Field Story
Feedback Howl Ruins the Keynote
A hotel ballroom with reflective marble floors and glass chandeliers produced feedback at 2.2 kHz and 3.8 kHz during a corporate keynote for 800 attendees. The problem detector identified the two feedback frequencies within seconds. Applying two narrow notch filters of -9 dB and repositioning the podium mic from omnidirectional to cardioid eliminated the issue entirely.
Hotel BallroomCoherence Measurement per IEC 61672-1
TL;DR
When verifying IEC 61672-1 frequency weighting accuracy via transfer function measurement, coherence provides the quality gate that validates your results. A coherence value below 0.95 at any test frequency indicates that the transfer function data at that frequency may be corrupted by noise or nonlinearity, making the weighting verification unreliable. SonaVyx coherence display alongside the transfer function magnitude lets you identify frequencies where the meter verification measurement is trustworthy and where additional averaging or improved test conditions are needed.
Coherence for Meter Verification
When measuring the frequency weighting response of a sound level meter, coherence tells you whether the measured transfer function accurately represents the meter response or is contaminated by measurement artifacts.
Quality Thresholds for Verification
- For meter verification, coherence should exceed 0.95 at all test frequencies
- This is higher than the 0.85 threshold for room measurement because the device under test should be a linear system
- Low coherence during meter verification indicates a test setup problem, not a room problem
- Check cables, connections, and signal levels if coherence is low
Frequency-Dependent Considerations
- At low frequencies, longer averaging is needed for stable coherence
- At high frequencies, ensure the signal generator has adequate energy
- The A-weighting curve attenuates low frequencies by 30+ dB, so input SNR must be high
- Use pink noise for broadband verification or swept sine for per-frequency analysis
Common Mistakes
- Accepting verification results with coherence below 0.95
- Not checking coherence at the frequency extremes where weighting attenuation is highest
- Using too few averages for stable coherence at low frequencies
SonaVyx Workflow
View coherence in the SonaVyx transfer function display. Verify levels with the SPL meter. Use the RTA for spectral monitoring. Check for anomalies with the problem detector. Run AI analysis for automated checks. See learning modules for calibration procedures.
Standard Reference
IEC 61672-1:
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Last updated: March 19, 2026