Monday March 01, 2010
A Practical Alternative to Relying on NRR
Download PDF
Measurement of Insert-type Hearing Protector Attenuation on the End-user: A Practical
Alternative to Relying on the NRR
Introduction
Hearing protectors are labeled with a Noise Reduction Rating (NRR) that is derived from
laboratory measurements of the attenuation provided to trained and motivated human subjects in
a ‘sterile’ environment. As intended, these measurements represent a best-fit condition. The
literature indicates that the average attenuation provided to end-users of hearing protectors is
often only a fraction of the labeled values. Because of this, various derating schemes are
employed, including the 50% derating suggested in the OSHA inspector’s Technical Manual and
the variable derating according to protector-type suggested in the 1998 NIOSH criteria document
on occupational noise exposure.
De-rating the labeled NRR provides a better estimate of the average attenuation realized by a
population of end-users. However, field measurements indicate that attenuation provided across
end-users is highly variable (standard deviation > 10 dB), so many wearers will receive much
greater attenuation than the average, and many will receive much less attenuation than the
average. Therefore, if hearing protectors are selected according to a de-rated NRR, some
individuals will still be under-protected and some will receive excessive attenuation, leading to
potential communication problems. A solution to this problem is to base the selection process on
objective data obtained by measuring the attenuation provided to each end-user of insert-type
hearing protectors.
Steel Industry Experience
A Pennsylvania steel company has shown that individual hearing protector fit-testing can be
effectively integrated into the annual audiometric evaluation. Almost all HPD wearers at this
plant wear insert-type devices, and the attenuation provided by these devices has been measured
on each end-user in 1998 and 1999. The results of the 1998 measurements show the wide
variability of attenuation provided by insert-type protectors and distinct characteristics of specific
devices.
Initially, the attenuation provided by insert-type hearing protectors was measured for 389 steel
workers in June of 1998. The workers were allowed to choose HPDs from a selection of several
single-size foam plugs and one single-size reusable plug. Before the attenuation measurement,
the employees were individually instructed to wear the protectors as they would while on-the-job.
The attenuation measurement session was performed immediately after the annual audiometric
examination, which included a video on proper hearing protector fitting techniques. Attenuation
measurements were made binaurally using one-third octave bands of noise as test stimulus at six
center frequencies: 250, 500, 1000, 2000, 3150, and 4000 Hz.
The attenuation measurement process involves using automatic audiometric techniques to
measure hearing thresholds both with and without the hearing protectors in place. The
attenuation provided the end-user at each test frequency is essentially the difference between
these two threshold measurements. The headphones used in this process are specially designed
to be unobtrusive and do not affect the fitting of the plugs. Eight employees were tested per
hour, which was also the rate of administering the annual audiometric examination.
A new rating system, the Personal Attenuation Rating (PAR), was devised for reporting the
results of this study. The PAR was developed as a simple single-number measure of hearing
protector attenuation provided to the end-user. The PAR is analogous to the NRR, i.e., the PAR
is equal to the A-weighted noise reduction provided in a ‘flat’ noise exposure.