Accuracy of pointing a binaural listening array

INTRODUCTION Arrays of microphones have been used to increase the detection of distant or faint sources of sound by applying directional gain to the listening process. They can also be used to restore listeners' situation awareness by replacing auditory direction information attenuated or distorted by protective gear (such as helmets or chemical protective masks) or enclosures. Scanlon and Tenney (1994) developed a binaural microphone array that can support both of these functions. This array has two arms (one for each ear) that are connected and hinged at one end. It was designed to support the detection, identification, and localization of sources of sound and can he either portable (e.g., mounted in a helmet or on a rifle) or used at a fixed listening station. The goal of this microphone array is to augment soldiers' auditory abilities at low levels of sound intensity, much as night vision devices support vision at low light levels. In general, listening through an array can be considered similar to unaided binaural hearing. Interaural time differences (ITDs) would be minimal for sources along the midline and maximal for sources located laterally. Interaural differences of level (ILDs) would also be minimal for sources directly ahead. For the array, however, differences of level for sources of sound away from the midline do not depend on a shadow created by the head but, rather, on the shape of the beam pattern of each arm and its relation to the position of the source of sound. Intensity differences will be minimal only when the angular separation of the arms is small or when a source is located along the axis bisecting the array. In addition, there may be interactions between the arms that make operation of the array different from human hearing (e.g., one arm can narrowly shadow the other or reflect energy to it). Input to a listener is led directly through a dosed ear mold; it is independent of head movement and of the filtering effect of the head. This produces lateralized sound images that move along an interaural axis as localization cues favor one ear or the other. Note that occasionally we use the term localization to refer to listeners' ability to use the array to point at an external source of a sound, although they accomplish this by centering an internalized image. The desirable angular separation of the arms of a binaural array and a possible trading of coverage and resolving power are important concerns for the design of an array. Throughout this paper, we refer to the angle formed by the two beams of the array as [Alpha] and the angle formed by the axis of the array and the azimuth of the source of a signal as [Theta]. These relations are depicted in Figure 1. If [Alpha] is small, the reduced angular coverage afforded by the array reduces the range of the listener's interaural cues. Conversely, if [Alpha] is large, pointing accuracy might suffer. In the present study, we measure the ability of listeners to point the array at a source of sound for several fixed values of [Alpha]. METHOD Participants Twelve participants served as listeners. All were between the ages of 18 and 25 years, and all had hearing sensitivity better than 15 dB HL for octave frequencies between 250 and 8000 Hz. The difference between pure-tone thresholds in both ears was no greater than 5 dB at any test frequency. All participants had otoscopically normal ears and no history of otologic pathology. The Binaural Array Linear arrays of microphones can be operated in two fashions. A broadside array has microphones aligned perpendicular to the direction of a source of sound, and an end-fire array has microphones aligned parallel to the direction of the source. Stadler and Rabinowitz (1993) discussed the operation of these configurations. Scanlon and Tenney's (1994) binaural array consists of two end-fire linear microphone arrays (one for each ear) hinged together at one end. …