Ohms are the units which measure resistance or impedance. Resistance or impedance are two terms which define the tendency for a substance or electrical component to attenuate or reduce a voltage when current is passed through it. The relationships between resistance, current and voltage are described by Ohm's Law, a simple mathematical equation. With regard to speakers, generally, the lower the impedance rating the more inefficient the speaker is. That is, speakers rated at 16 ohms tend to be more efficient than 8 ohms which tend to be more efficient than speakers rated at 4 ohms. All speakers have an impedance which varies with respect to the frequency being produced. Most speakers tend to have large impedance dips and increases near their woofer resonance point, depending on the design, as well as at other frequencies throughout their operating range. Since amplifiers have to work harder in order to drive inefficient speakers as well as speakers which have low impedance somewhere within their frequency range, the impedance characteristics of any speaker are most properly expressed on a graph which has frequency response on the X-axis and impedance (Ohms) on the Y-axis. Often two impedance ratings are quoted for a speaker, when a graph is not used to indicate a speaker's impedance behavior. These are the nominal impedance rating and the minimum impedance rating. The nominal impedance rating is meant to express the "average" or "typical" impedance behavior of a speaker while the minimum rating is meant to indicate the minimum impedance at any frequency within the operating range.
