PA system design checklist: zones to amplifier size

A zone is an operating unit, not a room on the drawing. A classroom wing may be one zone, a corridor another, and the sports field a third. The fire strategy may split the same floor into different compartments.

Write the zone schedule first: area, use, fire compartment, paging source and whether the zone needs a local volume control. The head end, paging console and amplifier channels all follow this list.

Coverage is a geometry problem. Use the ceiling height, the loudspeaker coverage angle and the room boundaries to set the spacing. The two rules that are easiest to check are: no listener more than 15 m from the nearest loudspeaker, and no more than 8 m from the last corridor loudspeaker to the corridor end.

These are starting rules, not a substitute for the project survey. A high ceiling, a glass wall or a noisy factory floor changes the spacing and the tap setting.

Each 100V loudspeaker has a tap setting in watts. The tap load is the sum of every loudspeaker on the circuit. This number, not the number of speakers, decides the amplifier size.

Split the circuits by zone and by cable route. A long corridor circuit with a high tap load may need a larger cable or a different circuit split. Record the tap setting for every speaker; a later change at the ceiling changes the load.

The amplifier must cover the connected tap load plus headroom for programme peaks and ageing. A common starting range is 1.25 to 1.5 times the connected load, but the final figure depends on the programme material and the standby policy.

For a 100V line, current equals amplifier power divided by 100 V. Check the voltage drop on long runs and the fuse or breaker for each output. A 500 W amplifier is not automatically the right answer for a 400 W load.

The fire interface is a hard-wired priority path, not a software setting. The fire alarm must override music and paging, and the evacuation message must reach the correct zones even if the network or the paging console is unavailable.

Agree the cause-and-effect matrix with the fire consultant. It should list every trigger, every zone and every loudspeaker circuit. Test the release door by door and zone by zone before handover.

A school needs four priority levels: evacuation first, live paging second, scheduled bells third, background music last. A hotel may add a ballroom override and a back-of-house paging group.

Write the priority table into the operating procedure. If the staff cannot explain which source wins, the system will be used badly even if the equipment is correct.

Commissioning is not a listening test. It is a set of records: circuit impedance, SPL, priority, fire release and documentation. A lobby that sounds good is not proof that the back-of-house circuit works.

Send the floor plan, the zone schedule, the ceiling heights, the room use and the fire strategy. If the building has an existing PA system, add the cable type and the amplifier model.

With those inputs we can return a zone schedule, a loudspeaker count, a tap-load calculation and a starting equipment list. Without them, a per-speaker price is only a budget.