Measurement of earplugs insertion loss using a classical impedance tube

This work presents a new method for assessing the normal incidence insertion loss (IL) of earplug (EP) by the use of a classical impedance tube. The proposed technique allows for fast and straightforward IL measurements and thus is particularly suitable for earplug testing in a design phase. Indeed, compared to standard ATF measurements, the proposed method does not require cumbersome experimental facilities (i.e., reverberant or anechoic chambers). In the proposed method, the EP (≈ 10 mm outer diameter) is inserted in a rigid sample holder which is itself placed in a larger diameter impedance tube (commonly 29 mm inner diameter). The transfer matrix of the system is measured according to a standardized procedure. The transfer matrix of the EP alone is then recalculated by eliminating the effect of the sample holder. Finally, the IL is estimated by coupling the EP transfer matrix to a one-dimensional model of the occluded ear canal taking into account for the tympanic membrane. The proposed method is tested numerically using a twodimensional acoustical finite element model (FEM) of a cylindrical impedance tube and is validated by comparison with IL simulations computed from another numerical model of an occluded ear canal.