Loudness: Independent of the direction of the incident sound

Up to now it has been accepted that loudness is completely determined by the sound levels at the eardrum. However, recent investigations on spatial hearing indicate that the outer-ear transfer functions, depending on the direction of the incident sound, have no effect on loudness and timbre. Addressing this discussion, a loudness comparison experiment was carried out, using two loudspeakers in an anechoic room. Situation 1: loudspeaker A and B are placed close together in front of the subject. Situation 2: loudspeaker A is placed in front of the subject, loudspeaker B beside the subject, at the same distance as A. In both situations the loudness of loudspeaker B was determined due to the reference loudspeaker A. The matching sound levels in the ear canals were measured using probe microphones. Furthermore, a corresponding loudness comparison test was carried out, using headphones and reproducing the same levels as in the experiment above, which means lateralization instead of localization. The result is: loudness is independent of the position of loudspeaker A and B. It is determined by the sound level of the loudspeakers, but not by the sound levels at the ear drums. The result supports the hypothesis that localization cues cannot be loudness cues. This is predicted by the so-called “association model” in which an adaptive filter with a transfer function inverse to the outer ear transfer function is postulated.