How to Test PA System Intelligibility per IEC 60268-16

What You Need

• A device with a microphone for capturing (phone, laptop, or tablet)
• A second device or the PA system itself to play the STIPA test signal
• The venue configured as it would be during normal use (seats occupied or empty per your test condition)
• SonaVyx STI tool open at /tools/sti

Step-by-Step Procedure

1. Understand the STIPA method. STIPA uses a specially modulated pink noise test signal with 14 modulation frequencies across 7 octave bands (125 Hz to 8 kHz). The Modulation Transfer Function (MTF) measures how well the modulation survives the room and PA system. An STI of 1.0 means perfect intelligibility; below 0.50 is considered "Poor."
2. Generate the STIPA test signal. In SonaVyx STI tool, select "Generate STIPA Signal." This creates the IEC 60268-16 Table F.1 compliant test signal. Route this to the PA system — either through a direct audio connection or by playing from a device connected to the mixing console.
3. Set playback level. Adjust the PA system to its normal operating level for speech. STIPA measurements should be conducted at the same level the system operates during normal use. A typical target is 65-75 dBA at the listener position.
4. Position the measurement microphone. Place your capture device at the listener position — seated ear height (1.2 m) for auditorium seating, or standing ear height (1.5 m) for areas where occupants stand. IEC 60268-16 requires measurements at the most distant listener position and at positions representative of the seating area.
5. Capture for 15 seconds minimum. Select "Quick" mode for a 15-second capture, or "Pro" mode for three averaged measurements. The Pro mode reduces measurement uncertainty and is recommended for compliance testing. Keep the room quiet during measurement — any extraneous noise corrupts the MTF calculation.
6. Read the STI result. SonaVyx displays the STI value (0 to 1.0), the qualitative rating (Bad/Poor/Fair/Good/Excellent), and the per-band MTF breakdown. For life-safety systems, EN 54-16 requires STI ≥ 0.50 in all measurement positions. For general PA, aim for STI ≥ 0.60.
7. Measure at multiple positions. A single measurement point is not sufficient. IEC 60268-16 recommends measuring at the worst-case position and several representative positions. Map the STI across the venue to identify weak zones.
8. Diagnose low STI. If STI is below target, check: excessive reverberation (RT60 > 1.5s reduces STI), poor signal-to-noise ratio (ambient noise too high), frequency response irregularities (missing 2-4 kHz range critical for consonant intelligibility), or time alignment issues (delay speakers out of sync).

STI Rating Scale (IEC 60268-16)

STI Range	Rating	Typical Application
0.00 - 0.30	Bad	Unusable for speech
0.30 - 0.45	Poor	Below minimum for PA
0.45 - 0.60	Fair	Minimum for public address
0.60 - 0.75	Good	Target for most venues
0.75 - 1.00	Excellent	Studios, critical listening

Common Mistakes

• Playing the test signal too loud. Cranking up the PA masks noise floor problems that exist at normal operating levels. Always test at the level the system actually operates.
• Ignoring ambient noise. STI measures the modulation transfer through the entire signal chain including room noise. A quiet room with a bad PA can score higher than a good PA in a noisy room. Record the ambient noise level separately.
• Single-point measurement. One position can pass while the back row fails. Always measure the worst-case position (farthest from speakers, under balconies, corners).
• Not accounting for room occupancy. People absorb sound and reduce reverberation. Unoccupied room STI is typically 0.05-0.10 lower than occupied. Note which condition you measured.

Tool Bridge

Open the SonaVyx STI Tool to generate the STIPA signal and measure intelligibility. Use the per-band MTF breakdown to identify which frequency ranges are degrading intelligibility.