The influence of spectral and spatial characteristics of early reflections on speech intelligibility

The auditory system employs different strategies to facilitate speech intelligibility in complex listening conditions. One of them is the integration of early reflections (ER’s) with the direct sound (DS) to increase the effective speech level. So far the underlying mechanisms of ER processing have mostly been addressed by manipulating the temporal characteristics of ERs. To achieve a more complete picture, the present study investigates the influence of the spectral and spatial characteristics of ERs on speech intelligibility. Speech intelligibility tests were performed with 9 normal-hearing and 8 hearing-impaired listeners in a virtual auditory environment. In this setup with 29 loudspeakers, the amplitude of the DS and the ERs could be varied independently. The ER pattern was taken from a classroom simulated with the room acoustic software Odeon. Thus, the spectral, spatial and temporal characteristics of the ERs were preserved. The DS of the speech signal was always presented from the front and the ERs were either presented from the front or spatially distributed. Speech intelligibility was measured monaurally and binaurally for different types of interferers. It was found for both groups of listeners that speech intelligibility improved with added ER energy, but less than with added DS energy. An efficiency factor was introduced to quantify this effect. The difference in speech intelligibility could be mainly ascribed to the differences in the spectrum between the speech signals with and without ERs. As the ERs were changed from spatial to frontal presentation the speech intelligibility increased in the same manner for monaural and binaural listening. This indicates that the integration of ERs and DS depends on the direction of the ER’s, but not on the listening mode (monaural vs. binaural). The direction-dependency could be explained by the spectral changes introduced by the pinna, head, and torso. The results will be important with regard to the influence of signal processing strategies in modern hearing aids on speech intelligibility, because they might alter the spectral, spatial and temporal cues important for the benefit from ERs.