Measuring a voice alarm system in a quiet, empty building at night without accounting for daytime crowd and HVAC noise produces artificially high STI values. Instead, IEC 60268-16 allows measuring the clean MTF in the empty room and mathematically combining it with the 7-octave-band ambient noise spectrum.
The -15 dB to +15 dB Octave SNR Window
- SNR ≤ -15 dB: Intelligibility is zero (STI = 0.00).
- SNR = 0 dB: Maximum achievable STI is capped at 0.50.
- SNR ≥ +15 dB: Background noise no longer degrades STI.