STIPA (Speech Transmission Index for PA)
Definition
STIPA
STIPA is a simplified STI measurement method defined by IEC 60268-16 Annex F that uses a specific test signal containing 14 simultaneous amplitude modulations across 7 octave bands. STIPA achieves equivalent accuracy to full STI measurement in just 15 seconds, making it practical for field verification of PA and voice alarm systems. SonaVyx generates and analyzes STIPA signals in your browser.
How STIPA Is Measured
The STIPA test signal is played through the PA system under test while a measurement microphone captures the received signal at the listener position. Each octave band (125 Hz to 8 kHz) carries two specific modulation frequencies from IEC 60268-16 Table F.1. SonaVyx demodulates the received signal in each band, compares the received modulation depth to the original, and computes the modulation transfer function. The resulting STI value represents PA intelligibility at that position.
Practical Example
A fire safety inspector must verify that a shopping mall voice alarm system meets EN 50849 requirements of STI above 0.50 in all zones. Using SonaVyx, the inspector plays the STIPA test signal through the PA system and measures at 6 positions per zone. Three zones show STI between 0.52 and 0.61, passing the requirement. One zone near the food court measures only 0.44 due to high background noise, requiring speaker repositioning or noise mitigation.
STIPA Test Signal Structure
The STIPA signal consists of 7 octave-band noise carriers (centered at 125, 250, 500, 1000, 2000, 4000, and 8000 Hz), each amplitude-modulated by two specific modulation frequencies. The 14 modulation frequencies are carefully chosen from IEC 60268-16 Table F.1 to avoid intermodulation artifacts between bands. The modulation depth is 0.55 (-5.2 dB modulation index), matching the modulation characteristics of male speech.
Measurement Duration
IEC 60268-16 specifies a minimum measurement duration of 15 seconds for single STIPA captures. This duration ensures at least two complete cycles of the lowest modulation frequency (0.63 Hz) are captured, providing statistically reliable modulation depth estimates. SonaVyx Pro mode performs three consecutive 15-second measurements and averages them for improved confidence, particularly important in spaces with fluctuating background noise.
STIPA vs Full STI
Full STI measurement evaluates the modulation transfer function at 14 modulation frequencies per octave band (98 total measurements), requiring several minutes per position. STIPA evaluates only 2 modulation frequencies per band (14 total), reducing measurement time to 15 seconds. The IEC 60268-16 Annex F validates that STIPA results agree with full STI within ±0.03 for typical PA system conditions, making it the preferred method for field measurements.
Common Applications
STIPA is mandated for commissioning voice alarm and public address systems under EN 50849 (emergency sound systems) and BS 5839-8 (voice alarm systems for buildings). Airport terminal PA systems, train station announcements, shopping mall emergency systems, and hospital paging systems all require STIPA verification. SonaVyx generates the IEC-compliant STIPA signal directly, eliminating the need for separate test signal sources.
Try It Now
Measure STIPA — free speech intelligibility test in your browser