Active Noise Reduction (ANR) in Hearing Protection: Does it Make Sense for Industrial Applications?

Active noise reduction (ANR) is a much-heralded technology that would seem to offer interesting promise for enhancing the degree of noise attenuation provided by conventional passive hearing protection. For over 20 years researchers, especially those in the military, have been aggressively pursuing development of ANR, yet its application to industrial noise exposures where employees are mobile and exposed to varying noise sources has been limited, and for good reasons. Nevertheless many hearing conservationists seek ANR devices as though they were the Holy Grail, capable of solving the nastiest of noise exposure problems. This paper will explore the gains available in the best ANR earmuffs developed to date, and compare the protection provided by conventional and ANR devices worn in 300 typical industrial environments, including some predominantly low-frequency noise exposures. In terms of reduction in the overall A-weighted noise levels, it will be shown that in all but a small percentage of industrial environments, a conventional earmuff will provide essentially equivalent protection. This may be surprising especially in light of the dramatic auditory effect one can perceive when switching an ANR earmuff on and off in a low-frequency dominated noise. An explanation for this discrepancy is provided. The reader is encouraged to review an accompanying paper by D. Gauger of Bose Corporation (Gauger, 2002) that describes valid and useful applications for ANR hearing protection technology, primarily in general aviation and military environments. INTRODUCTION Conventional passive hearing protection is a relatively mature technology that has undergone approximately one-half century of evolution since the end of World War II. Of more recent development has been the design of noise-canceling circuitry that can be incorporated into hearing protectors to sense the sounds that pass through the hearing protector, invert them in phase, and rebroadcast them via an earphone to provide active noise reduction. It is natural to presume that with today’s advanced electronic devices, passive protection could be improved upon by such noise-canceling circuitry in hearing protection devices (HPDs) especially for earmuffs, if not also earplugs. Noise-cancellation devices, commonly referred to as active noise reduction (ANR) HPDs, have been the subject of much research and many claims. As ANR developments have continued, not only are analog devices being produced, but digital systems as well. Naturally, hearing conservationists in their quest for maximum protection in severe noise environments have been prone to turn to this electronic technology expecting that it would have much to offer. The purpose of this paper is to provide them guidance for the typical industrial application, namely an employee freely moving through an industrial space, not constrained to a vehicle or instrument panel, with no readily available source of electrical power except for batteries, and with no built-in communication systems in his or her HPDs. Although ANR earplugs are currently under development, the focus of this paper will be on ANR earmuffs since the devices which are commercially available today are only of that variety. For a general discussion of ANR systems the reader is encourage to turn to Casali and Berger (1996) and Gauger and Sapiejewski (1999) as listed in the references. E-A-R 01-16/HP, Does ANR make sense? Page 2 MATERIALS The Earmuff I have been asked on a number of occasions to evaluate ANR systems and have also reviewed numerous studies in the literature. Based upon my experience, a single prototype ANR system that was measured in our laboratory was selected as it represents one of the highest attenuating ANR earmuffs evaluated to date. Real-ear attenuation at threshold (REAT) was measured for this device turned off as well as for a conventional earmuff using a similar sized and shaped cup, but lacking the ANR components. The laboratory-measured NRR of the conventional earmuff was approximately 26 dB. Additionally, microphone-in real ear (MIRE) measurements on a group of subjects were recorded for the ANR earmuff in the on and off conditions in order to estimate the degree of active noise reduction. The active reduction was added to the REAT values to estimate the total noise reduction provided by the ANR when turned on. The data for the ANR earmuff are presented in Figure 1. Notice that up through approximately 400 Hz ANR provides useful gains with a peak performance of nearly 20 dB at 63 Hz and 125 Hz. However, at 500 Hz there is a small net loss in performance and essentially no effect at the higher frequencies. 1 REAT cannot be used to evaluate the attenuation provided by ANR devices because they create broadband residual noise that is audible in a very quiet test chamber. This creates masking in the occluded condition that would spuriously elevate the measured attenuation values. 2 In this paper REAT values were utilized for the passive attenuation estimate for both earmuffs. The active performance of the ANR device was estimated by adding the MIRE values to the REAT. One could also argue that one should base all the measurements on MIRE data (for both the passive and active modes) since it is well documented that at low frequencies REAT values are spuriously elevated due to physiological noise masking (Berger and Kerivan, 1983). The error is approximately 3 dB @ 125 Hz for the muffs evaluated herein, which would cause the performance of the ANR earmuffs to look slightly more favorable in the subsequent analyses, but would not materially effect any of the conclusions. REAT was selected since those values are more commonly reported in general hearing conservation practice. Figure 1: Attenuation of high-quality ANR earmuff -10