AUDITORY CONTINUITY AND LOUDNESS COMPUTATION

Sequences composed of alternating bursts of different levels with no s ilences separating them can give rise to a perception of a continuous sound upon which is superimposed an intermittent stream. These experim ents sought to determine how the perceived loudness of the intermitten t stream depends on the level difference between higher-level and lowe r-level bursts in the sequence in cases in which continuity is either heard or not heard. In the main experiment, listeners were asked to ad just the level of continuous or intermittent comparison sequences to m atch the loudness of components that appeared to be either continuous or intermittent in an alternating-level reference sequence, thus urgin g them to focus on the two-stream percept. Loudness matches of the con tinuous comparison stimulus were close to physical levels of the lower -level bursts, whereas matches of the intermittent comparison stimulus were well below the physical levels of higher-level bursts. These res ults are discussed in terms of Bregman's [Auditory Scene Analysis (MIT , Cambridge, MA, 1990)] ''old-plus-new'' hypothesis: The loudness of t he intermittent stream should result from the subtraction of the lower level from the higher level under the assumption that the higher-leve l burst represents a simultaneous mixture of sounds including the cont inuation of the lower-level burst, Additional experiments verified tha t, in the absence of the continuity phenomenon, matched levels were ve ry close to the physical levels and that matches to fixed-level contin uous and intermittent sequences were precise. The matching results fro m the main experiment support predictions of neither classical loudnes s models that do not take auditory organization processes into account nor schema-based models that presume a selection of information from the higher-level bunt that does not affect the perceptual content of t his burst. The matched levels fell between predictions of models based on subtraction of acoustic pressure and acoustic power, but were very different from subtraction of loudness measured in sones, suggesting that loudness is computed subsequent to auditory organization processe s. (C) 1998 Acoustical Society of America.