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:
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.
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