Judgments of speaker size are largely determined by two acoustic variables: glottal pulse rate (GPR) and resonance scale (i.e., vocal tract length (VTL)). Both variables change with age (or height), but the rate is governed by different factors. The interaction of the variables was previously measured using absolute judgments of speaker size [D.R.R Smith and R.D. Patterson, J. Acoust. Soc. Am. 118, 3177-3186 (2005)]. The resulting size surface (over the GPR-VTL plane) bends down outside the normal range. In this paper, a method is developed for deriving the surface using size-discrimination data. In a two-alternative forced-choice experiment, listeners compared sequences of vowels scaled in GPR and VTL to represent speakers with slightly different sizes; they were required to choose the interval with the smaller speaker. Comparisons about a point in the plane reveal the gradient vector, and the vectors across the GPR-VTL plane can be integrated to estimate the size surface. The results indicate that the size surface would be essentially planar if determined by size discrimination. This indicates that relative size judgments are different from absolute size judgments, probably because some source knowledge is required for the absolute judgments.

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