Academic journal article Perception and Psychophysics

Sweep-Induced Acceleration in Loudness Change and the "Bias for Rising Intensities"

Academic journal article Perception and Psychophysics

Sweep-Induced Acceleration in Loudness Change and the "Bias for Rising Intensities"

Article excerpt

A pure tone changing continuously in intensity shows sweep-induced fading (SIF) of loudness as intensity sweeps down and may show a lesser degree of sweep-induced enhancement (SIE) as intensity sweeps up (Canévet & Scharf, 1990); the former effect has been called decruitment, the latter upcruitment. An opposite effect-upsweeps being judged to show more loudness change than downsweeps-has been reported by Neuhoff (1998). These disparate results might stem from several procedural differences. We found that differences in the sweep's duration and intensity level did not account for the disparity, nor did the presence of a steady tone preceding the sweep. In a second experiment, direct judgments of sweep size, such as those Neuhoff's (1998) listeners made, were affected not only by sweep size itself, but also by the intensity at the end of the sweep. The latter effect was especially marked for upsweeps. Neuhoffs (1998) proposed "bias for rising intensities" was found only with a method for judging sweep size that is more sensitive to end level than to sweep size.

There is now a substantial body of evidence illustrating the accelerated loss of loudness that occurs when a tone starting at a moderate level decreases in intensity in a continuous manner: The end level of such a sweep is judged to be softer than that level presented alone, a phenomenon that has been called decruitment (Canévet, 1986) but, in this report, will be called sweep-induced fading (SIF). We know that this occurs for pure tones, and, under certain conditions and perhaps to a lesser degree, for noise. We also know that for tones, the magnitude of the effect decreases sharply as the duration of the sweep decreases below the region of 10-20 sec, and that it disappears entirely by 1 sec (Teghtsoonian, Teghtsoonian, & Canévet, 2000). (These temporal spans might be termed macrodurations to distinguish them from those of 250 msec or less [micro-durations], used, for example, by Stecker & Hafter, 2000.) It is also known that the effect does not survive a test in the contralateral ear: When the sweep is presented to one ear and the end level alone is presented to the other ear, the latter is judged to be as great as if it had been presented without the contralateral sweep (Schlauch, 1992), indicating a phenomenon that occurs at the periphery. Yet Schlauch has also shown that the magnitude of the effect is diminished when the listener performs a second, distracting task, suggesting that we are dealing with a peripheral process governed, to some extent, by a central factor, attention. Theoretical speculation about SIF has focused primarily on various forms of adaptation (e.g., Canévet & Scharf, 1990; but see also Teghtsoonian et al., 2000), although there is still no definitive account of the process and how it occurs. Despite this uncertainty, the empirical phenomenon is robust and striking in its magnitude: A 40-dB 1 -kHz tone heard at the end of a 20-sec downsweep starting at 70 dB will sound only about a quarter as loud as that same tone heard alone.

Another approach to the study of continuously changing sound intensities has been reported by Neuhoff (1998). His listeners were asked to judge the size of an intensity sweep with a duration of 1.8 sec, spanning 15-dB ranges in the interval between 60 and 90 dB for both increasing and decreasing conditions. For a 1-kHz tone and a synthetic vowel, although not for a white noise, he found that upsweeps were consistently judged to show greater change than corresponding downsweeps did. He offered the speculation that such a perceptual bias for rising intensities might have survival value by making the approach of a fixed-level sound source more salient than the receding of that source over an equal distance. (The merits of that speculation are not the main concern of this report, although we will offer some comments on it in our Discussion section.) Yet, in the work on SIF, rising sweeps never show a larger range of judged loudness than falling sweeps do. …

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