The Effect of Relative Formant Amplitude on the Perceived Identity of Synthetic Vowels

The perceptual effect of varying the relative amplitude of the formants in two-formant synthetic vowels has been investigated. It was found that as the amplitude of the second formant was reduced the identity of each sound was firstly independent of the relative amplitude, then became ambiguous and finally changed to that of a vowel of correct first formant frequency. These experiments were performed with isolated vowel sounds of 240 and 480 msec. duration, and with vowels in an /h-d/ context. The shift in the mean second formant frequency of the vowel areas in the F1-F2 space which accompanied the change in perceived identity was gradual for isolated vowels but abrupt for /h-d/ words. A greater uniformity between the responses of different listeners was observed with the vowels in context. These results are interpreted to indicate that a perceptual mechanism behaviourally similar to peak-picking operates when the format amplitudes are within 28 db. but that various biases of the listeners, and a possible subsidiary mechanism which cannot be precisely evaluated, dominated responses when the formant amplitudes differed by more than 28 db.