IEC 60268-16 STIPA: The Simplified STI Measurement Method
TL;DR
STIPA (Speech Transmission Index for Public Address systems) is a simplified single-number measurement defined in IEC 60268-16 Annex F. Instead of measuring 98 MTF values, STIPA uses a specially designed test signal with 2 modulation frequencies per octave band (14 total, per Table F.1), simultaneously modulating all 7 bands. A 15-second measurement captures the modulation depth at each frequency. STIPA correlates within ±0.03 of full STI for typical PA system conditions. It is the standard method for fire alarm voice evacuation system commissioning per EN 54-16.
Why STIPA?
Full STI measurement requires either a complete 98-point MTF grid or a high-quality impulse response. STIPA provides a practical alternative that can be measured in 15 seconds using a standardized test signal played through the PA system. This makes it ideal for large-scale commissioning where hundreds of measurement positions are needed.
Test Signal Design (Annex F, Table F.1)
The STIPA test signal consists of 7 simultaneous octave-band noise signals (125 Hz to 8 kHz), each amplitude-modulated at 2 specific frequencies. The modulation frequencies are chosen so that no two bands share the same pair, and all 14 of the standard modulation frequencies (0.63 to 12.5 Hz) are represented:
| Band (Hz) | Mod freq 1 (Hz) | Mod freq 2 (Hz) |
|---|---|---|
| 125 | 1.00 | 5.00 |
| 250 | 0.63 | 6.30 |
| 500 | 0.80 | 8.00 |
| 1000 | 1.25 | 10.0 |
| 2000 | 1.60 | 12.5 |
| 4000 | 2.00 | 3.15 |
| 8000 | 2.50 | 4.00 |
The modulation depth is 0.55 (-5.2 dB relative to full modulation). Each band is independently modulated so crosstalk between bands can be detected.
Measurement Procedure
- Play the STIPA test signal through the PA system at normal operating level
- Record 15 seconds of the received signal at the measurement position
- Bandpass filter into 7 octave bands
- Extract the modulation depth at each of the 14 frequencies
- Compute MTF as the ratio of received to transmitted modulation depth
- Follow the standard STI calculation pipeline
Accuracy and Limitations
STIPA agrees with full STI within ±0.03 for most PA system conditions. However, accuracy degrades when:
- Strong discrete echoes (flutter echo) create MTF peaks at specific modulation frequencies not sampled by STIPA
- Nonlinear distortion (clipping) affects specific bands asymmetrically
- Very long reverberation times (T > 4s) where the 15-second measurement may be insufficient
For these edge cases, measurement from impulse response or extended-duration STIPA (3× averaged) provides better accuracy.
EN 54-16 Application
The European fire alarm voice evacuation standard EN 54-16 mandates STIPA measurement at every listener position during system commissioning. The minimum requirement is STI ≥ 0.50 (Fair). Many specifiers require ≥ 0.55 or ≥ 0.60 to provide a safety margin. See measurement procedures for equipment requirements.
SonaVyx STIPA
The STI tool generates the standard STIPA test signal and measures the received modulation transfer function. Quick mode uses a single 15-second capture; Pro mode averages three captures for improved reliability. Results include the qualitative rating and can be included in compliance reports.
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Last updated: March 19, 2026