Certain U. S. and international consensus standards governing hearing protection device (HPD) attenuation testing specify the use of a diffuse sound field to ensure the sound field remains uniform and random-incidence in an envelope about the subject’s head (ANSI, 1974; ANSI, 1984; British BSI 5108:1983; Canadian CSA Z94.2-M1984; ISO 4869- 1:1990; Swedish SS 882151). However, there are very few experimental data to support these restrictive requirements. The research presented herein investigated this issue by applying three different environments in tests of the attenuation of four different hearing protectors (three earmuffs and one earplug) at each of nine 1/3 octave band frequencies centered at 125, 250, 500, 1000, 2000, 3150, 4000, 6300, and 8000 Hz. One testing environment comprised a reverberant room with three loudspeakers, one firing in each room plane, that met all the specifications for testing under ANSI S3.19-1974 (ANSI, 1974). The other two environments progressively degraded the diffusivity of the sound field through the use of a single loudspeaker and room surface treatment with absorptive panels. A psychophysical real-ear-attenuation-at-threshold procedure was used to obtain attenuation data. The results showed small, but statistically significant, differences in attenuation among the three environments for specific test frequencies. Due to their statistical significance, these differences preclude direct comparison of attenuation data obtained in these different environments, especially when the data are used for purposes such as technical design research, product comparison and/or labeling, and testing standards development. However, being of small magnitude, these differences are not great enough to prevent obtaining an estimation of the attenuation that an individual is achieving with a particular device under these alternative environments. With this in mind, the use of an industrial audiometric test booth may be beneficial for determining an individual worker’s protection levels actually achieved on the job. In sum, the interpretation of the results differs depending upon the intended purpose of the testing. / Master of Science