# Impedance

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.

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