This project addressed two problems. Its first objective was to provide, for the first time, valid criteria with which to evaluate hearing capacity in individuals suffering from a hearing loss and who are employed in noisy environments in which detection of acoustic alarms is necessary. A secondary objective was to sensitize personnel involved in the rehabilitation of individuals suffering from hearing loss to the need to prescribe the use of hearing aids in workers employed at noisy workstations.

The noise attenuation associated with hearing aids which are turned off was evaluated in 1991, using a mock-up of a human head specifically designed for this purpose. Current models of hearing aids, including behind-the-ear, in-the-ear, and out-of-ear models, were also evaluated The influence of various characteristics of these hearing aids on noise attenuation was examined systematically. The results indicate turning off hearing aids constitutes a highly effective hearing protection device in noisy environments, as long as the tip of the hearing aid does not have any apertures. The project was completed in 1992. Hearing aids were found to aid in the detection of acoustic alarms in noisy environments. A very satisfactory signal-to-noise ratio can be obtained by using magnetic signals which can be detected by the hearing aid's induction coil, since the tip of hearing aid serves to attenuate ambient noise, while the hearing aid itself amplifies the signal. However, a number of technical details have to be addressed to ensure that the device transmits the range of frequencies corresponding to the sound spectrum of acoustic alarms.