Detection of changes in spectral shape: uniform vs. non-uniform background spectra

In most of the previous studies [(1987) Profile Analysis: Auditory Intensity Discrimination, Oxford Univ. Press] concerning the detection of a change in spectral shape, or 'profile analysis', the listener's task was to detect an increment to a single component of an otherwise equal-amplitude, multi-component background. We wished to determine whether listeners would exhibit sensitivity to changes in spectral shape if the spectrum of the background were not flat. The results of two experiments, like those of Green and Kidd [(1983) J. Acoust. Soc. Am. 73, 1260-1265], indicate that the spectrum of the background can deviate rather substantially from 'flat' before detection of the signal becomes more difficult. In a third experiment, we investigated how the function relating the threshold of the signal to the frequency of the signal would vary as the number of components in a flat multi-component background is altered. Our results indicate that as the number of components which compose the background is reduced from 21 to 3, thresholds for increments to the middle region of the spectrum are elevated rather considerably.