Characteristics of listening environments: benefits of binaural hearing and implications for bilateral management.

Acoustically, a room's boundaries can be thought of as dirty mirrors. By repeated reflections, these mirrors generate multiple sound images whose intensities decrease with increasing distance. The sound energy received from these images (conditioned by the inverse square law and atmospheric absorption) sums with direct sound from the source, but with delays that are directly proportional to distance. For a time-varying signal such as speech, it is important to distinguish between 'early' and 'late' reflections. Early reflections enhance perception and arrive from fairly well defined directions. Late reflections generate both energetic and informational masking and arrive from all directions. In effect, late reflections create a diffuse, self-generated noise that adds to the noise from other sources. For all sources, there are inter-aural differences in time of arrival, sound pressure level, and spectrum, together with intra- and inter-aural effects of head movement. These differences and effects are a rich resource of information about source direction and distance, and they facilitate source segregation, selective attention, and release from both energetic and informational masking. To the extent that this information can be preserved in hearing devices, and accessed and used by hearing-impaired listeners, the potential benefits should follow. But deficits of temporal resolution, spectral resolution, bandwidth, and processing skills, together with delays and other effects of signal processing, may limit the benefits - hence the need for prescriptive and performance measures of binaural function.