What are ohms, anyway?
Short answer: The ohm is the unit of measure for impedance, which is the property of a speaker that restricts the flow of electrical current through it. Typical speakers have impedance ratings of 4 ohms, 8 ohms or 16 ohms. The impedance of a speaker is a physical property that (ideally) does not change value, although from an engineering standpoint, there are many complex characteristics that make up speaker impedance For this reason, the rating of a speaker is called its ‘nominal’ value, which pretty much means “in name only”. For the average audio user, the nominal impedance is the dominant characteristic and for the purposes of this discussion, we will use the nominal value of the speaker’s impedance.
Why are ohms important?
(1) If you connect your amplifier to the wrong speaker impedance, you risk damaging the amp. In tube amps, too high a load impedance (or a disconnected load) can result in damage to the output tubes or output transformer, while in solid state amps, if the speaker impedance is too low, the amplifier will tend to overheat and more power is used up in the amplifier than is delivered to the speaker. Too many speakers on a solid state amp can burn up the power output section.
(2) The amplifier will deliver maximum power (volume) to the speaker when the speaker impedance matches (is equal to) the internal impedance (called the OUTPUT IMPEDANCE) of the amplifier. Too low an impedance will result in weak output and poor tone. If the speaker impedance is higher than that of the amplifier, its power output will again be less than it is capable of.