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 Ballroom

Phase Analysis per IEC 61672-1

TL;DR

IEC 61672-1 specifies not only the magnitude response of frequency weighting networks but implies phase characteristics through the filter design specifications. While most practitioners focus on magnitude verification, the phase response of the weighting filters affects the time-domain behavior of the meter, which is important for impulsive and time-varying sounds. SonaVyx phase analysis in the transfer function view lets you examine the complete complex response of the measurement chain, providing a more thorough verification of IEC 61672-1 compliance than magnitude-only testing.

Phase Response of Weighting Networks

The A and C weighting curves defined in IEC 61672-1 are implemented as analog or digital filters with specific phase characteristics determined by the filter topology.

A-Weighting Phase

  • The analog A-weighting filter has specific pole-zero locations that determine phase
  • At 1 kHz (reference), the phase is near zero
  • Below 100 Hz, the phase lead can exceed 90 degrees due to the high-pass rolloff
  • Digital implementations should match the analog phase response for equivalence

Practical Significance

  1. Phase affects the peak detection of transient signals
  2. Different implementations (analog vs digital) may handle impulsive sounds differently
  3. For Impulse time weighting, the phase response influences the peak reading
  4. Complete meter verification should include phase to ensure impulse response accuracy

Measurement Procedure

Measure the transfer function of the complete weighting network and display both magnitude and phase. Compare against the theoretical phase response calculated from the standard pole-zero specification.

Common Mistakes

  • Verifying only magnitude and ignoring phase response
  • Not accounting for digital filter latency in the phase measurement
  • Confusing filter phase with propagation delay

SonaVyx Workflow

View phase in the SonaVyx transfer function. Verify magnitude with the SPL meter. Use the RTA for spectral monitoring. Check for issues with the problem detector. Run AI analysis. See our learning modules for filter theory.

Standard Reference

IEC 61672-1:

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Last updated: March 19, 2026