The Speech Transmission Index is a measure of the reduction of speech modulation by noise and reverberation in the environment. The essence of this method of clarity evaluation can be made obvious by simply observing the time vs. amplitude response of recorded speech.
Figure 1 shows the time vs. amplitude response of anechoic (no echoes) speech. Note the lack of energy between the speech syllables. The dynamic range of the speech has not been reduced by the environment. A room will tend to “fill in the gaps” of the time vs. amplitude record, therefore reducing speech modulation.
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