Effects of Interaural Level and Time Differences on the Externalization of Sound

Distant sound sources in our environment are perceived as externalized and are thus properly localized in both direction and distance. This is due to the acoustic filtering by the head, torso, and external ears, which provides frequency dependent shaping of binaural cues, such as interaural level differences (ILDs) and interaural time differences (ITDs).

Further, the binaural cues provided by reverberation in an enclosed space may also contribute to externalization. While these spatial cues are available in their natural form when listening to real-world sound sources, hearing-aid signal processing - such as wide dynamic range compression - affects the ILDs and thereby potentially reduces the perceived degree of externalization.

In the present study, the effect of room reverberation on the spectro-temporal behavior of ILDs was investigated. This was done by analyzing speech played at different distances and recorded on a head-andtorso simulator in a standard IEC 268-13 listening room. Next, the effect of ILD fluctuations on the degree of externalization was investigated in a listening experiment with normal-hearing listeners.

The experiment was performed in the same standard listening room and a distant speech source was simulated via headphones using individual binaural impulse responses. The speech signal was then processed such that the naturally occurring ILD fluctuations were compressed. This manipulation reduced the perceived degree of externalization in the listening experiment, which is consistent with the physical analysis that showed that a decreased distance to the sound source also reduced the fluctuations in ILDs.