STIPA Measurement Methodology — IEC 60268-16 Speech Transmission Index for Public Address

A technical deep-dive into STIPA measurement methodology, the IEC 60268-16 standard, test signal requirements, measurement procedures, and result interpretation for voice alarm and PA system commissioning engineers. This guide complements the main STIPA Guide with additional technical detail on the measurement process and the physics behind speech intelligibility quantification.

Speech intelligibility — the ability of listeners to correctly identify spoken words — is degraded by room acoustics, background noise, distortion in the audio system, and time delays. STIPA (Speech Transmission Index for Public Address systems) provides a single number (0–1) that quantifies the combined effect of all these degrading factors on speech intelligibility in a specific electroacoustic system and acoustic environment. A value of 0.50 or above (classified as "Fair" per IEC 60268-16) is typically required for voice alarm systems to meet life-safety standards.

STIPA Test Signal & Measurement Procedure

The STIPA test signal consists of octave-band noise modulated at seven specific modulation frequencies per IEC 60268-16. The signal is emitted by a calibrated talkbox (BTB65 or BTB115) connected to the PA system amplifier input, so the test signal is broadcast through the installed loudspeakers. A calibrated Class 1 sound level meter with STIPA analysis capability measures the received signal at each measurement position and calculates the modulation transfer function (MTF) by comparing the modulation depth of the received signal against the transmitted signal. The MTF is then converted to an STI value using the weighting matrix defined in IEC 60268-16.

Factors Affecting STIPA Results

• Reverberation time — longer reverberation degrades modulation depth, particularly in mid-frequencies. Rooms with RT60 > 1 second are more challenging for STIPA compliance.
• Background noise level — higher background noise reduces signal-to-noise ratio, directly degrading the STI value. Measurement positions with high HVAC or crowd noise will have lower STI.
• Signal-to-noise ratio — the PA system must provide sufficient level above background noise at all measurement positions.
• System distortion and clipping — non-linearity in the audio chain introduces distortion products that degrade intelligibility, particularly with Full STI measurement.
• Time delays — excessive delays between zones in distributed speaker systems can cause comb filtering and intelligibility degradation.

STI Classification (IEC 60268-16)

• 0.00 – 0.30: Bad — speech barely intelligible, evacuation messages likely not understood
• 0.30 – 0.45: Poor — speech difficult to understand
• 0.45 – 0.60: Fair — adequate intelligibility; typical minimum required for voice alarm compliance (EN 54-16, BS 5839-8)
• 0.60 – 0.75: Good — speech clearly intelligible
• 0.75 – 1.00: Excellent — near-perfect speech transmission

STIPA vs Full STI — Technical Comparison

STIPA uses 14 modulation frequencies (2 per octave band across 7 octave bands). Full STI uses 98 modulation frequencies (14 per octave band). Full STI provides higher accuracy, particularly in systems with non-linear distortion and complex acoustic conditions. The Bedrock Elite i10 is the only instrument capable of Full STI measurement in under 60 seconds. Read the detailed comparison: Full STI vs STIPA: When to Use Each.

STIPA Instruments

• Bedrock Elite i10 — Full STI (under 60s) + STIPA, Class 1. The most accurate and capable instrument.
• Bedrock Elite i9 — STIPA, Class 1. For engineers who also need octave analysis and RT60.
• Bedrock Elite i5 — Class 2 STIPA meter and sound level meter. Simple field operation.

Full STIPA Guide  ·  Request a Quote  ·  Voice Alarm Testing