Drive units and complete loudspeaker systems are also rated in terms of their impedance.
This is the load presented to the amplifier, where a low impedance means the amplifier will have to deliver more current, and hence 'work harder'.
A common nominal impedance is 8 ohms. 'Nominal’ means that this is averaged over the frequency range of the drive unit or loudspeaker, and you will find that the actual impedance departs significantly from nominal according to frequency.
Normally this isn't particularly significant, except in two situations:
- At some frequency the impedance drops well below the nominal impedance. The power amplifier will be called upon to deliver perhaps more power than it is capable of, causing clipping, or perhaps the amplifier might even go into protection mode to avoid damage to itself.
- The output impedance of a power amplifier is very low – just a small fraction of an ohm. You could think of the output impedance of the amplifier in series with the impedance of the loudspeaker as a potential divider. Work out the potential divider equation with R1 equal to zero and you will see that the output voltage is equal to the input voltage. However, give R1 some significant impedance, as would happen with a long run of loudspeaker cable, and you will see a voltage loss. Make R2 - the loudspeaker impedance - variable with frequency and you will now see a rather less than flat frequency response.
To be honest, the above points are not always at the forefront of the working sound engineer's mind, but they are significant and worth knowing about.