Development of a new standard laboratory protocol for estimation of the field attenuation of hearing protection devices: sample size necessary to provide acceptable reproducibility.

The mandate of ASA Working Group S12/WG11 has been to develop "laboratory and/or field procedure(s) that yield useful estimates of field performance" of hearing protection devices (HPDs). A real-ear attenuation at threshold procedure was selected, devised, tested for one earmuff and three earplugs via an interlaboratory study involving five laboratories and 147 subjects, and incorporated into a new standard that was approved in 1997 [Royster et al., "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 S 12, Noise," J. Acoust. Soc. Am. 99, 1506-1526; ANSI, S12.6-1997, "American National Standard method for measuring real-ear attenuation of hearing protectors" (American National Standards Institute, New York, 1997)]. The subject-fit methodology of ANSI S12.6-1997 relies upon listeners who are audiometrically proficient, but inexperienced in the use of HPDs. Whenever a new method is adopted, it is important to know the effects of variability on the power of the measurements. In evaluation of protector noise reduction determined by experimenter-fit, informed-user-fit, and subject-fit methods, interlaboratory reproducibility was found to be best for the subject-fit method. Formulas were derived for determining the minimum detectable difference between attenuation measurements and for determining the number of subjects necessary to achieve a selected level of precision. For a precision of 6 dB, the study found that the minimum number of subjects was 4 for the Bilsom UF-1 earmuff, 10 for the E.A.R Classic earplug, 31 for the Willson EP100 earplug, and 22 for the PlasMed V-51R earplug.

Speech intelligibility with helicopter noise: tests of three helmet-mounted communication systems.

BACKGROUND: Military aviator helmet communications systems are designed to enhance speech intelligibility (SI) in background noise and reduce exposure to harmful levels of noise. Some aviators, over the course of their aviation career, develop noise-induced hearing loss that may affect their ability to perform required tasks. New technology can improve SI in noise for aviators with normal hearing as well as those with hearing loss. METHODS: SI in noise scores were obtained from 40 rotary-wing aviators (20 with normal hearing and 20 with hearing-loss waivers). There were three communications systems evaluated: a standard SPH-4B, an SPH-4B aviator helmet modified with communications earplug (CEP), and an SPH-4B modified with active noise reduction (ANR). RESULTS: Subjects' SI was better in noise with newer technologies than with the standard issue aviator helmet. A significant number of aviators on waivers for hearing loss performed within the range of their normal hearing counterparts when wearing the newer technology. The rank order of perceived speech clarity was 1) CEP, 2) ANR, and 3) unmodified SPH-4B. CONCLUSIONS: To insure optimum SI in noise for rotary-wing aviators, consideration should be given to retrofitting existing aviator helmets with new technology, and incorporating such advances in communication systems of the future. Review of standards for determining fitness to fly is needed.

Improved communications and hearing protection in helmet systems: the communications earplug.

Despite significant advances in hearing protection and compliance with protective standards, military personnel are still subject to noise-induced hearing loss in many combat and combat support operations. Although the Army has experienced a decrease of some 15% in primary hearing loss disability cases since 1986, a fiscal year 2000 report documents a 27.5% increase in audiograms, which demonstrated significant threshold shifts in assessed personnel (N = 841/1,077). Compensation for noise-induced hearing loss disability for the Army alone exceeded $180 million in 1998. Thus, communications and hearing protection remain critical issues for personnel involved in Army operations. Aircraft, ground vehicles, and weapons produce noise levels in excess of the limits defined in current hearing conservation standards. Performance of most helmets, improved over the years, remains marginal with regard to speech intelligibility. Furthermore, these helmets do not provide adequate hearing protection. The communications earplug, which consists of a high-quality earphone coupled with an earplug protector, provides the needed extra protection. It weighs less than 15 g and is comfortable when worn over extended periods. It is considered highly acceptable by seasoned Army aviators and crewmembers.