Spatial Unmasking of Speech Based on Near-Field Distance Cues

Abstract

These days it is recognised that for bilateral hearing loss there is generally benefit in fitting two hearing aids, one for each ear (see Byrne, 1980 and Feuerstein, 1992 for clinical studies, see Byrne et al., 1992, Durlach et al., 1981, and Zurek, 1981 for laboratory studies). Bilateral fitting is now standard practice for children with bilateral loss and as of 2005 bilateral fittings account for approximately 75% of all fittings (Libby, 2007). Nonetheless, it is only within the last half-decade that it has become possible to transfer audio signals between bilaterally-fitted hearing aids (Moore, 2007). This is primarily attributed to the technological advances in integrated circuit design, longer lasting batteries and also wireless intercommunication between the two hearing aids, e.g., using near-field magnetic induction (NFMI) communication. The possibility to exchange audio signals between bilaterally-fitted aids opens the door to new types of binaural signal processing algorithms to assist hearingimpaired listeners separate sounds of interest from background noise. In this chapter, we consider whether or not the manipulation of near-field distance cues may provide a viable binaural signal processing algorithm for hearing aids. More specifically, this chapter describes three experiments that explore the spatial unmasking of speech based on nearfield distance cues. In a typical cocktail party setting, listeners are faced with the challenging task of extracting information by sifting through a mixture of multiple talkers overlapping in frequency and time. This challenge arises as a result of interference in the form of energetic masking, where sounds are rendered inaudible due to frequency overlap, and informational masking, where sounds from different sources are confused with one another (Bronkhorst, 2000; Brungart et al., 2001; Kidd et al., 2008). Despite this, listeners are reasonably adept at parsing complex mixtures and attending to separate auditory events. One factor that influences speech intelligibility in mixtures is perceived spatial location. Many studies have established that sounds originating from separate locations are easier to distinguish than sounds which are co-located (Hirsh, 1950; Bronkhorst and Plomp, 1988; Ebata, 2003). Separating sounds in space can result in an increase in the signal-tonoise ratio at one ear (the ‘better ear’). Moreover, sounds that are spatially separated give rise to differences in binaural cues (interaural time and level differences, ITDs/ILDs) that can improve audibility by reducing energetic masking (Durlach and Colburn, 1978;