How many images are in an auditory scene?

If an auditory scene consists of many spatially separated sound sources, how many sound sources can be processed by the auditory system? Experiment I determined how many speech sources could be localized simultaneously on the azimuth plane. Different words were played from multiple loudspeakers, and listeners reported the total number of sound sources and their individual locations. In experiment II the accuracy of localizing one speech source in a mixture of multiple speech sources was determined. An extra sound source was added to an existing set of sound sources, and the task was to localize that extra source. In experiment III the setup and task were the same as in experiment I, except that the sounds were tones. The results showed that the maximum number of sound sources that listeners could perceive was limited to approximately four spatially separated speech signals and three for tonal signals. The localization errors increased along with the increase of total number of sound sources. When four or more speech sources already existed, the accuracy in localizing an additional source was near chance.