A PA system is judged by how well people understand it, not by how loud it gets. STI (Speech Transmission Index) is the metric. STIPA is the field test. Both decide whether your mosque, mall, or transit hub passes audit, and whether emergency announcements actually save lives.
Four ideas to carry into the next specification conversation you have.
0 = unintelligible. 1 = perfect. The test compares modulated signal at the speaker vs at the listening point.
EN 54-32, BS 5839, NFPA 72, KSA Civil Defense. Below 0.50 announcements fail and life-safety certification is withheld.
A standardised test signal (Bose STI-PA generator, NTI XL2) played through the PA, measured at listener positions. 30 seconds per point.
Spacing > 1.6× ceiling height = poor STI no matter how loud. Spacing ≤ 1× height = consistent STI > 0.60.
Per IEC 60268-16 (5th edition) and EN 54-32 thresholds for life-safety PA.
| STI | Intelligibility · class | Spec required |
|---|---|---|
| 0.00 – 0.30 | Bad · U | Reject |
| 0.30 – 0.45 | Poor · V | Background music only |
| 0.45 – 0.60 | Fair · U-V | General PA |
| 0.50+ | Required minimum · life-safety | EN 54-32 / KSA CD |
| 0.60 – 0.75 | Good · A-B | Mosque · classroom |
| 0.75 – 1.00 | Excellent · A-A+ | Theatres · courtrooms |
Five practical rules for PA in Saudi mosques, malls, and stations.
A mosque with hard marble walls and dome ceilings can have RT60 (reverberation time) of 4–6 seconds. Even a 100 W speaker delivering 85 dB cannot achieve STI 0.50 in that acoustic environment without distributed speakers, time-aligned, with delay processing. Spec acoustic treatment AND PA together.
One powerful speaker in the centre gives high SPL but creates strong direct/reverberant ratios that fall off rapidly. Twenty small ceiling speakers at 8 m spacing, each at 4 W, deliver consistent STI ≥ 0.55 across the entire floor. Saudi mosques and malls now spec distributed by default.
STI requires the speech signal to exceed background noise by ≥ 10 dB. A mall food court at 75 dBA needs 85+ dBA speech to hit STI 0.55. Without adaptive volume (ambient noise sensors driving amplifier gain), evening crowds drop STI below threshold.
An empty mall reverberates differently from a packed one. EN 54-32 mandates STIPA at "normal occupancy noise level", meaning ambient HVAC running, but the test is done empty using calibrated noise sources. Spec the test conditions in the commissioning plan.
KSA CD inspection for mosque, mall, and assembly occupancy reviews STI/STIPA reports from commissioning. The reports list 30+ measurement points with STI value per point, and an averaged class. Build commissioning includes this, not optional.
We design distributed PA systems for mosques, malls, and transit hubs to STI ≥ 0.55, deliver STIPA commissioning reports, and provide KSA Civil Defense documentation.
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