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3-7
As described in Chapter 2, speech consists of the frequency of the sound being uttered and the
amplitude modulation of that sound into the phonemes that create words. The STI (Speech
Transmission Index) method measures the modulation transfer function for 14 modulation
frequency bands spaced at 1/3-octave intervals from 0.63 Hz to 12.5 Hz, across seven frequency
bands from 125 Hz to 8 KHz. These 98 measurement points (7 x 14) are weighted and combined
to create a number between 0.0 (totally unintelligible) and 1.0 (perfectly intelligible). The
standardization of the carriers and modulation frequencies to be used, as well as the weighting to
be applied, has now been standardized by IEC 60268-16. The STI method is the most accurate of
all intelligibility measures, however the procedure can take as long as 20 minutes per test.
Alternate methods such as STIpa, STITEL, and RASTI (described below) are subsets of the
STI method with faster measurement times.
This is a special adaptation of the STI method for the measurement of speech intelligibility for
public address (PA) systems. It is a modification to the STI method that has been correlated to
within +/- 0.03 CIS of a full STI measurement. The STIpa method is implemented in the
Simplex/GoldLine intelligibility measurement system.
The STITEL method is a special adaptation of the STI method used for the measurement of speech
intelligibility for telephone systems. It reduces the 14 modulation frequencies to seven and applies
one to each of the seven octave bands simultaneously.
The Rapid Acoustics Speech Transmission Index (RASTI) is a simplified version of the STI
method that uses two octave bands instead of seven with four modulation frequencies measured in
one band and five in the other, for a total of nine measurements.
The “Articulation Loss of Consonants” (ALcons) is a measurement method for speech
intelligibility that is based upon the importance of consonant sounds in the overall perception of
speech. It uses specially chosen simple words in transmission tests. ALcons is expressed in
percent (%ALcons). This is a subjective test with a much greater potential for variations in score
than some of the other tests described.
The Phonetically Balance (PB) word score is described in ISO/TR 4870, and consists of specially
chosen words selected from a known population transmitted to a panel of listeners. This is a
subjective test with a much greater potential for variations in score than some of the other tests
described.
Measures of Intelligibility, Continued
The STI Method
STIpa
STITEL
RASTI
Percent (%) ALcons
Phonetically
Balanced Word
Scores