Speakers are commonly employed in electronic siren circuits, public address systems, background music systems, whole-house stereo, and more. As such, when an installer carelessly connects two or more speakers together, damage can occur to either the speakers themselves or the amplifier they connect to.

To properly connect your speakers, dealers must learn how audio sound circuits operate, as well as how to calculate the overall resistance in a circuit. The basis for both is the ever-popular formula commonly referred to as Ohm's Law.

"Ohm's law expresses the relationship between voltage, current and resistance in an electrical circuit. [It] states that circuit current is directly proportional to voltage and inversely proportional to resistance" (Electronics Math, Bill R. Deem, Prentice-Hall, Englewood Cliffs, N.J.)

For all practical purposes, the total resistance of a simple audio circuit with only one speaker (see drawing a) is equal to the resistance of the lone speaker. In this case the total resistance is 8 Ohms, which is the resistance of the speaker itself, or:
RT = R1

In a series circuit (see illustration b), total circuit resistance is easy to calculate by adding each speaker resistance together. In this case, total circuit resistance is determined as follows:

RT = R1 + R2
RT = 4 + 4
RT = 8 Ohms

Where dealers encounter problems, however, is where there are multiple speakers in parallel (see drawing c). Here, we have two 8-Ohm speakers in parallel.

In this case, there are different ways in which to determine total circuit resistance. Where there are two or more identical speakers in an audio circuit, for example, simply divide the value of one resistor by the number of resistors. Where there are two 8-Ohm resistors in parallel, we would divide 8 by 2 for a total resistance of 4 Ohms

Another way to determine total circuit resistance in a parallel circuit is to use the following formula:

RT = ______1______
_1_ + _1_
R1 R2

In the case of our two 8-Ohm speakers in parallel, to determine total circuit resistance, we plug our values in as follows:

RT = ______1______
_1_ + _1_
8 8

RT = ______1______
0.125 + 0.125

RT = ___1___

RT = 4 Ohms

For optimum efficiency, performance and longevity, the Ohms and Watts ratings of a speaker circuit must match that of the amplifier to which it is connected to. For example, an audio amplifier rated at 4-Ohms, 10-Watts will function very well when connected to a speaker circuit rated at 4 Ohms, 10 Watts. An identical match will also mean maximum clarity and longevity.

Where siren speakers are concerned, it may not be necessary to match the Ohms and Watts exactly. For example, where a siren driver's audio amplifier is designed for 4 Ohms, 15-Watts, installing two 4-Ohm, 15-Watt speakers in series would work just fine. This configuration creates a working load of 8 Ohms with a power potential of 30 Watts, although 30 Watts will never be realized on a continuous basis.