Most speakers have an impedance of either 4, 8 or 16 ohms (Ω).
To push more air, several speakers (always of equal impedance) can be
wired together in a cabinet.
The cabinet impedance depends on how the speakers
are wired and may differ from the
impedance of the speakers.
and their effect on cabinet impedance are displayed below.
An amp works best when driving a load with the same impedance as the
amplifier's "output impedance". If they don't match, there could be
If the speaker impedance is higher than the amp's, the amp might sound weak or dull.
If the speaker impedance is lower than the amp's, the amp might sound fantastic but
eventually burn out.
For these reasons, amplifier speaker jacks are
often marked with the minimum load you should connect (4Ω for example).
Speakers and cabinets should also be marked with an ohms value.
If not, you can easily measure the value with a multimeter:
Multimeters measure DC resistance, not AC impedance. However, the two values
correlate. The DC resistance is somewhat lower than the AC impedance so round up the meter reading to
the closest of 4, 8, or 16 ohms.
Some amps have an impedance selector switch on the rear panel that you
can use to match its
output impedance to different speaker impedances. Changing the
speaker impedance is one of the easiest and most dramatic ways to change the sound of a guitar amp.
WARNING: If your amplifier uses output tubes, be sure to have
speakers connected whenever the amp is turned on. Otherwise, the
amp's output transformer can easily be damaged, especially if you're
playing through it.