Ohms? Load? Impedance? Resistance? All of these terms are commonly used when talking about audio equipment in general and particularly in discussions about amplifiers and loudspeakers. Confusion frequently exists, however, when it comes to defining these words and using them in practical situations. This article will provide you with some useable definitions of these words and some basic information about their use in routine sound system set-up and operation.

The ohm is a unit of measurement much like the decibel or the kilogram or the inch. It is used to describe the quantity of "impedance"or "resistance" that a particular piece of equipment or cable may have. Although impedance and resistance are not interchangeable terms, they are similar in concept.

Figure 1

Let's look at several examples that will help us to define these words. In Figure 1, you see two identical water reservoirs with two different size outlet pipes. The water sitting in the reservoir represents its potential energy; whereas, the rate at which the water empties through the pipe represents its rate of flow. In electronics, the potential energy of the water sitting in the reservoir represents voltage and the rate at which the water empties through the pipe represents current.

If you look at Figure 1 and imagine that the release valves for each reservoir are opened simultaneously, it is obvious that reservoir II will be empty first. If each outlet pipe is left open for the same amount of time, more water will flow through the larger pipe in reservoir II because there is less resistance in the pipe. The smaller diameter pipe of reservoir I will offer a higher resistance and will impede the flow of water. Likewise, if the outlet pipe had any bends in it, the bends may not increase the pipes resistance but it could greatly impede the flow of water (i.e. its total impedance will be higher). In most electronic systems we are dealing with the total impedance rather than just the resistance. As the amount of impedance increases, the number of ohms needed also increases.

Figure 2

For our second example, lets look at Figure 2. The reservoirs in this example are very similar to those of Figure 1; but this time, instead of each reservoir having different sized outlet pipes, reservoir IV has two outlet pipes of the same size while reservoir III has only one outlet pipe of that size.  In this situation, the total area of the outlet pipes attached to reservoir IV are double that of the outlet pipe attached to reservoir III; therefore the total impedance to water flow will be half as much for reservoir IV as compared to reservoir III.

With those examples in mind, let's look at a practical example of impedance. In Figures 3 and 4, we have two different configurations of identical amplifiers and loudspeaker(s). The owner's manual for the amplifier tells us that the power amplifier can deliver 100 watts of power to a total load impedance of 8-ohms and 150 watts to a total load impedance of 4-ohms. The owner's manual for the loudspeakers (remember that each of the loudspeakers are identical) tells us that each loudspeaker has an impedance of 8-ohms.

Figure 3

The amplifier, in Figure 3, is connected to a single 8-ohm loudspeaker and "sees" a total load impedance of 8-ohms. Therefore, the loudspeaker will receive a maximum power of approximately 100 watts.

Figure 4

The amplifier in Figure 4 is connected to two loudspeakers wired in parallel. In most situations, the additional connectors on amplifiers or loudspeakers are labeled "loudspeaker out" or "external loudspeaker." This amplifier with two 8-ohm loudspeakers (or two outlet pipes) connected in parallel "sees" a total load impedance of 4-ohms. The amplifier will now deliver 150 watts to its loudspeaker load or 75 watts to each loudspeaker.

The basic guideline to follow in this type of situation is to make certain that the total load impedance of all loudspeakers connected to an amplifier is not lower than the minimum recommended amplifier load impedance. If, for example, the amplifier in Figure 4 had a minimum load impedance of 2-ohms (check the back of the amplifier or the manual for this information), we could have connected two more 8-ohm loudspeakers in parallel for a total of four loudspeakers.

Keep in mind that every time you double the number of loudspeakers (with the same impedance) connected in parallel, the total load impedance is cut in half. Remember to think of each additional loudspeaker connected to an amplifier channel as just one more water outlet pipe added to the same reservoir.

Obviously, if you start mixing different size outlet pipes (loudspeakers with different impedances) the formula gets more complicated.