Speaker Impedance varies with frequency. You will often see Impedance measured in ohms using Ω (the Greek letter Omega). Speaker impedance is different at different frequencies. You can see this by using a multimeter and moving the speaker cone. Most speaker impedance discussions concern nominal impedance which is the approximate DC resistance of the voice coil.
Impedance tells you how much current will pass through a speaker at a specific voltage. Impedance is a way of telling you how much of the voltage introduced at one end will really make it to the other end. A speaker’s impedance influences the ratio of voltage and current. Impedance also depends on other qualities of electricity, such as resistance, reactance, inductance, and capacitance.
Impedance restricts the flow of power from your receiver or amplifier. A lower impedance speaker will accept more power. For example, a 4 ohm speaker will use more power from your amplifier than an 8 ohm speaker. Is this good" Yes and no. A 4 ohm speaker will be louder than an 8 ohm speaker at the same setting, but it will also run hotter. Does that mean a higher impedance is better" No. Impedance that is too high restricts the flow of current, vital current that a speaker needs in order to play loudly.
Impedance is determined by resistance and another value, reactance. Reactance takes into account opposing forces and frequency. Reactance describes the electrical effect of the inductors and capacitors typically found in a speaker’s crossover network. Two elements of reactance, inductance and capacitance, correspond to frequency. Inductance is in proportion to frequency while capacitance is inversely proportional to frequency.
For most average speaker owners, a speaker impedance specification in the 6-8 Ohm range (the most common kind) represents a good compromise between current and voltage. Most amplifiers and receivers can safely drive speakers with 6-8 Ohm spec.