Coherence masking protection in speech sounds: the role of formant synchrony

Three experiments were performed to examine listeners' thresholds for identifying stimuli whose spectra were modeled after the vowels/I/ and /epsilon/, with the differences between these stimuli restricted to the frequency of the first formant. The stimuli were presented in a low-pass masking noise that spectrally overlapped the first formant but not the higher formants. Identification thresholds were lower when the higher formants were present than when they were not, even though the first formant contained the only distinctive information for stimulus identification. This indicates that listeners were more sensitive in identifying the first formant energy through its contribution to the vowel than as an independent percept; this effect is given the name coherence masking protection. The first experiment showed this effect for synthetic vowels in which the distinctive first formant was supported by a series of harmonics that progressed through the higher formants. In the second two experiments, the harmonics in the first formant region were removed, and the first formant was simulated by a narrow band of noise. This was done so that harmonic relations did not provide a basis for grouping the lower formant with the higher formants; coherence masking protection was still observed. However, when the temporal alignment of the onsets and offsets of the higher and lower formants was disrupted, the effect was eliminated, although the stimuli were still perceived as vowels. These results are interpreted as indicating that general principles of auditory grouping that can exploit regularities in temporal patterns cause acoustic energy belonging to a coherent speech sound to stand out in the auditory scene.