Effects of inherent envelope fluctuations in forward maskers for listeners with normal and impaired hearing

Abstract

Gaussian noise simultaneous maskers yield higher masked thresholds for pure tones than lowfluctuation noise simultaneous maskers for listeners with normal hearing. This increased masking effectiveness is thought to be due to inherent fluctuations in the temporal envelope of Gaussian noise, but effects of fluctuating forward maskers are unknown. Because differences in forward masking due to age and hearing loss are known, the current study assessed effects of masker envelope fluctuations for forward maskers in younger and older adults with normal hearing and older adults with hearing loss. Detection thresholds were measured in these three participant groups for a pure-tone probe in quiet and in Gaussian and low-fluctuation noise forward maskers with either 1 or 1/3 equivalent rectangular bandwidths. Higher masked thresholds were obtained for forward maskers with greater inherent envelope fluctuations for younger adults with normal hearing. This increased effectiveness of highly fluctuating forward maskers was similar for older adults with normal and impaired hearing. Because differences in recovery from forward masking between listeners with normal and impaired hearing may relate to differences in cochlear nonlinearities, these results suggest that mechanisms other than cochlear nonlinearities may be responsible for recovery from rapid masker envelope fluctuations.

Inherent envelope fluctuations in forward maskers: Effects of masker-probe delay for listeners with normal and impaired hearing

“…A masker bandwidth of 25-50 Hz produced the greatest difference in masked thresholds between GN and LFN at 1000 Hz, suggesting that a masker bandwidth near one-third an equivalent rectangular bandwidth (ERB; Glasberg and Moore, 1990) may reveal the maximal additional amount of masking yielded by GN as compared to LFN. Generally consistent with these findings for NH listeners, Svec et al (2015) showed that a 1/3 ERB GN forward masker not only produced more masking than a 1/3 ERB LFN forward masker, but that a greater difference (GN > LFN) was observed for the 1/3 ERB maskers compared to the slightly broader bandwidth masker of 1 ERB. However, this effect of masker bandwidth was unexpectedly similar for NH and HI listeners at the short masker-probe delay of 25 ms, inconsistent with the presumed effect of auditory filter bandwidths.…”

“…Masker bandwidth may be an important variable for understanding the masking effects of inherent envelope fluctuations. Although masker bandwidth did not explain differences in forward masking for NH and HI listeners in a previous study (Svec et al, 2015), forward-masked thresholds for maskers with different bandwidths at a range of masker-probe delays may provide additional information related to the theoretical implications posited by Hartmann and Pumplin (1988). Their results suggested that the temporal envelope of LFN is maximally flat (e.g., minimal fluctuations) when the bandwidth of the masker is within the boundaries of a critical band.…”

“…Presumably, these envelope detection cues are similarly robust for both NH and HI listeners at a relatively short masker-probe delay (25 ms), given that the masking effectiveness of noises that vary in their inherent envelope fluctuations do not change with hearing loss (Svec et al, 2015). Nonetheless, the slope of the recovery from forward masking is steeper for NH than for HI listeners, due to reduced cochlear nonlinearities with cochlear hearing loss (e.g., Ludvigsen, 1985;Oxenham and Moore, 1997).…”

“…When considering the known effects of hearing loss on recovery from forward masking, differences in cochlear nonlinearities cannot explain the similarities in recovery from inherent masker envelope fluctuations for NH and HI listeners observed in Svec et al (2015). However, more information is needed about the slope and time constant of recovery for forward maskers that vary in inherent envelope fluctuations.…”

“…Because of conflicting evidence about the effects of inherent masker envelope fluctuations after the offset of a masker (forward masking), Svec et al (2015) measured forward-masked thresholds for narrowband GN and LFN. The authors hypothesized that differences in the effects of masker envelope fluctuations between NH and HI listeners may partially explain why HI listeners do not "recover" after the offset of a masker, or during the valley of an AM masker, to the same extent as NH listeners.…”