Impedance is the measurement of resistance to electrical current flow in an alternating current circuit. A speaker's impedance is its resistance to current flow from the amplifier. Basically, it's a measurement of how hard it is for the amp to move the speaker. Impedance is measured in Ohms (Ω), with larger numbers representing higher impedances. Common values for speakers are 4 Ohms, 8 Ohms, and 16 Ohms.

Why is this important?

Since the speaker produces sound by transforming the electrical power from the amp to acoustical energy, the higher the speaker's impedance is, the less current will flow from the amp, and the volume will be reduced. Conversely, the lower the speaker's impedance is, the more current will flow from the amp, and the volume will be greater.

Amplifier designs take into account speaker impedances. All-tube amps are designed for operation at a specific impedance load, and many feature an impedance switch, allowing them to operate properly with different setups. You should always run your tube amp with the proper impedance selected. That is, if your speaker cab has an impedance of 8 Ohms, your amp should be set at 8 Ohms. If you are using two 8-Ohm cabs together, the resulting load will be 4 Ohms (See our section on Daisy Chaining Speakers) and your amp should be set at 4 Ohms. Connecting an improper impedance load will not only result in compromised tone and power, but also risks damage to both the amp and your speakers.

Hybrid amps and solid-state amps (including PA power amps) are slightly different. Their design allows an impedance load greater than the designated load without ill effects. However, connecting a load that is less than the minimum will cause the amp to overheat, resulting in the activation of the amp's protection circuitry (in the best case) or severe damage to your amp (in the worst case). For example, if your solid-state amp is designed for a minimum impedance load of 8 Ohms, you can safely connect a speaker or combination of speakers) at 8 Ohms, 16 Ohms, or even higher. However, you should not connect a speaker (or combination of speakers) that results in a load of 4 Ohms or less.