Tech Tip:Speaker Impedance
The simplified explanation: Impedance is the tendency of a circuit to resist the flow of AC electricity and is measured in ohms. As the impedance goes up, it's harder for the electricity to get through. A good analogy would be to water flowing through a pipe. As the pipe diameter gets bigger, the water flows through it more easily. The bigger the pipe, the lower the resistance or impedance.
An amplifier's output transformer is designed to work with a particular impedance level (or more than one if it has switchable settings). Matching the correct speaker impedance to the amp transformer is important. If mismatched, the transformer and other amp components can be damaged. Most speakers used today are either 4, 8, or 16 ohms.
The more technical explanation: Impedance is the combination of reactance (which changes with frequency), and resistance (which does not change with frequency). Thus the speaker's impedance actually changes depending on the frequency of the signal it is receiving. However, speakers are generally rated at a standard frequency level, resulting in the uniform 4, 8, and 16 ohm ratings.
It's when you wire several speakers together that things get freaky. Adding two speakers together, for example, can result in different impedances. It all depends on how they are hooked together, in series or in parallel. Two 8 ohm speakers wired together can result in 16 ohms (in series) or 4 ohms (in parallel). Here's why. In series, all the current is flowing through them one after another; thus, they represent a larger impedance than a single speaker, the sum of their individual impedances. If you wire them in parallel, the current has two different paths to choose from and will split up evenly between them (assuming the speakers have the same impedance). Thus, the total impedance will be less than the impedance of either single speaker. The formula for finding the impedance for speakers wired in parallel is the product of the two speaker ratings (8 x 8 = 64), divided by the sum (8 + 8 = 16). So two 8 ohm speakers would result in 4 ohms (64 divided by 16 = 4). Wiring parallel is the only way to add speakers together without adding reactive loads.