The present study examined the possible relationship between classification of professional singing voices and their vocal tract parameters including vocal tract length and volume, and vowel formant frequencies. Acoustic reflection technology (ART) was used to measure vocal tract length and volume of 107 professional singers: 32 tenors, 25 baritones, 27 sopranos, and 23 mezzo-sopranos. The first three formant frequencies (F1-F3) of the English vowels /a, æ, i/ produced by the professional singers were also obtained. Results indicated significantly shorter oral and vocal tract length, and smaller oral and vocal tract volume associated with sopranos when compared with mezzo-sopranos. Acoustically, sopranos had higher F1, F2, and F3 values than mezzo-sopranos. The present findings suggest that, in addition to vocal tract length, vocal tract volume may also affect formant frequencies, implying the possibility that classifying professional singing voices is based on both vocal tract length and volume information.