Masked loudness functions and their relation to intensity discrimination

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Abstract
One way to assess the relation between intensity discrimination and the slope of the loudness function is to measure the jnd for a sound that falls on two distinctly different loudness functions. Two such functions were generated by presenting a 1000-Hz tone in narrow-band noise (NBN, 925–1080 Hz) at 70 dB SPL and in wideband noise (WBN, 75–9600 Hz) at 80 dB SPL. Over a range from near threshold to about 75 dB SPL, the tone's loudness function is much steeper in the NBN than in the WBN. At 72 dB SPL, where the two loudness curves cross, we measured the tone's jnd in each noise by a two-interval forced-choice procedure. Despite the differences in slope (and in sensation level), the jnd was the same in both NBN and WBN. The value of ΔI/I was 0.22, which is close to that interpolated from Jesteadt, Wier, and Green [J. Acoust. Soc. Am. 61, 169–176 (1977)] for a 1000-Hz tone at the same loudness in quiet as our 72-dB tone in noise. These and other data suggest the possibility that the size of the jnd for intensity depends more on loudness magnitude than on the slope of the loudness function. [Supported by funds from the Medical Research Service of the VA to RH and by a grant (BMS 73-06944) from NSF to RT.]

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