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.