HOW TO READ THIS MAP
This map uses two microphones separated by a known distance to determine the direction of noise sources at each frequency band. Each colored line is a bearing — a compass direction the noise is coming from at that frequency.
Coherence (γ²)
Measures how synchronized the pressure waves are between the two microphones at each frequency. Think of it like two people watching ocean waves from different spots on the shore:
- γ² ≈ 1.0 — Both see the exact same wave pattern arriving in lockstep. That's a real, identifiable source — a machine, a fan, a compressor.
- γ² ≈ 0.0 — Each mic hears completely different, random noise. No identifiable source — just wind, traffic, general ambient.
- γ² ≥ 0.7 — High confidence: mechanical source confirmed at this frequency.
Only frequencies with significant coherence produce reliable bearing lines. Low-coherence frequencies are noise, not signal.
TDOA (Time Difference of Arrival)
The time delay between when a sound wave reaches mic 1 versus mic 2. This delay maps directly to a compass bearing:
- Sound from the left of the array arrives at the left mic first → positive delay.
- Sound from directly ahead (broadside) arrives at both mics simultaneously → zero delay.
- The delay is converted to an angle using: θ = arccos(Δt × speed_of_sound / mic_separation).
A two-mic array has front/back ambiguity — it can't distinguish sounds from above vs below the array axis. The "Disambiguate" control resolves this using level differences or forced quadrant selection.
What the Lines Mean
- Red/orange lines — VFD motor harmonics (66, 132, 198, 264, 330, 396 Hz). These are the frequencies generated by variable-frequency-drive HVAC fans.
- Blue lines — Other coherent frequencies (broadband mechanical noise, structural resonances).
- Wider wedge = lower coherence = less certain direction.
- Narrow wedge = high coherence = confident bearing.
- Faded lines = ambiguous mirror (front/back uncertainty).
Heat Map & Contours
- Heat map (colored wedges on map) — Accumulated bearing density over time. Brighter/warmer colors = more epochs with sources in that direction. Persistent sources glow; transients fade.
- Noise contours (when enabled) — Projected sound levels at various distances from the estimated source. Uses hemispherical propagation model. Red = louder, blue = quieter.
- Epoch counter — Number of 30-second measurement cycles contributing to the heat map.
Key insight: When multiple independent frequency bands — each measured with high coherence — all point to the same compass bearing, this constitutes strong evidence that a specific mechanical source at that location is responsible. The probability of unrelated noise sources at different frequencies coincidentally producing the same bearing is negligible.