Thresholds for formant frequency discrimination have been established using optimal listening conditions. In normal conversation, the ability to discriminate formant frequency is probably substantially degraded. The purpose of the present study was to change the listening procedures in several substantial ways from optimal towards more ordinary listening conditions, including a higher level of stimulus uncertainty, increased levels of phonetic context, and with the addition of a sentence identification task. Four vowels synthesized from a female talker were presented in isolation, or in the phonetic context of /bVd/ syllables, three-word phrases, or nine-word sentences. In the first experiment, formant resolution was estimated under medium stimulus uncertainty for three levels of phonetic context. Some undesirable training effects were obtained and led to the design of a new protocol for the second experiment to reduce this problem and to manipulate both length of phonetic context and level of difficulty in the simultaneous sentence identification task. Similar results were obtained in both experiments. The effect of phonetic context on formant discrimination is reduced as context lengthens such that no difference was found between vowels embedded in the phrase or sentence contexts. The addition of a challenging sentence identification task to the discrimination task did not degrade performance further and a stable pattern for formant discrimination in sentences emerged. This norm for the resolution of vowel formants under these more ordinary listening conditions was shown to be nearly a constant at 0.28 barks. Analysis of vowel spaces from 16 American English talkers determined that the closest vowels, on average, were 0.56 barks apart, that is, a factor of 2 larger than the norm obtained in these vowel formant discrimination tasks.
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