Effects of anechoic vs. reverberant sound-field, subject gender, and outlier dismissal on the real-ear attenuation of hearing protection devices
An experiment was conducted to determine the effects of different acoustic characteristics of the testing environment on the measured attenuation of hearing protection devices (HPDs) when using a standard real-ear attenuation at threshold (REAT) protocol. In the experiment, three earmuffs and three earplugs were tested in two diffuse sound fields implemented in two different sound environments with different loudspeaker configurations. In the first case, the testing environment was reverberant, with frequency-specific reverberation time characteristics as specified in ANSI $3.19- 1974, "Method for the Measurement of Real-Ear Protection of Hearing Protectors and Physical Attenuation of Earmuffs" and with three loudspeakers, one in each principal plane of the room. In the other case, a diffuse sound field was established within an anechoic chamber (free-field), as permitted by ANSI S12.6-1984 "Method for the Measurement of the Real-Ear Attenuation of Hearing Protectors" using four loudspeakers, one at each vertex of a tetrahedron with the subject's head center position at the centroid. Each of the environments met the requirements of the applicable ANSI standard as well as the requirements of standards adopted by several foreign countries. The experimental design allowed a direct comparison of the testing environments (reverberant vs. free-field) permitted by the two aforementioned ANSI standards. Results indicate that for both earmuffs and earplugs, the environment in which a REAT evaluation is performed has a statistically significant impact on the results of the evaluation. These results have implications for ongoing standards development efforts not only in the United States but also abroad since the testing environments investigated in this research are either required or allowed by several international standards. These international standards include: International Standard ISO 4869-1981, Canadian Standard CSA Z94.2-M1984, British Standard BSI 5108:1983, and Swedish Standard SS 882151 (1981). The impact of outlier dismissal on the results of REAT tests of HPDs was also investigated and found to have minimal impact on the results obtained in this experiment. However, this result is most likely case-specific and it is doubted that any generalizations can be made concerning outlier tests and their impact on HPD evaluations. As a side issue, it was also determined that ear canal size is highly correlated with attenuation achieved using premolded earplugs with attenuation decreasing with increasing ear canal size. No consistent gender effects were found in the analysis, indicating that gender alone may not be an important factor in determining how much attenuation can be obtained with a given HPD. Finally, the lack of a significant trial effect points to the absence of a strong practice effect over the three trials of a REAT evaluation, at least for subjects who are highly practiced in the REAT procedures.