The effects of controlled speech level input on the intelligibility testing of speech compression algorithms

Speech level measurements were made for a collection of Diagnostic Rhyme Test (DRT) word lists as used for intelligibility testing of voice communications systems. These measurements were used in the generation of a subsequent set of DRT source tapes for which all DRT list segments on the tapes were equalized with respect to speech level. The effects of this equalization on intelligibility scores was evaluated for unprocessed and processed speech (both parametric and waveform coding algorithms). The manipulation of the speech levels for the DRT lists did not result in significant, predictable, and replicable effects on either system mean scores or on the DRT score variability across lists. The absence of effects was noted for all speech processing conditions and for both male and female speakers. Practical advantages in the use of the equalized word lists are discussed.