Key Takeaways
- PA sizing rule of thumb: 10W of amplifier power per person for an outdoor rock concert; 5W per person for an indoor pop format — these are rough starting points, not final specifications
- Line arrays provide 3 dB/doubling-of-distance SPL roll-off versus 6 dB for point-source — halving the level difference between front and back rows at deep venues
- Cardioid subwoofer configurations reduce rear SPL by 15–20 dB — essential for indoor venues where low-frequency energy builds destructively
- Delay towers (speakers 30–60m from the main stage) are required for outdoor events deeper than 60 metres — without them the back audience hears the delay between the visual performance and the audio
- Monitor systems for artists have become more complex as in-ear monitors have replaced wedges — the monitor console requires as many outputs as there are performers with IEM systems
Scale-appropriate PA specifications
200–500 pax indoor: A compact line array (4–6 elements per side) with 2–4 subs per side. FOH console with 48 inputs minimum. Total amplifier output: 20–40 kW. SPL target at mix position: 100–105 dB SPL. This specification covers most club-format and smaller theatre concerts in India.
1,500–3,000 pax outdoor: A medium line array (8–12 elements per side) with a cardioid sub array (6–12 subs total). Delay speakers if site is deeper than 50 metres. FOH console with 64–96 inputs. Total amplifier output: 60–100 kW. SPL target: 103–108 dB at 50 metres.
5,000–10,000 pax outdoor: Large format line array (12–16 elements per side), large cardioid sub array, delay towers at 50–60 metres, additional front fill speakers. FOH and monitor consoles with 96–128 inputs each. Total amplifier output: 150–250 kW. SPL target: 105–110 dB at 100 metres.
The delay tower calculation
Sound travels at approximately 343 metres per second. At an outdoor concert, an audience member 100 metres from the main stage receives the PA signal approximately 290 milliseconds after it leaves the speakers — which aligns with the acoustic arrival from the stage. At 60 metres, this is manageable. Beyond 60 metres, the time alignment between the visual performance and the audio becomes perceptible as a delay. Delay towers (speakers on scaffold towers at 60 metres from the stage, delayed electronically by the PA processor to align with the main system's timing at that position) solve this. Outdoor concerts deeper than 60 metres that do not have delay towers produce an audience experience where the back third of the crowd watches a performer while hearing the sound 0.2–0.4 seconds later.