Research has demonstrated that the implementation of hearing protectors to prevent noise-induced hearing loss merits from individual verification at the workfloor. Therefore, the MIRE (Microphone In Real Ear) technique has been adapted for custom-made earplugs with an inner bore drilled over the plug's total length. So a microphone can be inserted to measure via this test bore the sound pressure level behind the hearing protector. The measurement setup appears to be practically feasible, but a clear discrepancy is expected between the sound pressure level at the microphone and the level of interest at the eardrum. To gain insight into this difference - henceforth called transfer function - research has first been conducted with a HATS (Head And Torso Simulator) equipped with the custom-made earplugs. More concrete, the transfer function between the sound pressure at the HATS "eardrum" and the MIRE microphone has been measured and numerically simulated with the FDTD approach (Finite-Difference Time-Domain). The transfer functions appear to be stable and reproducible, moreover their most distinct characteristics can be traced down to the main acoustical features of earplug and ear canal. Further the influence of an individual's specific geometry of the earplug - ear canal complex on the transfer functions has been addressed by performing measurement and simulations with human subjects similar to the research conducted with the HATS. The concordance between individualized simulations and measurements is very satisfying, but the computational cost of the FDTD technique is too large to be implemented in measurement equipment used to test hearing protectors at the workfloor. Therefore a filter approach has been developed based on the FDTD model. The filter transfer functions show good resemblance with the original measurements and simulations if the length of the test bore and of the residual part of the ear canal are taken into account.