PAGA and voice alarm testing / Guide
STI and STIPA explained: measuring the speech intelligibility of PA and voice alarm systems
An alarm that is loud enough is not necessarily understood. The speech intelligibility of a public address, voice alarm or PAGA system is rated objectively with the Speech Transmission Index (STI), defined in IEC 60268-16. This guide explains what the STI measures, how the STIPA method works on a live system, and what makes a system fail.
What the STI measures
Speech carries its meaning in rapid fluctuations of level. The STI rates how well the transmission path, meaning the sound system together with the space around the listener, preserves those fluctuations. It is expressed on a scale from 0 to 1: the higher the value, the better speech is transmitted. IEC 60268-16 also groups results into qualification bands, from A+ at the top to U at the bottom, so a requirement can be written as a band as well as a value.
Full STI, STIPA and indirect methods
- Full STI is the complete method, using the full set of modulation frequencies in each octave band.
- STIPA is the simplified method for public address systems. A modulated test signal is played through the system itself and analysed at the listener position. It is the method normally used to commission and verify PA/VA and PAGA systems.
- Indirect methods derive the STI from a measured impulse response of the system and space, with the speech level and background noise accounted for separately.
What lowers speech intelligibility
- Signal-to-noise ratio: speech that is not far enough above the background noise at the listener loses intelligibility, however clear the system is.
- Reverberation: in hard, enclosed spaces each syllable is still decaying when the next arrives, which smears speech.
- Echoes and delays: sound from distant loudspeakers that arrives late acts like an echo of the nearest loudspeaker.
- Coverage: listener positions between, behind or too far from loudspeakers receive too little direct sound.
- Distortion: clipping, poor equalisation or overdriven loudspeakers damage speech before it leaves the system.
Intelligibility is not the same as audibility
Alarm tones are checked for audibility: the tone needs a margin above ambient noise, commonly 10 to 15 dB depending on the standard the project follows. A voice message has to be understood as well as heard, so a system can pass the audibility check and still fail on STI, particularly in reverberant or noisy areas.
Which criterion applies
Requirements come from documents such as IEC 60268-16, ISO 7240-19, the EN 54 series with CEN/TS 54-32, BS 5839-8 and NFPA 72 (Annex D), and from the project or operator specification. Voice-alarm criteria in these documents commonly centre on an STI of 0.50, with individual positions not below about 0.45 — the governing figure is the one your project specification names.
Measuring in practice
A STIPA test plays the test signal through the system and measures at the positions where people listen, with background noise measured at the same positions under representative operating conditions and accounted for as IEC 60268-16 describes. Measurements are made with Class 1 sound level meters and analysers capable of STIPA, calibrated by an ISO/IEC 17025 accredited laboratory.
Geonoise carries out these measurements independently of the system supplier. See PAGA and voice alarm speech intelligibility testing for the service, the standards table and frequently asked questions, or use the enquiry form.
