The job of a public address system is to take a weak audio signal such as that produced by a voice, convert it into an electrical signal, increase the size of this signal and convert it back into sound energy at a significantly larger volume. A public address system is composed of a series of components, of which it is the job of the amplifier to increase the size of the audio signal and the speakers and then convert this signal from electrical energy to sound energy. If the amplifier and speakers used in a particular PA system do not match properly in terms of amplifier output power and speaker power rating, speaker damage may occur, which can lead to potentially expensive repair bills or even the need to replace the speakers. This article will deliberate the elementary theory behind speaker and amplifier ratings, the reasons behind why speaker damage occurs and how to combine an amplifier or speakers to produce the best possible sound quality and the least chance of damage to the speakers.
How amplifiers and speakers work together
Audio signal amplifiers take a small electrical signal and, by using a series of transistors, finally produce an electrical signal that recreates the voltage fluctuations of the original but of much greater power. The loudspeakers operate according to the principle of the electric motor through which the electrical pulses of the amplifier are channeled 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 attached, which in turn transfers energy to the surrounding air molecules and produces a sound.
How speakers and amplifiers are classified
Both amplifiers and speakers have classifications in terms of the power they can supply or their ability to cope with the power supplied to them. Power is measured in watts (W) and is the speed 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 converted per second or 10 W of power is the equivalent of 10 Joules of energy converted per second.
There are several methods in use to describe the power of an electrical signal, and not all are best used to evaluate the power of amplifiers and speakers:
Instantaneous power: refers to the power that is used at any particular time during operation, but since the power used to move the speaker cone in the required complex manner varies continuously and rapidly, this is not a useful measure to describe the capacity of an amplifier or speaker.
Peak Power is the maximum amount of instantaneous power present at the highest level during the signal. For amplifiers, maximum power is useful for describing the maximum instantaneous limit of its capacity for sounds such as drums and bass notes. The maximum power of the amplifier is limited by the available power supply and if the input level increases beyond a point where the amplifier reaches the limits of its supply of power, a form of signal distortion recognized as clipping occurs. For loudspeakers, maximum power is produced at the point where the speaker cone reaches its anterior or posterior point beyond which damage may occur.
RMS or medium power: this is the maximum continuous average power output capacity of an essentially undistorted signal at a specific load impedance (in this case, the load is the speaker) and is the most consistent method for comparing power levels between amplifiers and speakers.
Music or program power: often used in speaker ratings, these terms were conceived by manufacturers, since speakers are rarely used to produce pure tones (for which the average power is measured) instead of being used to reproduce power distribution sounds that change rapidly. The power of the music or the program is said to be approximately twice the equivalent average power.
often used in speaker ratings, these terms were conceived by manufacturers, since speakers are rarely used to produce pure tones (for which the average power is measured) instead of being used to reproduce power distribution sounds that change rapidly. The power of the music or the program is said to be approximately twice the equivalent average power.
Why do the speakers fail?
The speakers usually fail due to excessive power or a distorted signal supplied by the amplifier. Heat is a byproduct of the effect of the motor caused by the sending of an electrical signal through the speaker coil and if excessive power is sent to a speaker, the generated heat can damage or destroy the coil. Alternatively, if a low power amplifier is driven to the point beyond which it is capable of delivering, this “cropped” signal produces excessive high frequencies that can burn highs or horns.
How to combine amplifiers and speakers
When assembling a public address system, you may have to combine amplifiers and speakers with different methods (for example, the amplifier is rated by RMS Power and the speakers are rated with Music Power) and, as such, you may not be sure how These two measurements are related to each other and if the amplifier and the speakers match correctly.
If both amplifiers and speakers are classified in terms of RMS power and are not likely to be overloaded, you can compare them directly by matching a 100-watt amplifier per channel with 100-watt RMS qualified speakers. However, if the system is to be used for dance or heavy metal music where the amplifier is likely to be overloaded and a clipping occurs, a speaker system with a power of approximately twice the RMS amplifier is recommended.
If your particular speakers are categories in terms of music or program power, remember that this is approximately twice the average power or RMS and, as such, for low-level applications such as speech, the speakers should double the amplifier rating ( for example, 100 W amplifier at 200 W power speakers). For applications such as live or dance music where clipping can occur, Program Power speakers will require more than twice the RMS Power amplifier and possibly up to three times more for high power applications.
In general, your PA system must be designed so that the amplifier is powerful enough to never be driven to a cutout and the speakers powerful enough to satisfy the continuous power produced by the amplifier.