Festival PA Alignment: Transfer Function Saved a 3-Stage Event
TL;DR
A 5,000-capacity outdoor festival with three stages suffered from a devastating -12 dB null at 120 Hz caused by reversed subwoofer polarity and miscalculated delay tower timing. SonaVyx transfer function measurements at 12 positions revealed the sub/main phase cancellation and 15 ms delay error. After polarity correction, 4.2 ms sub delay, and 23 ms delay tower realignment, the crossover region measured within ±2 dB with strong coherence across all 12 positions. The fix took 45 minutes and required zero additional equipment.
The Challenge: Bass That Vanished at 120 Hz
Three weeks before a major outdoor music festival, the production manager raised an alarm during pre-production testing. Despite deploying 16 subwoofers across the main stage — over 32 kW of low-frequency power — the bass sounded thin and punchy rather than full and powerful. The stage-left side of the audience received noticeably less bass than stage-right, and the delay towers at 40 metres sounded detached from the main system.
The festival featured three stages across a 5,000-capacity outdoor venue. The main stage ran a line array system with ground-stacked subs in a left-right configuration. Two delay towers at 40 metres extended coverage to the rear. With headliners expecting world-class sound, the production team needed answers fast.
Measurement: SonaVyx Transfer Function Analysis
The system engineer deployed SonaVyx transfer function mode at 12 measurement positions: 4 across the front of house area, 4 at mid-field, and 4 in the delay tower coverage zone. Pink noise was used as the reference signal to provide broadband excitation.
The results were immediately clear:
- Main stage front: A -12 dB null centred at 120 Hz with coherence dropping to 0.35 at that frequency
- Phase display: 180-degree phase offset between sub and main contributions at the crossover region (80 to 160 Hz)
- Stage-left positions: Null was 4 dB deeper than stage-right, indicating the left sub array was the problem source
- Delay towers: Impulse response showed the delay tower signal arriving 15 ms after the expected time, with a secondary peak indicating an incorrect reference point calculation
The SonaVyx problem detector flagged both the polarity reversal and the comb filter pattern at the delay tower positions.
Diagnosis: Two Distinct Problems
The AI diagnostic engine identified the root causes:
- Reversed sub polarity on stage-left array: Eight subwoofers in the left ground stack were wired with reversed polarity at the amplifier patch. The NL4 cables had been swapped during a last-minute amplifier rack change. The phase display confirmed a consistent 180-degree offset from the stage-left position.
- Delay tower timing error: The delay time had been calculated from the stage lip to the delay tower, but the main array was flown 3 metres behind the stage lip. This 3-metre error added 8.7 ms of incorrect delay, compounded by a further 6 ms error in the speed-of-sound calculation (using an indoor temperature rather than the outdoor ambient of 28°C).
Solution: 45 Minutes, Zero Additional Equipment
The corrections were straightforward once the problems were identified:
- Polarity flip: Reversed the polarity setting on the stage-left sub amplifier DSP — a single button press per channel
- Sub delay: Added 4.2 ms to the sub array to time-align with the main array, verified via transfer function phase
- Delay tower correction: Recalculated delay from the main array position (not stage lip) at the correct outdoor temperature, resulting in 23 ms (down from the incorrect 38 ms)
Each correction was verified in real time with SonaVyx before/after comparison.
Results: Full-Bandwidth Coherent Coverage
| Metric | Before | After |
|---|---|---|
| 120 Hz level (average) | -12 dB null | +2 dB (constructive summation) |
| Crossover coherence | 0.35 | 0.92 |
| Sub/main phase @ 120 Hz | 180° | 12° (near zero) |
| Delay tower alignment | 15 ms late | ±1 ms of target |
| L/R SPL variation @ 120 Hz | 8 dB | 1.5 dB |
The festival weekend proceeded without a single low-frequency complaint. The headlining act's front-of-house engineer commented that the sub coverage was among the most even he had experienced at a festival. The entire diagnosis and correction process took 45 minutes with zero additional equipment required.
Lessons Learned
- Always verify polarity after any cable or amplifier change: A single polarity reversal can destroy an entire crossover region, and the problem is invisible without measurement.
- Measure delay from the actual source position: Using the stage lip or a schematic reference point introduces systematic error. The impulse response shows the true acoustic source position.
- Temperature matters outdoors: The speed of sound varies from 331 m/s at 0°C to 349 m/s at 30°C. At 40 metres, this creates a 2 ms timing error if the wrong temperature is used.
- Coherence is the key diagnostic: Low coherence at a specific frequency band is the most reliable indicator of phase cancellation between sources.
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Last updated: March 19, 2026