Sound transmission to and within the human ear canal

Sound transmission to the eardrum from various points in the external ear was measured by means of probe microphone technique. Twelve human subjects participated, and three directions of sound incidence were included. For the major part of the audio frequency range the transmission to the eardrum proved independent of direction from points at the centerline of the ear canal, including the entrance (open or blocked). The results further suggested that the region with independent transmission extends some millimeters outside the entrance plane. The transmission from the free field to the eardrum was divided into a directional-dependent part and two directional-independent parts: (1) the transmission from the free field to the blocked entrance, (2) a pressure division between the radiation impedance and the ear-canal input impedance, and (3) the transmission along the ear canal. All parts of the transmission were seen to be highly individual. The first part was shown to be uncorrelated with any of the other parts, whereas mutual dependence of parts (2) and (3) resulted in a smaller variation in the combined transmission than for the parts in separate. The standard deviation between subjects for head-related transfer functions (HRTFs) measured at the eardrum, the open entrance, and the blocked entrance was studied, and the lowest values were found for the blocked-entrance HRTFs. It is concluded, that the blocked entrance is the most suitable point for measurements of HRTFs and for binaural recordings, since sound at this point includes the complete spatial information, and in addition to that the minimum amount of individual information.