DETECTION AND IDENTIFICATION OF A SINGLE MODULATED CARRIER IN A COMPLEX SOUND

Hall and Grose [J. Acoust. Soc. Am. 90, 3028-3035 (1991)] reported tha t subjects had difficulty in deciding which carrier in a two-carrier c omplex sound was modulated. The present experiments examined how the a bility to identify a single modulated carrier was affected by the numb er of carriers in the complex and by harmonicity. Initially, threshold s were measured for detecting amplitude modulation of a single carrier in a complex sound. Thresholds were higher when that carrier was one of the inner carriers in a six-carrier harmonic or inharmonic complex than when it formed part of a two-carrier complex. Thresholds were onl y slightly, if at all, higher when the modulated carrier was varied ra ndomly from trial to trial than when its frequency was fixed within a block of trials. Next, subjects were required to decide whether the fr equency of a single modulated carrier (with a suprathreshold modulatio n depth) in a complex sound was the same as or different from the freq uency of a probe composed of a single modulated carrier. They generall y performed well above chance. Performance was not greatly affected by whether the probe was presented before or after the complex, but was generally slightly better for a modulation depth of 100% than for a de pth of 50%. Randomly varying the level of each carrier in the complexe s from one stimulus to the next, produced only a slight impairment of performance, indicating that short-term across-frequency differences i n level were not used to identify the modulated carrier in experiment 2. Overall, performance was best for the six-carrier harmonic complex, less good for the six-carrier inharmonic complex, and worst for the t wo-carrier complex. The results are interpreted in terms of perceptual grouping.