Uncertainty of hearing protector impulsive attenuation measurements

Abstract

Measurement of the impulsive protection of a hearing protection device (HPD) is currently standardized in ANSI/ASA S12.42-2010 as a measurement of impulse peak insertion loss (IPIL). IPIL is a time-domain metric, determined as the difference between open-ear and occluded-ear peak sound pressure levels of an impulse, measured without and with an HPD in place on an acoustic test fixture. To characterize potential level-dependent protection of an HPD, the IPIL test is repeated at several free-field impulse peak sound pressure levels, typically ranging from approximately 130-170 dB. Based on our experiences measuring IPIL with both a shock tube and a rifle serving as the impulse source, as well as a review of existing literature reporting IPIL studies, we discuss and attempt to quantify the uncertainty of such measurements. Sources of uncertainty include the repeatability of the impulse source, characteristics of the sound field created by the impulse source, spectral characteristics of both the impulse source and the HPD’s attenuation, variability between HPD samples and repeat fitting of the HPD to the acoustic test fixture, and variability in the measurement instrumentation and data analysis procedures. Our estimates will be compared to those provided in the uncertainty annex of S12.42.