Under IEC 60268-16, speech intelligibility is objectively quantified by measuring how much amplitude modulation is lost across 7 octave bands (125 Hz to 8,000 Hz) due to reverberation, echoes (>100 ms), distortion, and noise.
Reverberation and ambient noise reduce amplitude modulation depth—this reduction is measured by the Bedrock SM50.
The 7 octave bands (125 Hz to 8 kHz) of the STIPA method per IEC 60268-16.
Warning: Excessive SPL Degrades Your STI Score!
Optimal speech level is around 65 to 75 dB(A). Cranking up the PA system above 80–85 dB(A) triggers psychoacoustic auditory masking in the human ear, which the IEC 60268-16 algorithm penalizes with a lower STI score. Therefore, calibrated SPL measurement is mandatory.
STI scale from 0.0 (unintelligible) to 1.0 (excellent).