The mechanisms of passive ear defenders

In this paper an analytical model of the sound transmission characteristics of a conventional circumaural passive ear protector at low frequencies is presented. It includes the effects of porous sound absorption material within the device as well as leakage. The theoretical study shows that the presence of sound absorption material effectively acts to increase the volume compliance of the system, thus decreasing its resonance frequency as expected. The role of leakage on the other hand is mainly to shift the resonance to a higher frequency depending on the equivalent leakage diameter. Experimental results are provided in support of the theoretical predictions. The model is useful in providing an understanding of the mechanisms of passive ear defenders and can be employed to guide their design. The modelling can also be extended to study and aid the development of communication and active noise control ear protectors.