What makes one speaker an easy load and another hard to drive? There are a number of answers, but the two most important are impedance and sensitivity. First, impedance. Impedance is resistance that varies with frequency. The fact that the speakers’ impedance varies with the frequency it is reproducing, is one reason why the amplifier has such a tough job.
For example, let’s examine a speaker with a nominal rating of 8 Ohms. The term nominal, means average, thus is not only possible, but quite likely that significant impedance variations will occur throughout the speaker’s frequency range. Indeed, a variation as wide as 3 to 40 Ohms is possible. The amount of variation, in addition to how low or high the range, determines how difficult the speaker is drive, and thus defines the amplifiers role.