The job of a PA system is to take a weak audio signal such as that produced by a vocal, convert it into an electrical signal, increase the size of this signal and convert it back into sound energy at a significantly greater volume. A PA system is made up of a number of components of which it is the job of the amplifier to increase the size of the audio signal and the speakers to then convert this signal back from electrical energy to sound energy. If the amplifier and speakers used in a particular PA system are not properly matched in terms of amplifier power output and speaker power rating then speaker damage can occur resulting in potentially expensive repair bills or even the requirement for replacement speakers. This article will discuss the basic theory behind speaker and amplifier ratings, the reasons why speaker damage occurs and how to match an amplifier or speakers to produce the best possible sound quality and lowest possible chance of speaker damage.
How amplifiers and speakers work together
Audio signal amplifiers take a small electrical signal and by using a series of transistors ultimately produces an electrical signal that recreates the voltage fluctuations of the original but of a much higher power. Speakers work on the electric motor principle whereby the electrical pulses from the amplifier are channelled through a coil of wire creating magnetic energy in the form of an electromagnet. This coil is then attracted or repelled from a second fixed magnet creating vibrations in the paper cone to which it is fixed which in turn transfers energy into the surrounding air molecules resulting in sound.
How speakers and amplifiers are rated
Both amplifiers and speakers are given ratings in terms of the power they are capable of supplying or their ability to cope with the power supplied to them. Power is measured in Watts (W) and is the rate at which energy (measured in Joules) is converted from one form to another. For example 1 Watt of power is the equivalent of 1 Joule of energy being converted per second or 10 Watts of power is the equivalent of 10 joules of energy being converted per second.
There are a number of methods in use to describe the power of an electrical signal not all of which are best used to rate the power of amplifiers and speakers:
Instantaneous Power: This refers to the power being used at any particular instant during operation but as the power used to move the speaker cone in the complex manner required is continually and rapidly varying this is not a useful measure for describing the capability of an amplifier or speaker.
Peak Power is the maximum amount of instantaneous power present at the highest level during the signal. For amplifiers peak power is useful for describing the maximum instantaneous limit of its capability for sounds such as drumbeats and bass notes. Amplifier peak power is limited by the available power supply and if the input level is increased beyond a point where the amplifier reaches the limits of its power supply a form of signal distortion known as clipping occurs. For speakers the peak power occurs at the point where the speaker cone reaches its fore or rearmost point beyond which damage may occur.
RMS or Average Power: This is the Maximum Continuous Average power output capability of an essentially undistorted signal to a specified load impedance (in this case the load is the speaker) and is the most consistent method of comparing power levels between amplifiers and speakers.
Music or Programme Power: Often used in speaker ratings these terms were conceived by manufacturers as speakers are very rarely used to produce pure tones (for which average power is measured) instead being used to reproduce sounds of rapidly changing power distribution. Music or programme power is said to be approximately twice the equivalent average power.
Why speakers fail
Speakers most commonly fail due to either excessive power or a distorted signal being supplied by the amplifier. Heat is a by-product of the motor effect caused by sending an electrical signal through the speaker coil and if excessive power is sent to a speaker the heat generated can damage or destroy the coil. Alternatively if an underpowered amplifier is driven to the point beyond which it is capable of delivering, this "clipped" signal produces excessive high frequencies which can burn up tweeters or horns.
How to match amplifiers and speakers
When putting together a PA system you may have to match amplifiers and speakers rated with different methods (eg the amplifier is rated by RMS Power and the Speakers are rated with Music Power) and as such you may be unsure how these two measurements relate to each other and if the amp and speakers are correctly matched.
If both the amplifiers and speakers are rated in terms of RMS power and they are not likely to be overdriven you can compare them directly matching an amplifier of 100 Watts per channel with speakers rated at 100 Watts RMS. If however the system is to be used for dance music or heavy metal where the amplifier is likely to be overdriven resulting in clipping, a speaker system rated at approximately twice the amplifier RMS is recommended.
If your speakers are rated in terms of Music or Programme Power remember that this is approximately twice the average or RMS power and as such for low level applications such as speech the speakers should be double the amplifier rating (eg 100 W amp to power 200 W speakers). For applications such as live or dance music where clipping may occur speakers of Programme Power more than twice the amp RMS Power will be required and potentially up to three times more for high power applications.
Overall your PA system should be designed so that the amplifier is powerful enough so as to never be driven to clipping and the speakers sufficiently powerful so as to cater for the continuous power produced by the amp.