Subject recruitment
At each site, volunteers were recruited among noise-exposed workers with the only eligibility criterion being current enrollment in
the company hearing conservation program. Recruitment took place through distribution of educational materials about the study
and in person meetings between groups of workers and the investigators. Company management did not take part in subject recruitment sessions, and volunteers provided written informed consent under study protocols approved by the Yale Human Investigation Committee. There was no exclusion of individuals with preexisting hearing loss or other medical conditions. After providing consent, workers were fitted with noise monitoring devices consisting of a dosimeter that is carried in a pocket or worn on a belt or hard hat (Figure 1) and connected by wires to small microphones (one for the left ear and one for the right ear) that record noise exposure under hearing protection devices. Dosimeter settings are similar to those used for OSHA hearing conservation compliance (5-dB exchange rate, slow response, 80-dBA noise floor). Study volunteers were asked to use these dosimeters on a daily basis as part of their regular hearing protection and to download the noise exposure data from the dosimeter at the end of each work shift.