Festival Sound: Measuring Multi-Zone PA Systems

Festival PA Zones

A typical medium-to-large festival (5,000-30,000 capacity) deploys these zones:

• FOH mains: The primary left-right arrays covering the front 60% of the audience. Line arrays are standard, flown 8-15 meters high, with 8-24 boxes per side.
• Delay towers: 1-3 rows of towers or poles at 30-60 meter intervals covering rear audience areas where the mains have lost 6-10 dB of level.
• Side fills / out-fills: Speakers covering the extreme left-right areas outside the main array coverage angle.
• Sub arrays: Ground-stacked or flown subwoofers, often in cardioid configuration to reduce rear-stage energy.
• VIP / backstage: Independent zone with its own source, often at lower SPL.

Delay Tower Alignment

Each delay tower must be timed so that the audience between the mains and the tower hears a single coherent source. The alignment position is the midpoint of the delay tower's primary coverage zone — not the tower itself.

The Delay Calculation

Measure the impulse response from the mains at the alignment position. The delay finder reports the propagation delay — say 87.3 ms for a main PA 30 meters away. Then measure the IR from the delay tower at the same position — say 23.1 ms for a tower 8 meters away. Set the delay tower's electronic delay to 87.3 - 23.1 = 64.2 ms.

Temperature correction: speed of sound = 331.3 + (0.606 × T°C) m/s. At 35°C (common for daytime festivals), speed = 352.5 m/s. At 15°C (evening), speed = 340.4 m/s. Over 30 meters, this is a 1.0 ms difference — audible as reduced coherence in the overlap zone.

Precedence Padding

Many engineers add 5-15 ms extra delay beyond time-alignment to maintain the precedence effect (Haas effect). This ensures the listener perceives the sound as coming from the stage (mains) even though the delay tower is closer. The optimal padding depends on the distance ratio and the desired effect — 10 ms is a common starting point.

SPL Compliance Monitoring

Most festival permits specify maximum SPL at the nearest noise-sensitive receiver (residential boundary, hospital, school). Common limits:

Jurisdiction	Typical Limit	Metric	Period
UK (Code of Practice)	65 dBA / 75 dBC	LAeq,15min	Any 15 min
EU (Environmental Noise)	55-70 dBA	Lden	24-hour
US (municipal, varies)	70-80 dBA	LAeq,1hr	Daytime
Australia (EPA)	65 dBA / 70 dBA	LA10,15min	Day / Evening

The SonaVyx SPL meter provides A/C/Z weighted measurement with Leq, percentiles, and configurable logging periods. For compliance, place a measurement mic at the boundary and run continuous logging during the event. The noise dose calculator tracks crew exposure separately under OSHA or ISO 9612 standards.

Sub Array Alignment

Festival sub arrays in cardioid configuration use rear-facing subs with delay and polarity inversion to cancel energy behind the array. This protects the stage and nearby noise-sensitive areas. Achieving 10-15 dB of rear rejection at 60-100 Hz requires precise delay setting — typically 5-8 ms on the rear-facing subs, measured and verified with the transfer function at positions behind the array.

Zone-by-Zone Measurement Workflow

1. Mains only: Measure TF at FOH, mid-audience, and rear-audience. This is your reference.
2. Each delay tower individually: Measure TF at the tower's primary coverage zone. Set delay per calculation above.
3. Mains + each tower combined: Verify summation with TF. Coherence should exceed 0.7 in the overlap zone. Adjust delay in 0.1 ms steps if needed (the gradient method).
4. Side fills: Align to mains at the coverage overlap edge.
5. Subs: Align subs to mains at FOH. Verify cardioid rejection behind the array.
6. Full system: Final check at 5+ positions with RTA for tonal balance and SPL meter for level distribution.

Store traces at each step using the workspace trace memory (press S). This creates a complete measurement record for the technical report.

Monitoring During the Show

Temperature, humidity, wind direction, and audience density all change during a multi-hour festival. The audience absorption effect alone accounts for 2-4 dB at high frequencies. Run continuous SPL monitoring at both FOH and the boundary throughout the event. If boundary levels approach the limit, the sub array is usually the first thing to reduce — low frequencies propagate furthest.