To predict the exposure of an individual worker wearing hearing protection, a de-rating factor is sometimes applied to the noise reduction rating (NRR) allowing for an estimate of the attenuation provided by their HPD (Neitzel et al, 2006). At the same time, studies have found that the field effectiveness of hearing protection is highly variable, and that laboratory measurements of ‘ noise reduction rating ’ (NRR) may not correlate well with measures of attenuation in the field (Berger et al, 1998). Studies have also shown that HPDs are underused (Tak et al, 2009), that the effectiveness may vary widely between individuals, and that use may vary with ambient noise exposure levels, as suggested by the finding of higher hearing loss rates among workers with moderate ambient noise exposures compared to those with higher estimated ambient noise exposures (Rabinowitz et al, 2007). Because of these critical barriers to understanding true noise exposures, a principal way that worker overexposure to noise is detected is when periodic audiometric testing finds that the worker has had a substantial shift in their hearing threshold levels, by which time a significant amount of hearing has been irretrievably lost.