Impedance
Definition
Impedance is the opposition a speaker presents to alternating current from the amplifier. It is measured in ohms (Ω). A lower impedance draws more current at the same voltage.
Nominal impedance is a single number on the spec sheet, usually 4Ω, 6Ω, or 8Ω. The real impedance curve varies with frequency. Bass notes often dip below the nominal rating.
Formula
Power, voltage, and resistance relate as:
P = V² / R
At a given voltage, halving the load roughly doubles the current the amplifier must supply. That is why a 4Ω speaker is a harder load than an 8Ω speaker on the same amp.
In a stereo system
Match the speaker's nominal impedance to the amplifier's minimum rated load. An amplifier rated "stable into 4Ω" can drive 4Ω speakers. An amplifier rated 8Ω minimum should not be paired with 4Ω speakers.
Build HiFi flags impedance mismatch as a pairing error. Current delivery and damping factor both depend on this match.
