With louder and louder weapon systems being developed and military personnel being exposed to steady noise levels approaching and sometimes exceeding 150 dB, a growing interest in greater amounts of hearing protection is evident. When the need for communications is included in the equation, the situation is even more extreme. New initiatives are underway to design improved hearing protection, including active noise reduction (ANR) earplugs and perhaps even active cancellation of head-borne vibration. With that in mind it may be useful to explore the limits to attenuation, and whether they can be approached with existing technology. Data on the noise reduction achievable with high-attenuation foam earplugs, as a function of insertion depth, will be reported. Previous studies will be reviewed that provide indications of the bone-conduction (BC) limits to attenuation that, in terms of mean values, range from 40 to 60 dB across the frequencies from 125 Hz to 8 kHz. Additionally, new research on the effects of a flight helmet on the BC limits, as well as the potential attenuation from deeply inserted passive foam earplugs, worn with passive earmuffs, or with active-noise reduction (ANR) earmuffs, will be examined. The data demonstrate that gains in attenuation exceeding 10 dB above the head-not-covered limits can be achieved if the head is effectively shielded from acoustical stimulation.

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An experiment was conducted wherein masked thresholds (using ascending method of limits) for a backup alarm were obtained in pink and red noise at 85 and 100 dBA for 12 participants immersed in a probability monitoring task and wearing a conventional passive hearing protection device (HPD, an earmuff or a foam earplug), an active noise reduction (ANR) headset, or no HPD at all (only in 85 dBA noise). Results revealed statistically significant between-HPD differences in red noise (from 2.3 to 3.1 dB) and in the 100-dBA noise level (from 2.6 to 4.3 dB). An additional finding, which corroborates other studies using different protocols, was that masked thresholds in 85-dBA noise were significantly lower (from 3.2 to 4.4 dB) for the occluded conditions (wearing an HPD) than for the open-ear (unoccluded) condition. This result refutes the belief among many normal-hearing workers that the use of HPDs in relatively low levels of noise compromises their ability to hear necessary workplace sounds. Actual or potential applications of this research include (a) the selection of appropriate HPDs for low-frequency-biased noise exposures wherein signal detection is important and (b) gaining insight into the appropriateness of ANR-based HPDs for certain industrial noise environments.

The National Institute for Occupational Safety and Health and the Environmental Protection Agency sponsored the completion of an interlaboratory study to compare two fitting protocols specified by ANSI S12.6-1997 (R2002) [(2002). American National Standard Methods for the Measuring Real-Ear Attenuation of Hearing Protectors, American National Standards Institute, New York]. Six hearing protection devices (two earmuffs, foam, premolded, custom-molded earplugs, and canal-caps) were tested in six laboratories using the experimenter-supervised, Method A, and (naive) subject-fit, Method B, protocols with 24 subjects per laboratory. Within-subject, between-subject, and between-laboratory standard deviations were determined for individual frequencies and A-weighted attenuations. The differences for the within-subject standard deviations were not statistically significant between Methods A and B. Using between-subject standard deviations from Method A, 3-12 subjects would be required to identify 6-dB differences between attenuation distributions. Whereas using between-subject standard deviations from Method B, 5-19 subjects would be required to identify 6-dB differences in attenuation distributions of a product tested within the same laboratory. However, the between-laboratory standard deviations for Method B were -0.1 to 3.0 dB less than the Method A results. These differences resulted in considerably more subjects being required to identify statistically significant differences between laboratories for Method A (12-132 subjects) than for Method B (9-28 subjects).

An obvious and important question to ask in regard to hearing protection devices (HPDs) is how much hearing protection, commonly called attenuation or noise reduction, can they provide. With respect to the law, at least, this question was answered in 1979 when the U. S. Environmental Protection Agency (EPA) promulgated a labeling regulation for hearing protection devices (HPDs) that specified a descriptor called the Noise Reduction Rating (NRR) measured in decibels (dB). In the intervening 25 years many questions and concerns have arisen over this regulation. Currently the EPA is considering publication of a proposed revised rule. This report examines a number of the relevant issues in order to provide recommendations for a new label, new ratings, and a preferred method of obtaining the test results from which the ratings are computed. A wide variety of ratings are reviewed, from the putative gold standard, an octave-band calculation, to simplified ratings employing fewer numbers that can be applied to more common noise measures such as C-weighted or even A-weighted sound levels or exposures. Additionally, the most simplified method of all, namely a class or grading scheme is examined. The conclusion is that a Noise Reduction Statistic for use with A weighting (NRS A), an A-A' rating computed in a manner that considers both inter-subject and inter-spectrum variation in protection, yields sufficient precision for most situations. Justification for this recommendation stems from consideration of the inter-wearer variation in fitting, the variation in noise spectra, and the accuracy of the basic measurements of hearing protector attenuation and noise-exposure values. Furthermore, it is suggested that to provide additional guidance to the purchaser, two such ratings ought to be specified on the primary package label-the smaller one to indicate the protection that is possible for most users to exceed, and the larger one to indicate the protection that is possible to achieve by individual highly motivated expert users; the range between the two numbers conveys to the user the uncertainty in protection provided. Guidance on how to employ these numbers, and a suggestion for an additional, more precise, graphically oriented rating to be provided on a secondary label (the Noise Reduction Rating, graphical, NRS G) are also included. Another important consideration is the data from which the new rating is computed. Examination of potential types of data from U. S. or international standards reveals that ANSI S12.6-1997 Method-B data appear to provide the best correlation to field performance and hence the most useful ratings; however, concerns about the reproducibility of Method-B based results led us to also offer an alternative Method-A based value. Since insufficient data are available at this time to clearly distinguish between the two recommendations the need for an interlaboratory study is identified along with suggestions for how it might be conducted.