Absolute thresholds for frequency-modulated signals: effects of rate, pattern, and percentage of modulation.

Absolute monaural thresholds for continuous unmodulated and frequency-modulated signals were determined for five normal-hearing subjects using the method of adjustment. Three carrier signals (250, 1000, and 4000 HZ) were modulated at rates of 2 and 20 times per sec using frequency deviations of +/- 5, 10, and 20%, and both ramp and sinusoidal modulation. Thresholds measured at the slower modulation rate were 5 to 10 dB lower than those measured for unmodulated signals at the two higher test frequencies. Suggestions are presented as to how the clinician may minimize the possible discrepancy between modulated and unmodulated signals using currently available warble-tone equipment.