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 ANSI S1.4

TL;DR

ANSI S1.4 frequency weighting filters have defined phase characteristics that are inherent to the specified pole-zero design. Complete meter verification includes checking that the phase response matches the theoretical values, not just the magnitude response. Phase accuracy affects how the meter responds to transient and impulsive sounds, which is particularly relevant for the Impulse time weighting option. SonaVyx phase analysis provides the measurement capability for this thorough verification, displaying both magnitude and phase simultaneously for comparison against ANSI S1.4 theoretical curves.

ANSI S1.4 Phase Specifications

The A-weighting network specified in ANSI S1.4 is defined by its transfer function poles and zeros, which determine both magnitude and phase response. While the standard specifies magnitude tolerances explicitly, the phase is implicitly defined by the filter topology.

Theoretical A-Weighting Phase

  • At 1 kHz: approximately 0 degrees (reference frequency)
  • At 100 Hz: approximately +90 degrees (high-pass rolloff)
  • At 10 kHz: approximately -45 degrees (low-pass rolloff)
  • The exact values depend on the implementation (analog vs. digital)

Impact on Transient Response

  1. Phase determines the shape of the weighted impulse response
  2. Different filter implementations can produce different peak readings for the same impulsive sound
  3. This is particularly relevant for ANSI S1.4 Impulse time weighting
  4. Digital implementations should match analog phase behavior for equivalence

Verification Method

Measure the transfer function of the weighting network using a swept sine or noise signal. Extract both magnitude and phase. Compare phase against theoretical values calculated from the ANSI S1.4 pole-zero specification.

Common Mistakes

  • Assuming magnitude-only verification is sufficient
  • Not accounting for digital filter implementation delay in phase measurement
  • Comparing phase between different filter topologies without normalization

SonaVyx Approach

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

Standard Reference

ANSI S1.4:

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Last updated: March 19, 2026