Detection of amplitude modulation.

The detection of frequency modulation at high modulation frequencies seems to be based upon detection of the first lower modulation sideband in the presence of the carrier as masker. This conclusion followed from a series of interleaved experiments on modulation detection and sine tone masking. [W. M. Hartmann, J. Acoust. Soc. Am. 63, S66(A) (1978)]. The present paper reports a similar combined study of amplitude modulation detection and masking. The AM detection experiment used an 80 dB. 1000 Hz carrier with high and low modulation frequencies. The masking experiment used an 80 dB, 1000 Hz masker and a target tone with frequency equal to a sideband frequency from the AM experiment. Each experimental run consisted of an AM detection half and a masking half. Both halves employed a 2IFC staircase method. The results for three subjects show that for modulation frequencies of 200, 150, and 100 Hz, AM detection threshold occurs when the lower modulation sideband is at masked threshold. For modulation frequencies of 50, 25, and 12 Hz, AM is detected when the lower sideband is 5.2 dB (±1.3 dB) below masked threshold. These results agree with a duplex model of AM detection. In this model AM detection is based upon lower sideband detection for high modulation frequencies and upon detection of intensity fluctuations for low modulation frequencies. [Supported by the NSF.] [ABSTRACT FROM AUTHOR]