Field Story
Airport PA: When Lives Depend on Intelligibility
A major airport terminal expansion measured STI of 0.38 at gate areas, well below the mandated 0.50 minimum for emergency announcements. The impulse response showed a 3.1-second RT60 caused by the soaring glass atrium. Distributed ceiling speakers on 6-meter centers with carefully delayed zones raised STI to 0.56, passing the IEC 60268-16 intelligibility requirement.
Airport TerminalCoherence Measurement per IEC 60268-16
TL;DR
Coherence measurement directly predicts speech intelligibility performance per IEC 60268-16. Frequencies where coherence is low will have poor modulation transfer function (MTF) values, dragging down the overall STI score. There is a strong correlation between coherence and the MTF: both measure how well the linear relationship between source and receiver is maintained. SonaVyx coherence display helps you identify which octave bands are limiting STI performance, guiding your optimization strategy. Improving coherence through speaker positioning, acoustic treatment, or noise reduction directly improves speech intelligibility.
Coherence and STI Correlation
Coherence and MTF both quantify signal quality between source and receiver. Coherence measures the linear spectral relationship, while MTF measures the temporal modulation preservation. Both are degraded by the same factors: noise and reverberation.
Predicting STI from Coherence
- Coherence above 0.8 in a band typically corresponds to good MTF (above 0.4)
- Coherence below 0.5 typically corresponds to poor MTF (below 0.2)
- The relationship is not exact because coherence is frequency-domain while MTF is modulation-domain
- But coherence is faster to measure than full STIPA, making it useful for quick assessment
Identifying Problem Bands
- Measure coherence at each listener position
- Note which octave bands consistently show low coherence
- Low-frequency coherence drops indicate room mode problems
- Mid-frequency coherence drops indicate reverberation issues
- High-frequency coherence drops indicate noise or distance problems
Optimization Strategy
Improve coherence in the bands that matter most for STI. The 500 Hz to 4 kHz range carries the highest STI weight. Focus acoustic treatment and speaker optimization on these bands.
Common Mistakes
- Not connecting low coherence to poor STI performance
- Trying to fix low coherence with EQ when the cause is noise or reverberation
- Measuring coherence with insufficient averaging
SonaVyx Tools
View coherence in the SonaVyx transfer function. Measure STI with the STI tool. Check RT60 with the RT60 tool. Monitor levels with the SPL meter. Get AI recommendations from the diagnostic engine. See our learning modules for coherence interpretation.
Standard Reference
IEC 60268-16:
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Last updated: March 19, 2026