Subjects made forced choices to either side of a reference direction (0 ø, 30 ø, 60 ø, and 75 ø) to locate the image of pure tones presented through earphones. Various combinations of interaural intensity differences (liD's) and interaural time differences (ITD's) were used, including some combinations which do not occur in nature. Psychometric functions confirmed the expected apparent azimuth of ITD/IID combinations based on free-field diffraction theory and microphone measurements. The relative dominance of the intensive and temporal cues was examined at each frequency (500, 1000, and 8000 Hz). The shape of the psychometric plane for the bidimensional matrix at the 1000-Hz 30 ø condition showed possible nonlinearities in the trading relation. Data were also collected from the same subjects using the same signals presented by means of distant loudspeakers in an open field. When the free-field data were compared with the data from earphone-simulated sounds, they showed similar angular acuities, being approximately the size of those reported in the literature.
The present study attempts to simulate with earphonesthe localization of distant sound sources. The simulation is achieved by inserting in the earphone channels various combinations of interaural time difference (ITD) and interaural intensity difference (IID)predicted for various azimuth positions. A more traditional approach to earphone experiments on auditory localization has centered around the determination of a trading ratio.The trading ratio is defined as that ITD/IID where the two interaural differences will just balance each other in determining the subjective location of the sound image. One group of experiments had subjects adjust one interaural difference (either the IID or ITD) in an acousti• "pointer" stimulus until the apparent direction of the "pointer" matched the apparent direction of another "reference" stimulus (Feddersen el al., 1957; Moushegian and Jeffress, 1959). Various combinations of either IID's or ITD's or both were inserted in the "reference" stimulus and the subjects made a single azimuth match from which a trading ratio could be computed. Whitworth and Jeffress (1961) largeIy repeated this procedure, but they succeeded in obtaining two localization responses and two trading ratios, one for each of two separate spatial images associated with the single "reference" stimulus. The use of auditory "pointer" stimuli in these and many similar investigations has provided a large body of information about relative discriminations among the apparent directions of various soundso This information is often difficult to relate, however, to measures of azimuth in real space expressed in degrees or radians, since the apparent direction of the "pointer" stimulus is not measured.In another group of investigations a somewhat different technique was chosen to determine the trading ratio. An example of this technique is a study by Harris (1960)o Harris's subjects introduced enough ITD in an acoustic stimulus to just counteract a given IID already ...