UBC Theses and Dissertations
The forward masking effect of 2fl-f2 in normal hearing and noise-induced hearing loss subjects Carson, Arlene Jane
The present study investigated the combination tone 2fl-f2 in normal hearing subjects and subjects with high frequency noise-induced hearing loss by measuring the forward masking effect of 2fl-f2 exerted at its own frequency. Forward masking was plotted as the increase in masked threshold of a brief probe tone located at frequency 2fl-f2 as a function of the intensity of the primary tones, fl and f2. The masking effect of the lower primary alone, fl, was also measured to determine the possible contribution of fl toward the total masking obtained with the fl+f2 stimulus. Results of this investigation showed that 1) a forward-masking effect attributable to 2fl-f2 was seen for each normal-hearing subject, usually beginning at an fl+f2 intensity between 40 and 60 dB SPL. Average slopes of the growth of this masking were 0.8 between 60 and 70 dB SPL and 1.1 between 70 and 80 dB SPL. 2) there was little or no masking effect due to fl+f2 for hearing-loss subjects at intensities between 60 and 70 dB SPL, most likely explained by the fact that the quiet thresholds at the fl, f2, and probe tone frequencies were elevated for these subjects. This finding agreed with published studies by showing that the forward masking effect was strongly and inversely correlated with the quiet thresholds at the fl, f2, and probe tone frequencies (significant to p=.0l). 3) at an fl+f2 intensity of 80 dB SPL, several hearing-loss subjects displayed a masking effect attributable to 2fl-f2. It is not clear why this occurred for some of these subjects, because even at 80 dB SPL, the sensation level of the fl+f2 stimulus was only slightly supra-threshold. 4) for some normal-hearing and hearing-loss subjects a significant masking effect due to fl alone was noted. This finding made it difficult to assess the contribution of 2fl-f2 toward the total masking displayed with the fl+f2 stimulus for these subjects. 5) results with the fl and fl+f2 stimulus were highly variable for both subject groups. 6) the unresolved role of suppression in distortion product generation as well as a limited understanding of noise-induced hearing loss make it difficult to apply the present results toward one particular model of combination tone production.