The signal-to-noise ratio necessary for a constant performance level was determined for normally hearing and hearing-impaired subjects under three levels of reverberation (0.3, 0.6, and 1.2 s) with monaural and binaural hearing aids having directional and omnidirectional microphones. Results indicated (a) a significant binaural advantage (2-3 dB) which was independent of microphone type and reverberation time, (b) a significant directional microphone advantage (3-4 dB) which was independent of hearing aid arrangement (monaural or binaural) but dependent on level of reverberation, (c) a significant reverberation effect which was larger than either the binaural or directional microphone effect, and (d) additive binaural and directional microphone advantages. The results suggest that the signal-to-noise ratio is optimized when binaural hearing aids with directional microphones are used in rooms with short reverberation times.
Conventional and restricted-listening psychophysical tuning curve paradigms were used to evaluate the masking ability of forward maskers of different envelope characteristics, and the relationship between off-frequency listening effects and the envelope characteristics of both the variable masker and stationary masker used to restrict listening to a narrow region surrounding the probe. Maskers were selected so as to differ widely in the degree of fluctuation in their envelopes. The masker with the largest amplitude fluctuations exhibited greater forward-masking ability than other stimuli; this effect was observed on the high-frequency branch and within the tip region of the tuning curve. It is suggested that differences in masking ability may reflect the use of different signal detection cues. The results are also consistent with previous reports [D. Johnson-Davies and R.D. Patterson, J. Acoust. Soc. Am. 65, 756-770 (1979); B.J. O'Loughlin and B.C.J. Moore, J. Acoust. Soc. Am. 69, 1119-1125 (1981a)] which suggest that off-frequency listening is a factor contributing to the sharpness of psychophysical tuning curves. This effect is largely dependent, however, on the envelope characteristics of both variable and stationary maskers.
It has been suggested that the detection cues for simultaneously presented sinusoidal and noise maskers are different [D. Green, J. Acoust. Soc. Am. 41, 1517–1525 (1967) and D. Weber and R. Patterson, J. Acoust. Soc. Am. Suppl. 1 68, S37 (1980)]. The present study was designed to evaluate masking ability of forward maskers of varying temporal characteristics. Conventional forward-masked tuning curves were generated utilizing a sinusoidal probe and four types of stimuli for the masker: a sinusoid, quasi-frequency-modulated (QFM) tones, AM tones, and AM narrow-band noise 100 Hz wide. These maskers were selected as their envelopes possess a wide range of amplitude fluctuation. The AM and QFM tones (50 Hz wide) had identical spectral structures, yet distinctly different temporal characteristics. Differences in masker effectiveness were evaluated by comparing the tuning curves generated using the various types of maskers. Results indicate that stimuli with amplitude fluctuations that are large and random in nature (AM noise) exhibit greater forward masking ability that stimuli with either minimal or no envelope variation (QFM tone and sinusoid) or large, yet periodic, amplitude perturbations (AM tone). This difference in masking ability is exhibited on the high-frequency branch and within the tip region of the tuning curve. it is suggested that differences in masking ability may reflect the use of different signal detection cues.
Previous studies [D. Johnson-Davies and R. D. Patterson, J. Acoust. Soc. Am. 65, 765–770 (1979) and B. J. O'Loughlin and B.C. J. Moore, J. Acoust. Soc. Am. 69, 1119–1125 (1981)] suggest that off-frequency listening is a major factor contributing to sharpness of psychophysical tuning curves. The present study is designed to evaluate the contribution of off-frequency listening to the sharpness of psychophysical tuning curves as a function of temporal characteristics of both the variable masker and stationary masker used to restrict listening to a narrow region surrounding the probe using the restricted-listening tuning curve paradigm first employed by Johnson-Davies and Patterson (1979). Conventional forward-masked psychophysicai tuning curves initially were generated using stimuli of varying temporal characteristics (sinusoids, AM and QFM tones, and AM noise) as the variable masker. Restricted-listening tuning curves subsequently were obtained utilizing fixed-level stationary maskers 100 Hz wide of differing temporal characteristics (QFM tone, AM noise, and synthetic noise comprised of sinusoids of equal amplitude, random phase relation, and spaced 1 Hz apart) which were gated with the variable masker. Results suggest that the off-frequency listening effect observed utilizing a tuning curve paradigm is largely dependent on the temporal characteristics of both variable and stationary maskers. Assuming differing detection mechanisms for maskers of varying temporal characteristics, it appears that the different detection cues are not equally dependent on off-frequency listening.
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