Investigating Double Hearing Protection using Human Subjects

In the increasingly noisy military environment and with the growing need to protect the hearing of military personnel, the use of double protection, earplugs and earmuffs used in combination, has become more and more prevalent. Furthermore Active Noise Reduction (ANR) headsets and earplugs appear to offer a means of increasing the attenuation of these double protection systems. However, it has been shown that the attenuation afforded by double protection is not a simple additive process, the combined attenuation tends to be less than the sum of the individual earplug and earmuff attenuation figures. Also, at the levels of attenuation provided by double protection, bone conduction pathways start to play an important part in the sound level received at the ear. A fuller understanding of these processes is required if the full benefit of double protection is to be achieved in future devices. Most previous assessments of the attenuation the afforded by double protection and bone conduction limits have been carried out using the REAT (Real Ear At Threshold) test technique. However, this technique is limited for predicting the attenuation from active systems due to the masking effect of the residual electronic noise. Other work has used ATF (Acoustical Test Fixtures) for predicting the attenuation provided by ANR double protection. However, this can only take into account bone conducted