Twelve reproducible noises were used as stimuli in a 2-interval forced-choice signal-detection experiment. The noises were stored numerically in a PB250 computer that converted thein to sound during the experiment by means of a digital-to-analog converter. The 240 numbers specifying a noise were sampled at a rate of 2500 numbers/sec, generating a 96-msec stimulus. A sinusoidal signal with a period of 8 samples/cycle was added to one noise on half the trials. Spectral analyses were computed for all stimuli. On nonsignal trials, biases to particular noises were found that could be explained in part, but not entirely, by differences between the noise pairs in energy around the signal frequency. Performance on signal trials was related to the energy difference between the stimuli in the region near the signal frequency, but was not entirely accounted for by this variable. Special characteristics of certain noises appear to affect the subject's response when these noises appear in either signal or nonsignal trials. Except for bias effects, detection on signal trials with the same noise in both intervals was similar to that on trials with different noises. Implications for theories of signal detection are discussed.

In the presence auditory detection of a sinusoidal signal in the presence of random noise, it has previously been demonstrated that the presence of a “pedestal” or background sinusoidal of the same frequency and phase as the signal increases detectability. This increase was confirmed in a two interval forced choice experiment in which a 1000-cps sinusoid was present in our of two 0.1-sec intervals, and the noise plus pedestal were present in both intervals. Pedestals of moderate intensity in phase with the signal increased detectability. Pedestals 90° out of phase with the signal did not improve detectability. Very large pedestals decreased detectability regardless of phase. A simple energy detection model was developed and its performance compared with the experimental results. The model consists of a bandpass filter, a rectifier or square-law element, and an integrator. The probability of correct detection varies with signal in a manner similar to the auditory results. Detectability is improved by an inphase pedestal and approaches that of a correlation detector in the limit of infinite pedestal. Detectability is not improved by 90° out-of-phase pedestals. However, the model does not predict decreasing detectability with large pedestals.

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