up to 107
laboratory data themselves and not the ing conservation program, citations for dBC (to achieve a protected exposure
method of manipulating them into single failure to implement noise controls would of 85 dBA). Since 97% of industries have
or multi-number ratings.4 Even if one ig- be issued. TWAs below 100 dB, 12 this reasoning
nores the NRR altogether, and com- would suggest that almost all workers in
putes protection using the full comple- OSHA's written guideline is that if noise almost all environments will be protected
ment of laboratory-measured octave- controls are feasible, citations will be is- if only they are simply given hearing pro-
band attenuation values, the real-world sued when "hearing protectors alone tection.
predictive capability of the data is not may not reliably reduce noise levels" to
substantially improved. those specified in the standard. This is We have come to learn that such expec-
evaluated by dividing the labeled NRR tations are far from the truth. Fostering
Disregard Small Differences in NRRs by two (i.e. derating by 50%). Although such expectations leads to hearing con-
When comparing NRRs as a criterion interpretations and enforcement are servation programs in which inadequate
for selecting HPDs, one must consider regionally variable, OSHA is likely to find attention is paid to the aspects of a pro-
the inability of current labeled data ei- HPDs unreliable "when employee ex- gram that can make it work - training,
ther to accurately predict absolute attenu- posure levels border on 100 dBA"14, and motivation, supervision, and enforce-
when other indications, ment. 3
Figure 4 - Distribution of individual protection values (NRR with a
such as a high incidence
0-SD correction) for a group of real-world users with an NRR84 of of standard threshold HPDs vs. Noise Controls
4 dB, i.e. 84% achieve at least 4 dB of protection. The SD is 8 dB shifts (STSs), provides For the uninitiated, the real-world HPD
and the average NRR is 12 dB (NRR50). Note: 16% and 2% of supporting evidence of data in this EARLog may be shocking.
the users, respectively, exceed NRRs of 20 and 28 dB. HCP inadequacy. The inclination may be to dismiss hear-
ing protectors as ineffectual and resort
Mean Value only to engineering noise controls. This
Thus, for questions of would be a mistake. Although noise con-
-1 SD +1 SD HPD adequacy the NRR is trols are generally preferred, one must
+2 SD taken at its labeled value, consider that in many instances it is diffi-
Number of Workers
84% 50% but for questions of relative cult to achieve even 10 dB of noise re-
performance, the NRR is de- duction in a retrofit noise control appli-
16% rated by 50%. These cation, and 10 dB is what the more effec-
2% seemingly contradictory tive HPDs can provide for most of the
positions are confusing work force. Many of the types of prob-
not only to those trying to lems which afflict HPDs also impact the
comply with the law, but performance of engineering noise con-
NRR84 also to OSHA compliance trols. For example, one of the most com-
0 4 12 20 28 officers themselves. monly used treatments is an enclosure.
Individual Protection Value (dB)
is necessary. Most conventional hearing
Table I - Application of OSHA - specified computation for an HPD with a protectors, when properly sized and fit-
labeled NRR of 21 dB ted, and consistently worn, can fulfill that
NRR for use with NRR for use with requirement. As the sound levels in-
dBC measurements dBA measurements crease so that exposures exceed 95
ADEQUACY per hearing Cons. dBA, choices should be limited to the
Amendment, Appendix B 21 21 - 7 = 14 more protective devices. These have
been shown to be foam earplugs and
RELATIVE PERFORMANCE most earmuffs (as indicated in Fig. 1), or
21 / 2 = 10.5 (21 - 7) / 2 = 7
vs. noise controls a combination of the two.
EXAMPLE: Unprotected Time-Weighted Average (TWA) exposure = 100 dBA. The hearing protector selection process
To assess ADEQUACY per Appendix B, the protected TWA is computed as: should consist of more than merely scan-
100 dBA - 14 = 86 dBA, current hearing protection is legally adequate. ning manufacturers' specification sheets
To assess RELATIVE PERFORMANCE, i.e. comparison of field attenuation to engineering or and price lists. Wear test the products
administrative controls, the protected TWA is computed as: 100 dBA - 7 dB = 93 dBA; you intend to use, both on yourself (for
since this exceeds 90 dBA, feasible engineering controls must be implemented. extended periods of a few hours or more)
NOTE: when using muffs and plugs together, OSHA adds 5dB to NRR of higher-rated HPD. For field and on small groups of employees. By
effectiveness, 5dB is addd after 50% adjustment, as in: developing your own firsthand knowl-
NRR plug = 29; NRR muff = 21; field NRR for use with dBA = [(29 - 7)/2] + 5 = 16 dB. edge and combining it with employee
feedback, you not only improve the like-
lihood of selecting products your em-
ployees will accept, but you also will
If it is not well fitted, or left partially ajar, or ated. A 50% derating such as OSHA re- better motivate your workers by involv-
circumvented by an inconvenienced quires for evaluation of relative perfor- ing them in their own hearing conserva-
employee, or its gaskets and seals age, mance, is justifiable in order to reduce tion program.
deteriorate, or break, then its perfor- the existing NRRs from the unattainable
mance will be degraded in a manner References
to the achievable. Furthermore, treating 1. ANSI (1974). "Method for the Measurement of Real-Ear
similar to that observed for poorly fitted earplugs and earmuffs differentially (i.e. Protection of Hearing Protectors and Physical Attenuation of
and misused HPDs. Likewise, an em- Earmuffs," Am. Natl. Stds. Inst., S3.19-1974, NewY NY.
