Elliott H. Berger

Hearing protection: Surpassing the limits to attenuation imposed by the bone-conduction pathways

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.

Development of a new standard laboratory protocol for estimating the field attenuation of hearing protection devices. Part I. Research of Working Group 11, Accredited Standards Committee S12, Noise

This paper describes research conducted by Working Group 11 of Accredited Standards Committee S12, Noise, to develop procedures to estimate the field performance of hearing protection devices (HPDs). Current standardized test methods overestimate the attenuation achieved by workers in everyday use on the job. The goal was to approximate the amount of attenuation that can be achieved by noise‐exposed populations in well‐managed real‐world hearing conservation programs, while maintaining acceptable interlaboratory measurement variability. S12/WG11 designed two new laboratory‐based protocols for measuring real‐ear attenuation at threshold, with explicit procedures for subject selection, training, supervision, and HPD fitting. After pilot‐testing, S12/WG11 conducted a full‐scale study of three types of earplugs and one earmuff tested by four independent laboratories using both protocols. The protocol designated as ‘‘subject‐fit’’ assessed the attenuation achieved by subjects who were experienced in threshold ...

Technology Advancements in Hearing Protection Circa 1995: Active Noise Reduction, Frequency/Amplitude-Sensitivity, and Uniform Attenuation

Conventional hearing protection devices represent a mature technology that has been widely used since the late 1950s. When worn consistently and correctly such devices can provide suitable hearing protection in many, if not most noise-hazardous or aurally annoying situations. However, such devices have often been implicated in compromised auditory perception, degraded signal detection, and reduced speech communication abilities. In some instances this can create hazards for the wearer, or at the very least, resistance to use by those in need of hearing protection. Recent technological developments have been used to augment hearing protectors in an attempt to alleviate these problems for the user while providing adequate attenuation. Operational characteristics, design alternatives, performance data, and applications for active noise reduction, active sound transmission, frequency selectively, adjustable attenuation, amplitude sensitivity, and uniform attenuation features in hearing protectors are discussed, and recommendations are provided.

Diffuse field response of the ear

The ear canal pressure developed in a diffuse‐field facility complying with ANSI S12.6‐1984 was measured on 20 adult subjects (11 male and 9 female), with two replications on each ear for 19 of the 20 subjects. A soft 1‐mm‐o.d. silicone rubber probe tube was inserted until the subject reported the “thump” of the tube bumping the eardrum, and then withdrawn a couple of mm. Preliminary experiments indicated that this technique did not always produce the same results, with some curves on the same ear of the same subject showing substantially less “eardrum SPL” in the 8‐ to 20‐kHz region than others. It was sometimes (but not always) possible to confirm with otoscopic examination that the lower‐SPL curves corresponded to probe‐tube locations away from the eardrum. Based on the preliminary experiments, data from the “best run of the best ear” (i.e., highest level above 8 kHz) were averaged. The resulting curve was similar to the curve obtained on a KEMAR manikin in the same sound field.

Results of the National Institute for Occupational Safety and Health—U.S. Environmental Protection Agency Interlaboratory Comparison of American National Standards Institute S12.6-1997 Methods A and B

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).

A new hearing protector rating: The Noise Reduction Statistic for use with A weighting (NRSA).

An important question to ask in regard to hearing protection devices (HPDs) is how much hearing protection they can 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 specifying a Noise Reduction Rating (NRR) measured in decibels (dB). In the intervening 25 years many concerns have arisen over this regulation. Currently the EPA is considering proposing a revised rule. This report examines the relevant issues in order to provide recommendations for new ratings and a new method of obtaining the test data. The conclusion is that a Noise Reduction Statistic for use with A weighting (NRSA), an A–A’ rating computed in a manner that considers both intersubject and interspectrum variation in protection, yields sufficient precision. Two such statistics 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 a larger o...

ASSESSMENT OF THE PERFORMANCE OF HEARING PROTECTORS FOR HEARING CONSERVATION PURPOSES

Rating hearing protector performance for hearing conservation purposes can be most accurately accomplished when not only laboratory, but also field performance data are taken into consideration. Although standardized laboratory test data have been commonly available since the 1950s (ANSI Z24.22‐1957; ANSI S3.19‐1974), it is only since the mid 1970s that significant work has been conducted in the area of field performance evaluation of hearing protectors. Currently, a number of field techniques are available, including: real‐ear attenuation at threshold and/or mid‐line lateralization tests using actual noise exposed employees as subjects, either at their workplaces or at special test clinics; dosimetry studies via miniature microphones; and temporary threshold shift evaluations of hearing protection users in noisy industries. These real world methods will be reviewed and contrasted to the laboratory techniques, and key points illustrated with representative data.

Empirical evaluation using impulse noise of the level‐dependency of various passive earplug designs

An objective in the development of hearing protection devices (HPDs) has been the design of a passive earplug that provides modest or no attenuation at low sound levels, with greater protection at high sound levels. This raises the issue of not only how to construct such a device, but also how to evaluate it. There is the related question of whether conventional HPDs are actually level independent. Passive level dependency is typically accomplished via an orifice that causes sound transmission to decrease as input level increases. We utilized an impulsive noise source (explosives) with peak levels from 110 to 190 dB SPL to measure the insertion loss of a variety of commercially available and developmental earplugs. The tests were conducted at frontal incidence in a reflection‐free outdoor environment using the Institute of Saint‐Louis acoustical test fixture specifically constructed for HPD attenuation measurements. Conventional foam and premolded earplugs exhibited attenuation that was essentially consta...

Ongoing experience with laboratory‐based subject‐fit REAT methods for measuring hearing protector attenuation

As data have accumulated indicating the difficulty in basing estimates of field‐delivered (i.e., real‐world) protection for groups of users upon classical laboratory‐derived optimum attenuation data, more attention has been directed towards standards intended to provide better simulations of the real world. The development of ANSI S12.6‐1997 Methods for Measuring the Real‐Ear Attenuation of Hearing Protectors, is one approach towards addressing this problem. The standard includes both experimenter‐supervised fit and subject‐fit methods. The latter method, designated as Method B, requires the use of audiometrically experienced subjects who are naive in the use of hearing protection. This method is intended to approximate the upper limits to the attenuation that can be expected for groups of occupational users. It yields mean attenuation values, more so for earplugs than earmuffs, that are substantially lower and standard deviations values that are higher than previously found using ANSI standards. This pap...

Representative 24‐h Leqs arising from a combination of occupational and nonoccupational noise exposures

The noise exposure of occupational populations and their resultant noise‐induced hearing losses have been well studied in the literature. However, the same cannot be said for those exposed off‐the‐job and/or recreationally. To better understand the ‘‘normal’’ development of hearing loss with age (presbycusis plus sociocusis) data on societal noise exposure is required. This was recently high‐lighted by Berger et al. [Combatting Noise in the ’90s: A National Strategy for the U.S. (ASHA, Rockville, MD, 1991)]. As a result of his participation in this conference, the first author became interested in this problem and began an informal study to collect 24‐h noise exposure data on primarily nonoccupationally noise‐exposed friends and co‐workers living in Indianapolis, IN. Data from the subjects who were asked to ‘‘wear’’ dosimeters 24 h/day for week‐long periods, will be presented and anecdotal findings will be reviewed. Thus far, the average 24‐h Leq across the available sample of 14 people, is 77 dBA with th...