How Much Power?

by Patrick H. Quilter

You’ve just purchased new loudspeakers for your church praise band or for a public address system in your church hall. Now you need a power amplifier to drive them. How much power do you really need?

The answer to that often-asked question depends on several factors. How much power can your speakers handle? How efficient are your speakers? What is the room size? How loud do you need to be? And, what is the type of program? A simple speech system can get by on much less power than a full-range system for music and chorus.

Most system designs start with the speakers. Speaker selection is actually the most critical part of the system design. Their frequency range, coverage pattern, and power requirements basically determine the amplifier requirements. Most loudspeaker manufacturers give two figures for power handling: continuous and peak. A reasonable amplifier rating will fall between these two figures. Thus, a speaker rated at 200 watts continuous and 800 watts peak should operate safely with a 400-watt amplifier.

Speaker efficiency is another critical factor. Speakers have a "sensitivity rating" which tells you how much sound comes out when using a standard input power. A more efficient speaker delivers more sound for a given wattage. Every increase of 3 dB in a speaker’s sensitivity is equivalent to a "free doubling" of your amplifier power. For example, to produce the same sound level, a speaker with a sensitivity rating of 94 dB requires twice the power of a speaker rated at 97 dB, and four times the power of a speaker rated at 100 dB. However, more efficient speakers often sacrifice smoothness to "cut through" better. Therefore, a medium efficiency speaker, coupled with a somewhat more powerful amplifier, may be the best choice.

When considering room size, you will need two to four times the power to fill a space that is twice as long. If a 100-watt system sounds good in a small church, a church with twice the length will require 200-400 watts to maintain the same sound level in the back. Keep in mind, however, that large spaces need speakers with special "coverage patterns" to protect the people in front from excessive sound levels. A professional audio consultant can be hired to design a system that ensures even coverage, but one basic rule is to mount the speakers as high as possible, so some of the sound travels over the people in front in order to reach the back.

Most speakers reach a point of diminishing returns at about 400 watts. If you need additional volume, you will likely need to add more speakers, and their coverage patterns become even more important.

Remember that the dynamic range of today’s audio source equipment is extremely high. The amp that you select must be capable of handling the program peaks without clipping, but the system must be protected against severe "blasts" of sound. Many amplifiers offer clip limiting features which protects against the worst peaks. The signal chain feeding the amplifier should also have some sort of program limiter to protect both the system – and the congregation’s hearing – from unintended bursts of sound.

Patrick H Quilter is the founder and Chief Technical Officer of QSC Audio Products, Inc. For more information on QSC products, visit www.qscaudio.com, or e-mail: info@qscaudio.com.