Impedance is a measure of the resistance that an electrical circuit puts up against current when voltage is applied. In the context of loudspeakers, impedance is usually expressed in ohms and should be compatible with the output of the amplifier or receiver. A low impedance (for example, 4 ohms) may provide greater volume, but may also place greater demands on the amplifier. A higher impedance (for example, 8 ohms) may be less demanding on the amplifier, but may provide less volume than a lower impedance speaker with the same amplifier output.