Exploring the benefit from enhancing envelope ITDs for listening in reverberant environments

Jessica J. M. Monaghan and Bernhard U. Seeber
MRC Institute of Hearing Research, Nottingham, NG7 2RD, UK

Reflections in rooms alter the temporal envelope of the direct (target) sound: the modulation
depth of the sound received at the ears is reduced, onsets are made less steep and binaural 
ear signals are decorrelated. Our previous work indicated that envelopes must have a minimum 
modulation depth, gradient and coherence in order to achieve good discrimination of 
interaural time differences (ITDs) from envelopes. ITD discrimination thresholds, therefore, 
increase greatly in the presence of reverberation. This is particularly relevant for cochlear 
implant (CI) listening since most current devices transmit ITDs only in the sound envelope, 
leading to impaired sound localization in rooms.

The aim of the present project is to ameliorate the detrimental effect of reverberation on 
envelope ITD discrimination by selectively enhancing envelope characteristics. The problem is 
to enhance only those parts of the reverberant sound envelope that are perceptually relevant 
for correct localization of the direct sound while not increasing the salience of the 
reverberation nor impairing speech understanding. These parts are likely the onsets when the 
direct sound is dominant, and a number of methods for onset selection were explored: 1) using 
explicit knowledge of the position of onsets in the direct sound, 2) using explicit knowledge 
of the instantaneous direct-to-reverberant ratio (DRR) of the signal, 3) using explicit 
knowledge of the ITD of the direct sound to identify the moments with matching instantaneous 
ITD in the reverberant signal. Reverberant sounds were processed with a sine-wave vocoder 
modified to steepen selected envelope onsets and discrimination thresholds for ITDs in the 
direct sound were measured in normal-hearing listeners. 

Preliminary results indicate that selective sharpening of envelope onsets can improve 
envelope ITD discrimination thresholds in reverberation when compared to standard processing. 
The possible threshold reduction depends on the DRR and generally on the number of onsets 
processed. Possible detrimental effects of envelope sharpening on speech understanding will 
be ascertained.

Supported by the Intramural Programme of the MRC.