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

STI Measurement per IEC 61672-1

TL;DR

IEC 61672-1 defines the electroacoustic performance of sound level meters used in STI measurement chains. The standard specifies frequency weighting accuracy, dynamic range, and detector response characteristics that influence the quality of STIPA signal capture and MTF derivation. For STI measurement, the instrument must accurately capture the modulated STIPA signal across seven octave bands simultaneously, requiring flat frequency response and adequate dynamic range at each band. SonaVyx implements IEC 61672-1 compliant digital processing to ensure the internal measurement chain contributes minimal error to your STI results.

IEC 61672-1 for STI Measurement

IEC 60268-16 references IEC 61672-1 for the performance requirements of the measurement instrument used for STIPA signal capture. The meter must be able to accurately resolve the modulation depth of the STIPA signal across all octave bands.

Frequency Response Requirements

  • The meter must have flat response (Z-weighting) across 125 Hz to 8 kHz for STIPA capture
  • IEC 61672-1 Class 1 or Class 2 tolerances apply
  • The octave band filters used for STI analysis must also comply
  • Microphone calibration can correct for known deviations

Dynamic Range Considerations

  1. The STIPA signal has a crest factor of approximately 10 dB
  2. The measurement system must handle this without clipping or noise floor issues
  3. IEC 61672-1 specifies the linear operating range
  4. Set the measurement level to use the middle of the dynamic range

Detector Response

For STIPA measurement, you need access to the raw signal or a fast-responding detector. The IEC 61672-1 Fast time weighting (125 ms) is too slow for some STIPA modulation frequencies (up to 12.5 Hz). Digital processing of the raw audio stream, as used in SonaVyx, bypasses this limitation.

Practical Implications

Software-based STI measurement tools like SonaVyx process the raw audio stream directly, which avoids the detector response limitations of analog sound level meters. The IEC 61672-1 requirements then apply primarily to the analog front-end (microphone and preamp).

Common Mistakes

  • Using a meter with insufficient bandwidth for the 8 kHz octave band
  • Not verifying that the analog front-end has adequate dynamic range
  • Applying A-weighting to the STIPA signal capture (Z-weighting is required)
  • Ignoring microphone frequency response deviations in the STI octave bands

SonaVyx Implementation

The SonaVyx STI tool processes raw 48 kHz audio for maximum accuracy. Calibrate with the SPL meter before measurement. Measure room reverberation with the RT60 tool. Capture impulse responses with the IR tool for the indirect method. Check system response with the RTA. See our learning modules for STI measurement tutorials.

Standard Reference

IEC 61672-1:

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Last updated: March 19, 2026