STIPA Measurement Guide — Speech Transmission Index for Public Address Systems (IEC 60268-16)
STIPA (Speech Transmission Index for Public Address systems) is the internationally recognised method for measuring speech intelligibility in voice alarm, PA, and public address systems. Standardised in IEC 60268-16, STIPA provides a single number (0–1) that quantifies how clearly spoken words are transmitted through an electroacoustic system in a specific acoustic environment. A STIPA value of 0.50 or above ("Fair") is typically required for voice alarm systems to comply with EN 54-16, BS 5839-8, and ISO 7240-19.
What is STIPA and How Does It Work?
STIPA uses a special test signal — an octave-band modulated pink noise that mimics the spectral and temporal characteristics of speech — broadcast through the PA or voice alarm loudspeakers using a calibrated talkbox. A calibrated sound level meter with STIPA measurement capability (the receiver) measures how much the original signal's modulation has been degraded by the room acoustics, background noise, and the PA system itself. The degradation is calculated across seven octave bands and fourteen modulation frequencies. The result is expressed as a Speech Transmission Index (STI) value from 0 (unintelligible) to 1 (perfect), classified as Bad, Poor, Fair, Good, or Excellent.
The measurement requires two pieces of equipment: a talkbox (signal transmitter) and a STIPA-capable sound level meter (receiver). The talkbox connects to the PA amplifier input, sending the IEC 60268-16 test signal through the actual PA loudspeakers. The sound level meter is positioned at the listener location and measures the received signal after it has been affected by the room acoustics and system characteristics.
STI Classification per IEC 60268-16
0.00 – 0.30: Bad — speech barely intelligible
0.30 – 0.45: Poor — speech difficult to understand
Voice alarm system commissioning — EN 54-16, BS 5839-8, and ISO 7240-19 require demonstration of adequate speech intelligibility (typically STI ≥ 0.50 "Fair") at all critical listening positions
PA/GA system acceptance testing — IEC 60849 (public address systems for emergency use) and project specifications require STIPA verification
Annual inspections and maintenance — periodic verification that voice alarm systems continue to maintain required intelligibility levels over time
Railway & airport PA systems — EN 50849 and specific transport authority standards require ongoing STIPA verification
PAGA systems — offshore, industrial, and hazardous area public address and general alarm systems require STIPA commissioning per relevant safety standards
Building handover — PA and voice alarm systems must be commissioned and documented before building handover, with STIPA results as evidence
Full STI vs STIPA — Which to Use?
STIPA uses 14 modulation frequencies across 7 octave bands, providing a practical and widely accepted measurement method for field testing. Full STI (Speech Transmission Index) uses 98 modulation frequencies — seven per octave band — providing higher accuracy, particularly in systems with non-linear distortion, clipping, or complex acoustic conditions. Traditional Full STI measurement took 10–15 minutes per position; the Bedrock Elite i10 is the only instrument capable of measuring Full STI in under 60 seconds, making it practical for every measurement position in a building. Read the full comparison: Full STI vs STIPA: When to Use Each.
STIPA Measurement Equipment
Bedrock Elite i10 — Class 1 analyzer with Full STI (under 60 seconds) and STIPA. The most capable STIPA/STI instrument available. All modules included.
Bedrock Elite i9 — Class 1 sound level meter with STIPA measurement. All modules included. Ideal for engineers who also need octave analysis and RT60.
Bedrock Elite i5 — IEC 61672-1 Class 2 STIPA meter and sound level meter. Simple operation, IEC 60268-16 compliant. The most affordable STIPA meter in the Bedrock Elite range.
Bedrock SM50 — Class 2 sound level meter with STIPA capability for less demanding applications.