Speaker impedance is one of those numbers printed on the back of most speakers that rarely gets explained. This guide covers what it means, why it matters when connecting a speaker to an amplifier, and what happens when you get it wrong.
What impedance means
Impedance is a measure of how much a speaker resists the flow of electrical current from an amplifier. It is measured in ohms (Ω). Most consumer speakers are rated at 4 Ω, 6 Ω, or 8 Ω. Some professional speakers are 16 Ω.
Unlike a simple resistor, speaker impedance varies with frequency — the rating on the cabinet is a nominal figure, not a fixed value. A speaker rated at 8 Ω might dip to 5 Ω at certain frequencies. Manufacturers rate speakers at their nominal (typical) impedance.
Why the amplifier cares
An amplifier is designed to drive a specific range of speaker impedances. Most home amplifiers and AV receivers specify a minimum load — commonly 4 Ω or 6 Ω. Connecting a speaker with lower impedance than the minimum draws more current than the amplifier was designed to supply. This causes it to run hot, and if protection circuits do not intervene, it can damage the output stage.
Connecting a speaker with higher impedance than the rated load (say, a 16 Ω speaker on an 8 Ω amplifier) is generally safe but you will get less power delivered to the speaker and therefore less volume.
A practical rule
Match or exceed the amplifier's minimum impedance rating. If your amplifier says “minimum load 6 Ω”, use 6 Ω or 8 Ω speakers. Do not use 4 Ω speakers with it unless the manual explicitly permits this.
Wiring multiple speakers
Connecting two 8 Ω speakers in parallel (both positive terminals together, both negative terminals together) gives a combined impedance of 4 Ω. This can fall below some amplifiers' minimum. Connecting them in series (positive of amplifier to positive of speaker 1, negative of speaker 1 to positive of speaker 2, negative of speaker 2 back to amplifier) gives 16 Ω, which is always safe but means less power per speaker.
Frequently asked questions
Can I use an 8 Ω speaker where a 6 Ω is recommended?
Yes. Higher impedance than specified is safe. The amplifier will deliver slightly less power, which usually means slightly lower maximum volume. In practice the difference is small.
My amplifier gets hot with my new speakers. Is impedance the issue?
Possibly. Check the impedance rating of your speakers against the amplifier's minimum load specification in the manual. If the speakers are below the minimum, they are drawing more current than the design allows. Some amplifiers have thermal or current-limiting protection that will cause them to shut down before damage occurs — this is the correct response to an overload.
Does impedance affect sound quality?
Only indirectly, through its interaction with the amplifier. A correctly matched pair will perform within the amplifier's design parameters. A mismatched pair (load too low) may cause the amplifier to clip earlier, which does audibly degrade sound quality — and can damage speakers.
Always check your amplifier's manual for its minimum impedance specification before connecting new speakers.