Insert earphone calibration for hear-through options

A hear-through option in earphones exists when the earphone has a microphone mounted on the outside of the earphone thus allowing the user to hear the surroundings - or not. Although routing the microphone signal to the earphone driver is relatively straightforward, the physical presence of the earphone will modify the natural acoustics of the external ear, and some form of compensation is required to make the microphone-earphone combination acoustically transparent. Here we derive a general expression for the design target response of insert earphones with hear-through options emphasizing the differences that originate from non-individual approximations. Measurements are performed on a selected number of insert earphones and compared to the proposed design target response.