Perceiving fractal noises in pitch, loudness, and duration

Abstract

Three experiments examined discrimination among a family of fractals possessing different power law spectra. In experiment 1, random numbers sequences were generated so that their spectra had slopes (in log‐log plane) or 0 (white noise), − 1 (flicker noise), and − 2 (brown noise). The distributions were used to produce sequences that could vary in pitch, loudness, or duration. Listeners categorized each sequence as being of one of the three noise types. Thurstonian scaling revealed that noise segment classification could be accomplished on the basis of pitch and loudness variation, but not on the basis of duration changes. Experiment 2 explored simultaneous pitch and loudness variation, finding dimensional redundancy for white/flicker discrimination suggestive of a pitch‐loudness interaction. Experiment 3 examined sensitivity for discrimination of noises as a function of the power law exponent. Listeners heard two sequences of tones and indicated whether they had the same or different power law exponent. The range of exponents spanned the interval [0, −3], with the stimuli under comparison differing by 0.2, 0.4, 0.6, or 0.8. ROC analyses revealed some variation in sensitivity to the fractal exponents as a function of position in the [0, −3] interval.