ork,
a somewhat larger derating for earplugs)
ployee may complain, "I can't hear my may be warranted. However, protector 2. ANSI (1984). "Method for the Measurements of the Real-Ear
Attenuation of Hearing Protectors," Am. Natl. Stds. Inst.,
machine when I wear these earmuffs," brand-specific deratings are premature S12.6-1984, NewY NY
ork, .
but in terms of noise reduction and until additional real-world attenuation 3. Berger, E.H. (1979-1992). EARLog Series, #1 - #20. Aearo
change in sound quality, there is little Company, Indpls., IN.
results become available.
difference between putting the box on 4. Berger, E.H. (1983). "Using the NRR to Estimate the Real-
the employees head (e.g. donning an World Performance of HPDs,"S&V 17(1), 12-18.
earmuff), or putting the box around the Rather than relying upon labeled NRRs
5. Berger, E.H. (1988). "Can Real-World Hearing Protector
machine. in selecting HPDs, factors that should Attenuation Be Estimated Using Laboratory Data?", S&V
be considered are comfort, field attenu- 22(12) 26-31.
Most engineering control procedures, ation, human engineering, compatibil- 6. Berger, E.H. (1992). "Development of a Laboratory Procedure
for Estimation of the Field Performance of Hearing
except for some source noise control ity, durability, cost, styling and availabil- Protectors", in Proc., 1992 Hearing Conservation
accomplished through equipment rede- ity. It is not possible to rank order these Conference, Off. Eng.Serv., Lexington, KY, 41-45.
sign, required maintenance and periodic items in a manner suitable for all appli- 7. Berger, E.H ., Franks, J.R., and Lindgren, F (1996).
.
"International Review of Field Studies of HPD Atten.," In
adjustment or replacement to remain cations, but most would agree that com- Scientific Basis of NIHL, eds. Axlesson, et al., Thieme Med.
effective. Furthermore, except for enclo- fort and field attenuation should be Pub., NewY NY 361-377.
ork, .
sures, retrofit noise reductions of 10 dB weighed most heavily. General esti- 8. Berger, E.H., Kerivan, J.E., and Mintz, F (1982). "Inter-
.
or more are often difficult to achieve and mates of attenuation can be gleaned from LaboratoryVariability in the Measurement of Hearing
Protector Attenuation", S&V 16(1), 14-19.
maintain. Thus HPDs can be, and often data such as provided in Fig. 1. 9. EPA (1979). "Noise Labeling Requirements for Hearing
are, required as an effective adjunct to Protectors", Fed. Regist. 44(190), 40CFRPart 211, 56130-
engineering controls in the majority of The practice of purchasing only HPDs 56147.
industrial noise environments. that will meet the highest noise attenua- 10. ISO (1992). "Acoustics - Hearing Protectors - Part 2;
Estimation of Effective A- Weighted Sound Pressure Levels
tion requirements within a plant, on the When Hearing Protectors Are Worn", Int. Org. for Std., ISO/
Recommendations DIS 4869-2.2, Switzer land.
presumption that control of HPD usage
Existing North American hearing protec- 11. Lundin, R. (1992). "Properties of Hearing Protector Rating
is impossible and therefore any device
tor attenuation data and the associated Methods", Proc., 1992 Hearing Conservation Conference.
labeled NRRs provide a very poor indi- which is purchased may end up being Off. Eng. Serv., Lexington, KY 55-60.
,
cator of the protection that will typically used anywhere within the plant, should 12. OSHA (1981). "Occup. Noise Exposure; Hearing Cons.
Amend.," Fed. Regist. 46(11), 4078-4181.
be achieved by the majority of employ- be discouraged. Instead HPDs should
13. OSHA (1983). "Occup. Noise Exposure; Hearing Cons.
ees in industrial and military hearing be approximately matched to the noise Amend.,"Fed. Regist. 48(46), 9738-9785.
conservation programs. Efforts must be exposure requirements of groups of simi- 14. OSHA (1992). "Instruction CPL 2.45B, June 15", Field
expended to develop more realistic stan- larly-exposed individuals, and control of Operations Manual, 5th Ed., Gov. Inst., Inc., Rockville, MD,
p. IV-33 - IV-36.
dardized testing procedures so that pub- usage within the plant should become
15. OSHA (1990). "Instruction CPL 2-2.20B, Feb. 5,"Industrial
lished attenuation values can provide part of the educational process. Hygiene Technical Manual, Gov. Inst., Inc., Rockville, MD,
p.4-1 - 4-15
useful estimates of actual protection. In
fact, such work is currently underway.6 For the majority of industrial noise expo- 16. Royster, L.H., Royster, J.D., and Cecich, T.F (1984). "Eval.
.
sures, those up to equivalent eight-hour of the Effectiveness of Three HPDs at an Indust. Facility
with a TWA of 107 dB", JASA 76 (2), 485-497.
levels of about 95 dBA, 10 dB of actual
Meanwhile, labeled NRRs must be der- delivered on-the-job protection is all that
E·A·RLogŪ is a registered trademark of Aearo Company. Copyright 2000. First printing 1993
LIT. CODE 30314 2/99AG