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How much power do you need to fill a venue with sound?

How much power do you need to fill a venue with sound?

Small venues need small amplifiers. Large venues need racks and racks of amps. But how do you know how much power is enough?

by David Mellor, Course Director of Audio Masterclass

Working in live sound can encompass venues of all sizes from a small and intimate bar all the way up to the biggest sports stadium seating 100,000 or more.

So is there a way to calculate how much power you need to fill the venue with sound?

Firstly, we need to be a little more precise about this. "Fill the venue with sound" needs quantifying. One way to do this would be to say that the system should be capable of a level of 100 dB SPL over the whole seating area of the venue. Or a map of levels could be produced that allows some areas to be louder than others.

So now let's consider the variables in the system...

Let's say that you have 1000 watts of power available. This will be supplied to loudspeakers that have a certain efficiency rating. If a loudspeaker can convert 1000 watts of electrical power to 20 watts of sound power, it is doing pretty well at 2% efficiency. The rest of the energy is wasted as heat.

So now we have 20 watts of sound power to play with. All loudspeakers focus their output to a greater or lesser extent. The more focused the output, the higher the level in the direction of 'throw'.

Now for the difficult part - reflections from the room...

Any room (in acoustics, 'room' means an enclosed space of any size) holds and contains sound energy to an extent. A reverberant room will allow sound energy to bounce back and forth. A well-damped room will absorb sound energy. The reverberant room will be louder for the same sound power input because you get the opportunity to hear the same sound several times as it bounces back and forth.

Plainly, we are talking about some difficult calculations here. But there is an alternative... good old-fashioned 'rule of thumb'!

I don't think you will find a better rule of thumb than that provided by the experts at Crown Audio who collectively probably have at least as much and possibly more experience than anyone else in providing amplification for venues of varying sizes, and for different purposes.

So here it is...

  • Nearfield monitoring: 25 W for 85 dB SPL average (with 15 dB peaks), 250 W for 95 dB SPL average (with 15 dB peaks)
  • Home stereo: 150 W for 85 dB SPL average (with 15 dB peaks), 1,500 W for 95 dB SPL average (with 15 dB peaks)
  • Folk music in a coffee shop with 50 seats: 25 to 250 W
  • Folk music in a medium-size auditorium, club or house of worship with 150 to 250 seats: 95 to 250 W
  • Folk music at a small outdoor festival (50 feet from speaker to audience): 250 W
  • Pop or jazz music in a medium-size auditorium. club or house of worship with 150 to 250 seats: 250 to 750 W
  • Pop or jazz music in a 2000-seat concert hall: 400 to 1,200 W
  • Rock music in a medium-size auditorium, club or house of worship with 150 to 250 seats: At least 1,500 W
  • Rock music at a small outdoor festival (50 feet from speaker to audience): At least 1,000 to 3,000 W
  • Rock or heavy metal music in a stadium, arena or amphitheater (100 to 300 feet from speaker to audience): At least 4,000 to 15,000 W

Crown also provide a calculator, but this does not account for the directional properties of the loudspeakers, nor for reflections in the room. Still, it makes for a good starting point. (You could bear in mind that they want to sell you more amplifiers!)

Opinions from live sound practitioners would be welcome.

By David Mellor, Course Director of Audio Masterclass
Wednesday January 12, 2011

Readers' comments on this article...

Brian Masiello, Cts, Boston, Ma, USA
Monday September 19, 2011

Sorry for the late reply, but I only discovered your website recently. There are some great articles and very useful information! In regard to providing proper amplifier power ratings for a venue, please bear in mind that many sound system designers and installers will want to provide an amplifier that is slightly more powerful than the associated loudspeaker can handle. Here's why. Overdriving an amplifier will produce an output signal that is distorted. If you were to view this on an oscilloscope, you would see that this distorted signal appears like a square wave. The motion of a loudspeaker's cone will resemble the signal sent from the amplifier's output. For example, feeding a sine wave to a loudspeaker allows the loudspeaker's cone to move back and forth smoothly. But when fed a square wave, the cone protrudes out, holds there slightly, then pushes in, holds there slightly, repeat, so on and so forth. This puts quite a strain on the loudspeaker, and there is no better way to shorten the loudspeaker's lifespan than by feeding it a square wave. Therefore, it is advisable to provide an amplifier that will deliver the acceptable power level "cleanly" to the loudspeaker. Keep in mind that the amplifier does not need to be cranked up. Also, while Crown's recommendations may be a good starting point, when selecting an amplifier, it is even better if you can start by defining your goal (similar to their first two recommendations). Ask yourself, "what is maximum sound pressure level (SPL dB) that I will want to deliver to my audience?". By gathering the loudspeaker efficiency, listener distance, and my SPL goal, I can quickly calculate the amplifier power rating required (of course, bear in mind that this is a "quick check" and other factors, such as room acoustics, should also be considered for more precise calculations).
Fleeltygralse, Puerto Galera, Philippines
Sunday May 08, 2011

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