The ear sits 8 cm off-center, so source-ear distance changes with azimuth

Evidence

The time of arrival of the direct sound varies with direction, which is because the centre position of the head of the subject is at the centre of a circle along which the source moves, leaving the ear out of that position by about 8 cm. Thus, the distance between the source and the ear canal changes when the source moves. The source is closest to the ear when in the direction of about 90◦, where the earliest start of the response is seen. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 1416

Categorization

• DEFINE: Explains why HRIR arrival time shifts with source direction: geometric offset of the ear from the head’s rotational center.