Field Story

Justice Delayed by Poor Acoustics

A courtroom renovation replaced carpet with hardwood flooring, dropping speech intelligibility at the jury box from STI 0.72 to 0.49. The judge could not be understood clearly, risking mistrials. Impulse response measurement pinpointed strong early reflections from the new floor and side walls. Targeted absorptive panels behind the bench and on the side walls restored STI to 0.67.

Courtroom

STI Measurement per ANSI S1.4

TL;DR

ANSI S1.4 sound level meter specifications affect STI measurement accuracy because the modulation transfer function depends on precise amplitude measurement across frequency bands. The meter class determines the uncertainty in level measurement at each octave band, which propagates into the MTF calculation and the final STI value. Class 2 tolerances can introduce plus or minus 0.02 STI uncertainty, while Class 1 reduces this to plus or minus 0.01. SonaVyx digital processing minimizes these errors, but understanding the ANSI S1.4 contribution to STI measurement uncertainty helps you report results with appropriate confidence intervals.

ANSI S1.4 and STI Accuracy

STI is calculated from the modulation transfer function measured in 7 octave bands. The accuracy of the amplitude measurement in each band, as specified by ANSI S1.4, directly affects the MTF values and the resulting STI score.

Level Measurement Accuracy per Band

  • STI uses octave bands at 125, 250, 500, 1000, 2000, 4000, and 8000 Hz
  • ANSI S1.4 Class 2 tolerance ranges from plus or minus 1.4 dB at 1 kHz to plus or minus 5.0 dB at extremes
  • The 125 Hz and 8 kHz bands have the widest tolerances
  • The 500-2000 Hz bands (most heavily weighted for speech) have the tightest tolerances

Impact on MTF Measurement

  1. The MTF is derived from the ratio of modulated to unmodulated signal energy
  2. Amplitude measurement errors affect both the modulated and unmodulated components
  3. In the worst case, Class 2 errors could shift STI by plus or minus 0.03
  4. This is significant near threshold values (e.g., 0.50 for voice alarm compliance)

Practical Recommendations

For critical STI measurements (fire safety, building compliance), use the highest quality measurement chain available. Calibrate the complete system at 1 kHz and verify the frequency response against ANSI S1.4 tolerances at the seven STI octave band frequencies.

Common Mistakes

  • Not accounting for measurement uncertainty when reporting pass/fail compliance
  • Using uncalibrated microphones that add unknown frequency response deviations
  • Reporting STI to two decimal places when the measurement uncertainty is plus or minus 0.03
  • Ignoring the contribution of microphone self-noise to MTF degradation at low levels

SonaVyx Approach

The SonaVyx STI tool uses digital processing that exceeds ANSI S1.4 Class 2 specifications for the internal signal chain. Import microphone calibration for the SPL meter to minimize hardware contribution to error. Verify room conditions with the RT60 tool. Check system response with the transfer function. Use the IR tool for the indirect STI method. See our learning modules for measurement uncertainty guidance.

Standard Reference

ANSI S1.4:

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Last updated: March 19, 2026