Reflections and reverberations are a prominent aspect of sounds in natural environments. Human observers with normal hearing have a remarkable ability to extract the relevant information (and ignore irrelevant information) from complex environments. Other descriptions of listeners' auditory processing abilities is that they can attend to individual sources within multiple source environments and can separate the properties of the acoustic environment (the room) from the properties of the source. These are difficult tasks for automated processing of acoustic signals ( Thrust area I(D)). It seems likely that part of the difficulty is due to the fact that most objects are comparable to the wavelengths of sound so that they effectively diffract and diffuse the sound changing phases and amplitudes as a function of frequency without completely blocking the energy from any sources. Thus the received signals are a combination of filtered versions from multiple bearings and are not conveniently resolved.
The auditory system seems to be able to capitalize on differences in multiple aspects of the stimuli to achieve its performance. We plan a psychophysical study to explore human processing abilities in reverberant environments. This will be interpreted in association with a parallel study of physiological mechanisms in the inferior colliculus that is already underway and that is funded from a different source. We speculate that the functional processing capabilities of the natural auditory system might provide a useful processing scheme for signal processing in extreme reverberant environments such as underwater sound processing in coastal waters.
Up to the present, most psychoacoustical and physiological studies of auditory processing of these complex sound fields have focused on the case of a single sound with a single reflection. It has been observed that when the sound and its reflection are close together in time, the total sound is perceived as a single object (perceived as fused) and the location of this fused sound is perceived at the location of the direct or leading sound. This is referred to as ``the precedence effect'' or ``the law of the first wave front''. There has been minimal exploration of the functional basis for this effect. It appears that listeners are insensitive to the interaural time delay of the reflected or lagging sound but not insensitive to the overall intensity of the lagging sound.
We are planning a series of experiments to establish a functional description of the effects of leading sounds on the sensitivity to stimuli that occur later in time. The leading sounds will be described as multiple clicks or click trains with varying intensities and directions. The target sounds will be time-limited probes that will used to probe the detectability of the target as well as the sensitivity of the auditory system to parameters of the the target sound.
Specifically, psychophysical measurements will be obtained for the following quantities: (1) the detection threshold of the target sound as a function of time after the onset of the leading sound, and (2) the sensitivity of the observer to changes in the interaural time delay, the interaural intensity difference, and the overall intensity of the target sound, again as a function of time after the target sound.
These experiments will be measured for a leading sound that is a single burst of varying duration, that is a single burst with an intensity that decreases with time from a single direction, and that is a sequence of burst that are from a common direction and that are from a random set of directions (modeling diffuse reverberation). The target sound will be chosen to be a delayed version of the leading sound (the simple precedence case) and to be an uncorrelated burst with similar spectral characteristics (an independent noise burst), and as a burst with a different spectral characteristics.
The results of this set of studies should allow a description of the dependence of auditory sensitivity to stimulus changes during the time following stimulus onset. We believe that this study will suggest a model of post-onset suppression of sensitivity to some stimulus attributes. This model will describe a mechanism that would assist in the location of sources in reverberant environments. These studies are companion studies to a set of physiological experiments that are currently underway with closely related stimuli. In fact, our goal is a model that incorporates results from the psychophysical studies and the parallel physiological studies. For ongoing stimuli, these results may be appropriate as descriptions for schemes that allow extraction of information about source and environment using local maxima in the waveform as analogous to stimulus onsets.