Gaussian noise (GN) simultaneous maskers yield higher masked thresholds for pure tones than low-fluctuation noise (LFN) simultaneous maskers for listeners with normal hearing. This increased residual masking is thought to be due to inherent fluctuations in the temporal envelope of Gaussian noise, but these masking effects using forward maskers have been previously unexamined. Because differences in forward masking due to age and hearing loss are known, the first study measured forward-masked detection thresholds for younger and older adults with normal hearing (NH) and older adults with hearing loss (HI) for a 4000 Hz pure-tone probe at a single masker-probe delay in narrowband noises with maximal (GN) or minimal (LFN) inherent envelope fluctuations. As predicted, results suggested that no effect of age was observed. Surprisingly, forward-masked threshold differences between GN and LFN, an estimate of the magnitude of the effect of inherent masker envelope fluctuations, were not significantly different for older HI listeners compared to younger or older NH listeners. Due to the surprising similarities between listeners with normal and impaired hearing, the second study was designed to assess effects of hearing loss on the slopes and magnitudes of recovery from forward maskers that varied in inherent envelope fluctuations for masker-probe delays of 25, 50, and 75 ms. In addition to measuring these effects centered at 4000 Hz, forward-masked thresholds were also measured at 2000 Hz, a region of better hearing for the HI listeners. As hypothesized, regardless of masker fluctuations, slopes of recovery from forward masking were shallower for HI than NH listeners in all conditions. At 4000 Hz, additional residual masking was greater in HI than NH listeners at the longest masker-probe delays; whereas, no differences in additional residual masking between HI and NH listeners were observed for 2000 Hz. These results suggest that the masking effects from inherent envelope fluctuations persist to a greater degree and duration in regions of greater hearing loss. This persistence of the effects of inherent masker envelope fluctuations over time led to the consideration of measuring amplitude-modulated (AM) forward masking to estimate the contribution of modulation masking persistence. Studies measuring forward-masked modulation detection thresholds (MDTs), in which an AM masker preceded an AM signal, have recently revealed an effect termed “AM forward masking” (Wojtczak and Viemeister, 2005; Wojtczak et al., 2011). The third study was designed to assess differences in AM forward masking at 1000 and 4000 Hz for NH and HI listeners. In line with predictions, an unmodulated GN masker yielded significantly more masking than an unmodulated LFN, suggesting that inherent masker envelope fluctuations contributed to the amount of AM forward masking across listener groups. Contrary to predictions, results suggested there were no differences in the amount of AM forward masking between NH and HI listeners, suggesting there is little effect of hearing loss on recovery from AM forward masking. Considering the combination of forward masking, AM forward masking, and listener uncertainty, the persistence of masker envelope fluctuation effects likely lead HI listeners to experience a shallower slope and longer time course of recovery than NH listeners. When attempting to recognize a speech segment after the offset of a masker when the signal-to-noise ratio is particularly unfavorable, the persistence of masking may play a role in the difficulty that HI listeners have relative to NH listeners for speech-in-modulated-noise recognition.
University of Minnesota Ph.D. dissertation. August 2015. Major: Speech-Language-Hearing Sciences. Advisor: Peggy Nelson. 1 computer file (PDF); ix, 867 pages.
Temporal Masking Contributions of Inherent Envelope Fluctuations for Listeners with Normal and Impaired Hearing.
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