STI Testing Field Guide — IEC 60268-16 Procedures

What Is STI and Why Does It Matter

The Speech Transmission Index (STI) quantifies how well a transmission channel — whether a room, a PA system, or a combination — preserves the modulations in speech that carry intelligibility. Defined in IEC 60268-16, STI produces a single number from 0 (completely unintelligible) to 1 (perfect intelligibility). It is the internationally recognized metric for verifying emergency voice alarm systems, classroom audio, courtrooms, transport terminals, and any space where people must understand speech.

The STI Rating Scale

IEC 60268-16 defines five qualitative categories that this guide presents in a color-coded reference card:

• Excellent (0.75-1.00): Near-perfect intelligibility. Typical of well-treated studios and close-talk scenarios.
• Good (0.60-0.75): Reliable intelligibility for most listeners. Target for PA systems in conference rooms and classrooms.
• Fair (0.45-0.60): Intelligible with effort. Acceptable minimum for general announcement systems. Most listeners will understand most words but may miss some.
• Poor (0.30-0.45): Significant intelligibility problems. Many words are missed. Unacceptable for safety-critical systems.
• Bad (0.00-0.30): Speech is not intelligible. System requires immediate remediation.

Compliance Requirements

Many building codes and safety standards mandate minimum STI values:

• EN 54-16 (Emergency Voice Alarm): STI greater than or equal to 0.50 at all listener positions
• BS 5839-8 (UK Fire Alarm Voice): Minimum 0.50 STI
• NFPA 72 (US Fire Alarm): References intelligibility requirements
• IEC 60849 / EN 60849 (Sound Systems for Emergency): STI greater than or equal to 0.50
• ANSI/ASA S12.60 (Classrooms): While specifying RT60 and background noise, implied STI greater than 0.60 when both criteria are met

STIPA Measurement Procedure

STIPA (STI-Public Address) is the standardized method for field measurement defined in IEC 60268-16 Annex F:

1. Signal: Play the standard STIPA test signal through the system under test. The signal contains 7 octave bands (125 Hz to 8 kHz), each modulated at 2 frequencies (14 modulation frequencies total from Table F.1).
2. Duration: Minimum 15 seconds of continuous capture per position. For higher accuracy, use 3 consecutive measurements and average.
3. Positions: Measure at the worst-case listener positions first — rear of the room, under balconies, at the edges of coverage. Minimum 6 positions for system commissioning.
4. Background noise: Record ambient noise level before measurement. The system must achieve adequate signal-to-noise ratio at all positions.
5. Processing: Extract the modulation transfer function per band, apply the male speech weighting factors from Table 4, and calculate the composite STI with redundancy corrections.

Interpreting Per-Band MTF Results

The overall STI number can mask band-specific problems. The guide includes interpretation guidance:

• Low MTF at 125-250 Hz: Typically caused by excessive reverberation at low frequencies. Treatment: bass absorption or system high-pass adjustment.
• Low MTF at 2-8 kHz: Often caused by poor high-frequency coverage, absorption, or air absorption at long distances. Treatment: speaker aiming, EQ, or additional coverage speakers.
• Uniformly low MTF: Usually indicates excessive reverberation across all bands or insufficient signal level. Measure RT60 and SPL to diagnose.

Measure STI in Your Browser

Open the SonaVyx STI Tool to run STIPA measurements with the standard IEC 60268-16 test signal, automatic MTF extraction, and per-band analysis. Use the Room Analysis Workflow to combine STI with RT60 and frequency response for a complete acoustic assessment.