Christine M. Rankovic
Dept. Speech-Language Pathol. and Audiol., Northeastern Univ., 360 Huntington Ave., Boston, MA 02115
Harvey Fletcher developed articulation theory to enable the prediction of
the speech transmission capacity of imperfect communication systems, thereby
eliminating the need for the time-consuming speech recognition testing that is
otherwise required to differentiate among systems. Two assumptions underlie the
theory: (1) a speech signal in each of a number of contiguous frequency bands
comprising the audible spectrum contributes independently to the articulation
score (percent-correct for lists of nonsense syllables); and (2) the separate
contributions are additive. The approach of Fletcher and his colleagues was to
define the articulation index, A, an entity for which these assumptions hold
true. The equations for calculating A contain variables representing hearing
acuity, critical bandwidth, masking, loudness, perception, and frequency
importance weights so that the communication system's frequency response and
gain are translated into perceptual terms. In addition to the derivation of the
theory, Fletcher's 1953 book provides a method for calculating A using charts
and tabled data, as well as an extensive validation study that included a
variety of suboptimal systems. Fletcher's presentation of articulation theory
is far more comprehensive than two popular versions [ANSI S3.5-1969; N. R.
French and J. C. Steinberg,