Relation between the growth of loudness and high-frequency excitation

To test the conjecture that cochlear impairment and a masking noise in normal ears produce analogous effects on the growth of loudness, loudness was measured for 21 listeners, all with bilateral symmetrical high-frequency cochlear losses. Beyond the normal-hearing region, thresholds increased by 30-49 dB/octave for 10 listeners in group I and by 51-71 + dB/octave for 11 listeners in group II. Measured by absolute magnitude estimation and production at two frequencies where thresholds were normal, the dynamic range over which loudness grows was not altered by the adjacent hearing loss. This was not true however, for the slope. Whereas the overall shape and slope of the loudness functions were essentially the same for group I, the mid-to-high level slope for group II was noticeably flatter for a tone at the cutoff frequency than for a lower-frequency tone. In contrast, markedly steeper loudness functions, characteristic of loudness recruitment, were obtained for both groups at a frequency in the region of impaired hearing. The reduced rate of loudness growth observed in the region of normal hearing for listeners with steep low-pass losses, like the reduction usually found for a tone masked by an adjacent high-frequency noise, is ascribed to the tone's restricted excitation pattern. These results indicate that high-frequency auditory cues are effectively eliminated by a steeply sloping high-frequency hearing loss just as they are eliminated by the addition of an external noise.