Attenuation of hearing protectors at 85 dB SPL investigated by commercial "insertion gain" method

Attenuation of hearing protectors is conventionally measured as the difference between the subject's pure-tone hearing threshold with, and without, hearing protection under free field conditions (loudspeaker) in a sound attenuated (an-echoic) room. This REAT procedure is not objective as it involves the response of a subject. The attenuation can not be measured at noise hazard levels: > 85 dB SPL. Commercial computerized "insertion gain" equipment has recently been developed to improve the individual fitting of hearing aids. By "insertion gain" method a slender silicone probe-tube connected to an outside microphone, is inserted in the ear canal, registering the sound pressure level in front of the tympanic membrane. Thus one can objectively register the sound pressure level difference (gain) obtained close to the tympanic membrane with a given hearing aid in a non-attenuated room. We wanted to study whether the same procedure could be used to assess the attenuation of hearing protectors at noise hazard levels with acceptable measurement variability. With a commercial computerized "insertion gain" equipment (IGO-HAT 1000. Madsen Electronics) we registered the "insertion loss", the difference in sound pressure level measured < 5 mm in front of the tympanic membrane for 20 consecutive sessions (fittings) in the same subject with versus without the same hearing protector. Below 6000 Hz standard deviations were remarkably low--around 2 dB--for the two protectors tested.