The Case of the “Missing MLD”

Abstract

The decrease in the size of the MLD with signal frequency is presumably caused by the inability of the auditory fibers to follow details of the waveform at high frequencies. The ear appreciates slow frequency envelope variations even at high carrier frequency, however, as the residue attests. Using amplitude modulation, we attempted to obtain an MLD by manipulating the phase of a 250-Hz envelope on a 2000-Hz carrier. Essentially no MLD was observed. Attempting to create a more salient low-frequency pitch sensation we used a high-pass filtered pulse train. Delaying the pulse train, 250 ips, by half a period in the opposite ear had essentially no effect on the detectability of the binaural stimulus in wide-band masking noise. Our procedure is different from that used by Small who showed that simply inverting the waveforms at the two ears produced no MLD. These results replicate some of a more extensive series of measurements carried out by Dr. G. Bruce Henning. [Research supported by the National Institutes of Health, Public Health Service, U. S. Department of Health, Education, and Welfare, and by a research grant awarded to the Center for Human Information Processing.