PURPOSE: to compare perception and production of voiced constraint in the speech of two subjects, one in normal acquisition and another with phonological impairment, and analyzing acoustic and perceptual research methods of the voiced constraint. PROCEDURES: an instrument of minimal voiced pair versus voiceless constraints were created to expel the speech production and perception of the subjects. The data acoustical analyses were recorded on MiniDisc Sony MZ-R70, in acoustic treated room, and by Sona-Graph 5500 program of Kay Elemetrics analysis. Two clinical and phonological acquisition researchers made perception judgments about speech of the children in terms of voiced and voiceless constraint. We also evaluated the voiced constraint perception by the children with Levi's instrument (Levi, 1994). RESULTS: both children had appropriate perception of voiced constraint and its production in some contexts. The normal phonology acquisition had more contexts with adequate production. There were 10% of disagreement between the judges about children' voice constraint. CONCLUSION: the perception and acoustical analyses are complementary in speech evaluation. There are some voiceless events in the typical acquisition child's speech.