The maths behind sizing a PA system
Stand next to the speakers at soundcheck and everything sounds fine. The question that actually decides whether a PA is right for your event is what it sounds like at the back, and that is a different piece of maths entirely.
PA sizing is really two separate problems: coverage (does the sound reach the back row) and loudness (is it strong enough once it gets there). Coverage follows the inverse-square law: outdoors, with nothing to reflect sound back, level falls around 6dB every time the distance from the speaker doubles. Loudness does not scale the way the wattage spec implies either, doubling an amplifier's power only adds about 3dB; a genuinely louder-sounding system needs roughly ten times the power. That is also why a point-source pair of speakers suits a wedding or corporate crowd and a line array suits a deep festival crowd: the physics decides the shape of rig you need, not just the guest count.
Ask a sound engineer how big a PA an event needs and they will not start with the guest count. They will ask how far the back row is from the speakers, how the crowd is shaped, and what the venue's noise limit is. Guest count only tells you roughly how many people need to hear something; it says nothing about the distance the sound has to survive to reach them, and distance, not headcount, is what the maths actually runs on.
Problem one: the inverse-square law, or why the back row is the real target
Sound radiates outward from a source in every direction, and as that wavefront expands, the same total energy is spread across a larger and larger area. Under free-field conditions, meaning open air with nothing nearby to reflect sound back, that spreading follows the inverse-square law: sound pressure level drops by around 6dB every time the distance from the source doubles. A system reading 100dB two metres from the mix position is only around 82dB at 16 metres, a drop that has nothing to do with the equipment and everything to do with geometry.
That is also why a rig that sounds fine standing at the desk can go quiet fast towards the back of a marquee: the mix position is close, the back row is not, and the gap between the two is exactly where the 6dB steps add up. Sizing coverage properly means measuring the real distance to the furthest guest, not judging by ear from the front.
Problem two: why more watts is not the fix people expect
The instinct, once coverage falls short, is to book a bigger, higher-wattage system. That helps, but far less than the spec sheet suggests, because decibels are a logarithmic scale, not a linear one. Doubling an amplifier's power output adds only around 3dB, an increase a listener can just about notice but would not describe as louder in any meaningful sense. Reaching a genuinely perceived doubling of loudness, roughly a 10dB jump, takes not double the power but around ten times it. A system rated at 2,000 watts is not "twice as loud" as one rated at 1,000 watts, it is a small, useful step up; getting to genuinely double the perceived volume from that 1,000-watt starting point would take something closer to 10,000 watts.
This is precisely why a good supplier will not simply offer you "a bigger PA" when coverage is the problem. More power raises the ceiling a little; it does not fix a rig that is fundamentally the wrong shape for the room.
Point source or line array: matching the rig to the crowd
The two main topologies of PA solve the distance problem in different ways because they throw sound in different shapes. A point-source rig, the familiar pair of powered speakers suppliers quote for most weddings, parties and corporate functions, radiates sound in a roughly spherical pattern with a horizontal spread around 90 to 100 degrees. That wide, even spread suits a crowd that is roughly as wide as it is deep, which is most marquees, barns and function rooms up to a few hundred guests.
A line array stacks several speaker elements in a vertical column so their outputs combine into a single, more coherent wavefront. Instead of spreading wide and losing energy quickly with distance, that combined wavefront stays narrower and carries further with less drop-off, which is why festivals, outdoor stages and any deep, long crowd rely on line arrays rather than a bigger point-source rig. The choice is not really about total power at all, it is about which shape of coverage the crowd in front of the stage actually needs.
The other ceiling: what the venue will actually let you run
None of this maths happens in a vacuum, because UK noise rules cap how loud an event is allowed to be before the geometry even comes into it. There is no single fixed decibel limit in law for a private party or wedding: councils judge outdoor and amplified noise against the Environmental Protection Act 1990's test of "statutory nuisance", a case-by-case judgement rather than a number. What many licensed venues do set themselves is a sound limiter, commonly somewhere in the 90 to 100dB range measured at the mixing position, as a condition of keeping their entertainment licence, and 11pm is the point most councils expect outdoor amplified sound to stop or drop hard. A rig sized perfectly for the back row is no use if the venue's limiter cuts it off long before it gets there, which is why the curfew and limiter setting belong in the brief to the supplier from the start, not discovered on the night.
