Do you need to fill a room? Ben Duncan has the figures at his eartips.

AN AMPLIFIER putting out so many watts will develop a certain relative amount of sonic power in a room or auditorium. What we want to know, of course, is how many watts produce how much loudness? Unfortunately, it's not just a matter of simply checking the wattage.

Much depends on speaker efficiency — that is, how effectively the speakers turn electrical watts into sheer acoustic power (the 'loudness'). More subtly, it also depends on how the acoustic power is distributed. So there are three main factors at play:

1) HOW MANY WATTS of electrical amplifier power do you have?
2) How EFFICIENT are your speakers? This is measured in dB (decibels) per watt at one metre, and the result is often called SPL (Sound Pressure Level).
3) How do these speakers PROJECT the sound into the room — like a laser beam or a lawn sprinkler?

Beginning with amplifier wattage power, you'll need to increase this at least three times to attain any useful extra level. In fact, just to double the sound level you'll need around ten times the power. The upshot is that fine gradations in power — say the difference between 75 watts and 120 watts — are pretty meaningless. At the same time, amplifier costs per watt are very nearly proportional to power rating, meaning that multiplying PA power by 10-fold isn't by itself a very cost-effective means of increasing loudness.

Speaker efficiency is commonly ignored. A typical studio monitor speaker converts just 0.5% of the amplifier's electrical watts into acoustic watts. Translated into figures, that would mean one watt of electrical power from the amplifier creates 95dB of sound at one metre distance, and pro rata for higher power levels.

Highly tuned PA speakers can improve on this by 10dB, giving 105dB at one metre distance for the same watts-worth. The ramifications of this become clearer once we've sussed that a 10dB increase in sound level dBs represents a doubling in loudness. So by simply opting for efficient speakers (lots of dBs per watt, measured at one metre) we can score the same effect as having ten times the amplifier power. But — and this is important — hiring or buying sensitive speakers needn't necessarily entail shelling out ten times as many pound notes; even if they cost twice as much, it's worthwhile.

The directivity of your speakers and the way their dispersion relates to the space you're trying to fill is no less a vital part of the equation. Broadly, some speakers boast lots of dBs per watt only because the majority of the sound is focussed straight ahead. In speaker terminology, it's called 'on-axis response'.

In a highly reverberant space, like an empty venue, the torrent of sound hitting the back wall soon bounces around and evens out. But later on, come the acoustic deadening effect of tightly packed punters' flesh, the only people who will hear properly are those pushy enough to manoeuvre themselves bang in line with the speakers.

The results of good and bad directivity are magnified as soon as we start assembling a PA stack using two or more cabinets. If sound splaying out of adjacent speakers overlaps too much, some sonic cancellation is bound to result. This explains why a typical towering PA stack of the 1970s totalling around 10kW (10 kiloWatts = 10,000 watts) would be outblasted by a modern 2kW PA. In the modern set-up, the speakers' dispersion pattern has been tuned so the individual speakers all work together to get sound out into the room, instead of fighting agin t'other.

Despite these factors, the real life language of buying and hiring PAs revolves around the question, "How many watts?"

The classic guidelines are:

• 100-200 watts: Fine for vocals and the odd instrument (also sax, clarinet) in a pub or club or village hall, for crowds of 50 to 100.

• 250-500 watts: the same — for a bigger club, say a 150-seater. You'll also need this kind of power in a smaller venue if you put keyboards through the PA.

• 500 watts-1kW: this is the bare minimum power if you put the drum kit through the PA, whether the venue holds 50 or 250. This is because a kit can be played louder than most PAs below 500 watts! But even with 1kW, beware of using too many mikes — it's a good idea to limit yourself to just two on the kit, one overhead and one on the bass drum.

• 1-5kW: You'll need this sort of power for audiences in the 250 to 500 region. At these powers there's enough to mike up the whole band and put six or more mikes on the kit.

• 5-10kW: The minimum power for a small festival, and enough indoors for a 1000-seater (a cinema, say).

The power needed also depends on how many there sure of you. You can safely halve the watts above for a solo or duo performance. Or, if your music is essentially LOUD, or it's a ten-piece band, doubling the figures above wouldn't go amiss.

Ultimately, finding out 'how many watts' comes from experience of hiring (and firing) a variety of PAs since their effective loudness has only a vague relationship to their power output. It's more frustrating having too little power than having some extra in hand, so in cases of blind uncertainty it's best to err on the side of loudness. You can do this by doubling the wattage you first thought of.