Quantitative assessment of spatial sound distortion by the semi-ideal recording point of a hear-through device

A hear-through device combines a microphone and earphone in an earpiece so that when worn, one per ear, it can work as an acoustically transparent system allowing for simultaneous individual binaural recording and playback of the real sound field at the ears. Recognizing the blocked entrance to the ear canal as the ideal recording point – i.e. all directional properties of the incident sound field are recorded without distortion - it is critical for such device to be sufficiently small so that it can be completely inserted into the ear canal. This is not always feasible and the device may stretch out from the ideal position and thus distort the captured spatial information. Here we present measurements that quantify by how much the directional properties of the sound field are distorted by semi-ideal hear-through prototypes built by mounting miniature microphones on the outer part of selected commercial insert earphones. This includes an analysis of the magnitude by which spatial information is distorted and the extent to which these distortions are direction dependent. Potential strategies for compensating these distortions are also considered.