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 Terminal

EQ Recommendation per IEC 60268-16

TL;DR

EQ optimization for speech intelligibility per IEC 60268-16 focuses on maximizing the modulation transfer function across the seven STI octave bands. Unlike music-focused EQ that targets flat response, speech EQ prioritizes the 500 Hz to 4 kHz range where consonant energy determines intelligibility. SonaVyx AI diagnostic engine generates EQ recommendations specifically targeting STI improvement: reducing low-frequency energy that excites room modes, boosting the speech presence range where signal-to-noise ratio can be improved, and shaping the spectrum to match the speech-weighted importance factors defined in IEC 60268-16.

EQ for Speech Intelligibility

IEC 60268-16 weights the seven octave bands by their contribution to male speech intelligibility. EQ optimization should focus resources on the bands with the highest STI weight.

Speech-Weighted EQ Strategy

  • 500 Hz, 1 kHz, 2 kHz: highest STI weight, prioritize flat or slightly boosted response
  • 4 kHz: important for sibilants and consonant distinction
  • 125 Hz, 250 Hz: lower STI weight, reduce if they excite room modes
  • 8 kHz: lowest weight but contributes to presence and clarity

Noise Masking Compensation

  1. Identify bands where ambient noise reduces SNR below 15 dB
  2. Boost the signal in these bands if headroom allows
  3. Alternatively, high-pass filter to reduce low-frequency noise excitation
  4. Re-measure STI after EQ changes to verify improvement

What EQ Cannot Achieve

If STI is limited by reverberation (not noise), EQ has limited effect. Reducing low-frequency energy can help by exciting fewer room modes, but the primary solution is acoustic treatment or increased speaker directivity.

Common Mistakes

  • Applying music-optimized EQ curves to speech PA systems
  • Boosting bass for perceived fullness at the expense of intelligibility
  • Not measuring STI after EQ changes to verify improvement

SonaVyx Workflow

Get speech-optimized EQ from the SonaVyx AI diagnostic. Measure STI with the STI tool. Check response with the transfer function. Verify levels with the SPL meter. Measure RT60 with the RT60 tool. Follow the PA tuning workflow.

Standard Reference

IEC 60268-16:

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Last updated: March 19, 2026