What this means for your event
Measure the real distance from where the speakers will sit to the furthest guest before assuming any system is "big enough". For a few hundred guests in a marquee or barn, a point-source pair with the right coverage angle for the room usually beats a bigger, louder rig aimed the wrong way. For a deep outdoor crowd, ask specifically whether the supplier is quoting a line array, because that is the topology built for throw distance, not just output. And get the venue's noise limit and curfew in writing before the band is booked: it shapes the system choice as much as the room does. The PA, sound & AV hire guide covers pricing by job, from a simple speech system through to an engineered festival rig, and pairs well with the generator hire guide for powering it on site.
PA and sound coverage questions, answered properly
Why does a PA that sounds loud enough at the front go quiet halfway across a marquee?
Because sound loses energy over distance whether or not anyone turns anything down. Under free-field conditions, the kind an open marquee or an outdoor site gives you, sound pressure level drops by roughly 6dB every time the distance from the speaker doubles. A system reading 94dB at 2 metres from the desk is down to about 88dB at 4 metres, 82dB at 8 metres and 76dB at 16 metres, a drop nobody chose, it is just what distance does. A rig specced by standing next to it at the mix position will always undersell the back of the room.
Is a bigger, more powerful PA system automatically louder?
Not by nearly as much as the wattage number suggests. Loudness in decibels is a logarithmic scale, and doubling an amplifier's power only adds about 3dB, an increase most people can just about notice but would not describe as "twice as loud". Getting to a perceived doubling of loudness, roughly a 10dB jump, takes ten times the power, not two. That is why upgrading from a modest system to one rated at double the wattage on paper rarely feels like the leap the spec sheet implies.
What is the difference between a point-source and a line-array PA, and which do I need?
A point-source rig, the pair-of-powered-speakers setup used for most weddings and corporate events, radiates sound in a roughly spherical pattern with a horizontal coverage angle around 90 to 100 degrees. That is the right shape for a room or a marquee of a few hundred guests, where the crowd is roughly as wide as it is deep. A line array stacks multiple speaker elements vertically so the sound combines into a narrower, more directional wavefront that carries further with less drop-off, which is the shape a long, deep festival crowd or an outdoor stage audience actually needs. The choice is about the shape of the crowd and the distance to the back row, not simply the guest count.
Is there a legal noise limit for a wedding or private party PA in the UK?
Not a single fixed number. The Environmental Protection Act 1990 works on whether noise is a "statutory nuisance", a judgement call by the local council rather than a decibel line in the sand, and the Noise Act 1996 only sets a fixed limit, 34dB(A), for noise measured inside a neighbouring home overnight. In practice, many licensed venues fit their own sound limiter, commonly set somewhere in the 90 to 100dB range at the mixing position, as a condition of their entertainment licence, and 11pm is the point at which most councils expect outdoor amplified sound to stop or drop hard. Ask the venue for their limiter setting and curfew in writing before booking a band.
Why does an outdoor event need more PA power than an indoor one for the same size crowd?
Walls help, even though nobody thinks of them that way. Indoors, sound that misses a listener on the first pass bounces off walls and ceiling and reaches them anyway, reinforcing the direct sound rather than being lost. Outdoors there is nothing to bounce off: every bit of sound that is not aimed where a guest is standing is gone for good, which is the free-field condition the inverse-square law describes exactly. The practical result is that the same crowd, covering the same distance, needs a noticeably higher-output system outdoors than it would under a roof.
Sizing the rest of the event?
Work out marquee area, power and toilets with the free calculators, then build the full kit list, PA included, in one tap.