The Influence of Masker Type on the Binaural Intelligibility Level Difference

The Binaural Intelligibility Level Difference (BILD) was first described by Licklider (1948). It is a manifestation of binaural unmasking, the advantage of binaural over monaural hearing of a signal S against the background of a spatially separated noise N. In headphone experiments, often a design with a N0S0 presentation vs a N0Sπ presentation is used, in which the noise is presented homophasic and the signal either homophasic or antiphasic. The BILD is then defined as the difference in the speech reception threshold (SRT) in the N0S0 and N0Sπ presentation mode. Blauert (1997) provides an overview of experimental work on BMLD and BILD. Estimating the BILD requires SRT measurements in the N0S0 and N0Sπ presentation modes. Since the diotic N0S0 stimuli contain no binaural information, they can be considered as an estimation of monaural speech perception (Siegel and Colburn 1983). The BILD for a stationary masker is known to be about 4–7 dB (e.g. Blauert 1997; Johansson and Arlinger 2002). We are interested in the BILD for fluctuating maskers because of their relevance for daily life. Assessing speech intelligibility in the presence of a fluctuating masker in a N0Sπ condition, several components should be taken into account (see Fig. 1): point of departure is the diotic speech reception threshold (SRT) for stationary noise (Fig. 1a.). If temporal modulations are introduced there will be release of masking (masking release, MR), the SRT reduces (Fig. 1b.). A typical value is of this masking release is 10 dB. On the other hand, if an interaural phase shift is introduced in the speech-signal the SRT will be reduced because of binaural unmasking (Fig. 1c.). A typical value of this binaural unmasking is 5 dB. The question addressed is this chapter is whether there is an interaction between the diotic (“monaural”) release of masking and the binaural unmasking. So, can the reduction in SRT in the condition with modulated noise and interaural phase shifted speech be predicted by adding the values of masking release and binaural unmasking, i.e. 10 + 5 = 15 dB or is it different?