Forensic Acoustics Case Study

DOSE / RESPONSE
We recognize that over the past forty years rigorous, scientifically defensible dose/response relations have been established between noise exposure and the development of threshold deficits. These relationships have been widely published as International (ISO 1999 xx) and National (ANSI S3.44 xxx) Standards. These documents provide a basis on which to compare the hearing thresholds exhibited by an individual exposed to noise with the hearing thresholds of a large population of individuals similarly exposed. Furthermore, because the standards are based on a large body of data, it is possible to predict the hearing thresholds of individuals who are more and less sensitive to the effects of noise exposure.

Keep in mind, however, that the standards do not predict the thresholds of any particular noise exposed worker. They only permit comparisons between an individual and a group.

An example of the data provided in the standards is shown in the first figure. The figure shows the expected hearing threshold for a male, age 50, exposed to 90 dB TWA for 20 and 30 years. The curves shown represent the population of average susceptibility to noise and the most sensitive 10 percent of the population.

We also know, from the age adjustment tables in the OSHA Hearing Conservation Amendment, that as a population ages, its hearing, on average, becomes poorer. This suggests that if audiograms from a large group of workers of widely varying ages were compared, we should expect to find a range of hearing thresholds. It follows that if the workers were exposed to levels of noise insufficient to cause hearing loss, workers of different ages should exhibit different audiometric profiles. Any departure from these finding are cause for consternation.