Tyco 579-769 specifications Measures of Intelligibility, Continued

Measures of Intelligibility, Continued

The STI Method	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.
STIpa	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.
STITEL	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.
RASTI	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.
Percent (%) ALcons	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.

Phonetically

Balanced Word

Scores

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.

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