Loudspeakers work with inductive or capacitive technology. The loudspeaker impedance (Z) is therefore frequency-dependent and has different values over the frequency range of the loudspeaker. To protect the power amplifiers, DIN and IEC defined the term nominal impedance as early as the 1980s. However, it is not used by the manufacturers in accordance with the definition.
In the manufacturers' specifications, the loudspeaker impedance (Z) is given as the average value for the AC resistance of a loudspeaker or speaker cabinet. It is 4 ohms or 8 ohms. The value is very important in that the sound power output falls inversely proportional with the speaker impedance for the same voltage; (`P = U^2/R`). This means that an 8-ohm loudspeaker emits only half the sound power of a 4-ohm loudspeaker.
When looking at a moving coil loudspeaker, it can be seen that the impedance of the loudspeaker coil at DC current corresponds to the ohmic resistance of the coil and is a few ohms. As the frequency increases, the impedance also increases and has a maximum at the free-air resonance, which depends on the speaker design, and may well be 20 ohms or more. The free-air resonance is at low frequencies in the lower audible range. After the more or less pronounced resonance impedance, the impedance drops to the frequency where voltages across the coil inductance and the transformed diaphragm mass just cancel each other out. After that, the impedance increases due to the coil inductance, which is a few milli-Henry (mH).
The loudspeaker impedance is important for series and parallel connection of loudspeakers, because the resulting impedance should not fall below the value required by the power amplifier. The loudspeaker impedance is matched to the internal resistance of the power amplifier by means of voltage matching.