Masking patterns for synthetic vowels in simultaneous and forward masking

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Abstract
Two synthetic vowels /i/ and /ae/ with a fundamental frequency of 100 Hz served as maskers for brief (5 or 15 ms) sinusoidal signals. Threshold was measured as a function of signal frequency, for signals presented [to subjects] immediately following the masker (forward masking, FM) or just before the cessation of the masker (simultaneous masking, SM). Three different overall masker levels were used: 50, 70 and 90 dB SPL [speech perception level]. To compare the data from SM and FM, and to compensate for the nonlinear characteristics of FM, each signal threshold was expressed as the level of a flat-spectrum noise which would give the same masking. The internal representation of the formant structure of the vowels, as inferred from the transformed masking patterns, was enhanced in FM and blurred in SM in comparison to the physical spectra, suggesting that suppression plays a role in enhancing spectral contrasts. The first 2 or 3 formants were usually visible in the masking patterns and the representation of the formant structure was impaired only slightly at high masker levels. For high levels, filtering out the relatively intense low-frequency components enhanced the representation of the higher formants in FM but not in SM, indicating a broadly tuned remote suppression from lower formants towards higher ones. The relative phase of the components in the masker had no effect on thresholds in forward masking, indicating that the detailed temporal structure of the masker waveform is not important.

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