Measurement of hearing protector insertion loss at ultrasonic frequencies

Abstract

The preferred method for assessing the attenuation of hearing-protective devices (HPDs) involves real ear attenuation at threshold (REAT) determinations, usually obtained in accordance with ANSI Standard S12.6-1997 or ISO Standard 4869-1, where measurements are normally made in the range 125–8000 Hz. In certain circumstances, the use of an acoustic test fixture (ATF) with built-in artificial ears is an acceptable substitute for subjective testing. In this case, the metric obtained is “insertion loss”; the difference between the open-ear and occluded-ear sound levels measured using the ATF microphone. REAT attenuations are not generally interchangeable with insertion loss measurements; however, their similarity often promotes data comparison. Experience has shown that subjective thresholds obtained above 8000 Hz are highly variable due primarily to decreased hearing sensitivity and to the complexity of high-frequency wave patterns. With an ATF, measurements may be carried out over an extended frequency range while minimizing the problems associated with subjective evaluations. In this study, the high-frequency insertion loss of two earmuff and three earplug HPDs was measured objectively at frequencies up to 22.4 kHz. One of the earplugs provided consistently better performance than the other devices for both grazing and direct sound incidence. In addition, two lightweight materials (copier paper and foamcore sheeting) were evaluated for high-frequency noise reduction when placed in the direct path between the sound source and the artificial ear of the ATF. Of these, the foamcore was the more effective barrier. ©