Detection of simple and complex tones in the presence of a 64-dB SPL uniformly masking noise was examined in two experiments. In both experiments, the signals were either pure tones (220, 1100, or 3850 Hz) or an 18-tone complex consisting of equally intense components between 110 and 7260 Hz. In experiment 1, psychometric functions were obtained for detection in a 2I, 2AFC task. Results for eight normal listeners show that the psychometric functions are parallel for simple and complex tones. As expected, the masked thresholds for the pure tones are 43-44 dB SPL independent of frequency; the masked threshold for the complex tone is about 37 dB SPL per tone. These results indicate that the simultaneous presence of signal energy in many auditory channels aids detection. In experiment 2, psychometric functions were obtained with all four signals presented in random order within a block of trials. Results for four normal listeners show that the psychometric functions are parallel to one another and to those obtained in experiment 1. The thresholds are elevated to about 46 dB for the pure tones and to 40.5 dB for the complex tone. These results are nearly, but not quite, consistent with a multiband energy-detector model using an optimum decision rule; it appears that listeners may only make an unweighted sum of decision variables across an optimum selection of channels.
The ratios between the modulation index (eta) for just noticeable FM of a sinusoidally modulated pure tone and the degree of modulation (m) for just noticeable AM at the same carrier and the same modulation frequency were measured at carrier frequencies of 0.125, 0.25, 0.5, 1, 2, 4, and 8 kHz. Signal levels were 20 dB SL and 50 dB SPL or 80 dB SPL. At low modulation frequencies, for example, 8 Hz, AM and FM elicit very different auditory sensations (i.e., a fluctuation in loudness or pitch, respectively). In this case, eta and m show different values for just noticeable modulation. Since both stimuli have almost equal amplitude spectra if eta equals m (m less than 0.3), the difference in detection thresholds reflects differences in the phase relation between carrier and sidebands in AM and FM. With increasing modulation frequency, the eta-m ratio decreases and reaches unity at a modulation frequency called the "critical modulation frequency" (CMF). At modulation frequencies above the CMF, the same modulation thresholds are obtained for AM and FM. Therefore, it can be concluded that the difference in phase between the two types of stimuli is not perceived in this range. At center frequencies below 1 kHz, where phase errors caused by headphones and ear canal presumably are small, the CMF is useful in estimating critical bandwidth.
Psychometric functions for detection of pure (220, 1100, or 3850 Hz) and complex tones in the presence of a 64 dB SPL uniformly masking noise were measured in a 21, 2AFC paradigm. The complex tone consisted of 18 equally intense components spaced about one critical band apart between 110 and 7260 Hz. In experiment 1, all levels for a single signal were presented in mixed order within a block of trials. Results for eight normal listeners show parallel psychometric functions for simple and complex tones. Thresholds are 43–44 dB SPL for the pure tones and about 37 dB SPL per tone for the complex tone. In experiment 2, all levels and signals were presented in random order within each block of trials. Results for four normal listeners show psychometric functions parallel to those in experiment 1. Thresholds are about 46 dB for the pure tones and 40.5 dB for the complex tone. These results are consistent with a multiband energy-detector model in which the decision is based on an unweighted sum of decision variables across an optimum selection of channels. [Supported by Deutsche Forschungsgemeinschaft and NIH-NINCDS R0 1NS 18280.]
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