How much Wattage is actually necessary for a PA System?
Decibels and watts
When the amount of watts concerned doubles, there is approximately a 3dB gain. In theory, 3dB is on average the smallest difference in volume that one can notice, when not particularly listening for a difference. This means that the difference between a 500W PA and a 1kW PA is only 3dB. If you were to put both systems on full blast, the perceived volume difference would probably not be as you would expect. This often leads to the misconception that having double the wattage is a major upgrade from something half the wattage, when in fact, you will probably only notice a small difference. So is it really worth getting the 1kW system with impressive specifications, as opposed to the 500W system? Sound equipment manufacturers that have a product with many wattage versions never increase their price proportionately to the output you're actually getting.
Inverse square law
The inverse square law is as follows: For every doubling of distance, the perceived volume level decreases by 6dB. This means, that if you're experiencing 106dB SPL at 1 meter from a loudspeaker, if you step another meter backwards you will experience 100dB SPL, and another 2 meters backwards will result in 94dB SPL, and so it goes. Determining how loud you want your musical performance, speech or amplified sound of any kind to be, you need to assess the distance at which listeners will be listening from, to determine just how much amplification you need in order to reach them with the appropriate volume.
The amount of wattage required for your PA system is also determined by the sensitivity/efficiency of your loudspeakers. A very efficient loudspeaker might for example, have an efficiency rating of 105dB at 1W/1m. What this means is that if you were to stand one meter away from the loudspeaker, (producing 1 watt of acoustic energy) you would experience a sound pressure level of 105dB SPL.
No 2 systems are the same
One 200W system will not produce the exact same volume as another 200W system. A wattage rating does not guarantee a certain output, but is only a theoretical value. This is because every link in the audio process chain is as important as the one before. Every part of the audio chain can and will distort the signal in some way, and based on the signal sent through one component, the end result may be softer signal, regardless of how it's amplified or processed.
A PA system will typically be built up of the following components:
- A source, such as a microphone or a CD player, which can be digital or analogue.
- A preamp, which brings all the signals plugged into it, to the same level. This is necessary because each type of source gives a different signal level based on its voltage sensitivity. For example, a microphone level gives off a tiny signal compared to a line level source.
- A power amp, which amplifies the combined signal sent from the preamp.
- Loudspeakers, which take the electrical energy sent through the sound component chain and turn it into acoustic energy which is output through the loudspeakers.
- Interconnects, which consist of all of the cabling between the audio components.
All of this brings an overall question to making a PA related decision. How much wattage do you really need? Assess your performance environment, your signal chain, (loudspeakers in particular) and the nature of your amplified sound.
Author: Duran (Port Elizabeth, South Africa)
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Edited by: Rajesh Bihani ( Find me on Google+ )