The role of spectro-temporal fluctuations in interaural level differences for the externalization of sound

Real-world sound sources are usually perceived as externalized and are thus properly localized in both direction and distance. This is largely due to 1) the acoustic filtering produced by the head, torso, and pinna, which results in modifications of the signal spectrum and thereby a frequency- dependent shaping of interaural cues and 2) interaural cues provided by the reverberation inside an enclosed space. In the present study, first the effect of room reverberation on the spectro-temporal behavior of interaural level differences (ILDs) was investigated. This was done by analyzing dummy-head recordings of speech played at different distances in a standard IEC 268-13 listening room. Afterwards the effect of ILD fluctuations on the degree of externalization was investigated in a psychoacoustical experiment with normal-hearing listeners. The experiment was performed in the same standard listening room and individual binaural impulse responses were used to simulate a distant sound source delivered via headphones. The ILDs were altered using a Gammatone filterbank for analysis and resynthesis, where the envelopes of the left- and right-ear signals were modified such that the naturally occurring variation of the ILDs was restricted. 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. An understanding of such properties could be important, e.g. in relation to hearing-instrument signal processing since processing schemes such as dynamic range compression and noise reduction may alter the ILDs.