An increasing number of earphones and other hearing devices contain functionalities that are based on a so-called hear-through feature, which allows the user to hear the acoustic environment through the device. Ideally, the user would perceive the hear-through sound identical to listening with the open ear, which is referred to as acoustic transparency. In technical terms, this means that the sound transmission to the eardrum should be as similar as possible between the open ear and through the device. In this study, we evaluate the acoustic transparency of the hear-through feature of seven commercial hearables as well as two research hearing devices by means of technical measurements on a dummy head. A variety of artefacts, including frequency response deviations, comb filtering artefacts, and destruction of spatial cues, were revealed and quantified, and surprisingly large differences between current devices are noted. The corresponding subjective sound quality has been assessed in a companion study.
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