STIPA Speech Intelligibility — IEC 60268-16
STIPA (Speech Transmission Index for Public Address) per IEC 60268-16 measures speech intelligibility using a test signal with 14 modulation components across seven octave bands. It computes the Modulation Transfer Function and produces a single 0-1 index rating from Bad to Excellent.
What Is STIPA?
STIPA per IEC 60268-16 clause F.1.2 measures how well a PA system preserves modulation patterns critical for speech. It transmits a standardized test signal and analyzes modulation depth reduction caused by reverberation, noise, and distortion.
The STIPA Test Signal
IEC 60268-16 Table F.1: seven octave bands (125 Hz to 8 kHz) modulated at two frequencies each. The 14 modulation frequencies (1.0 to 12.5 Hz) represent rates important for speech perception. Minimum 15-second measurement duration is required.
MTF and STI Calculation
MTF quantifies modulation preservation (0 to 1.0). IEC 60268-16 clause 4 defines it as received/transmitted modulation depth. The 14 MTF values become apparent SNRs, weighted per Table 4 male speech factors with redundancy corrections, producing a single 0-1 STI.
Rating Scale
IEC 60268-16 Table 1: Bad (below 0.30), Poor (0.30-0.45), Fair (0.45-0.60), Good (0.60-0.75), Excellent (above 0.75). Emergency PA systems typically require minimum 0.50 per BS 5839-8 and EN 54-16.
Key Factors
RT60 is the primary enemy: doubling RT60 approximately halves MTF at low modulation frequencies. Background noise adds unmodulated energy. An SNR improvement of 10 dB increases STI by 0.10-0.15 per clause A.4.2.
Try It Now
Measure STIPA in your browser with IEC 60268-16 analysis