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Speaker Impedance

What’s impedance?  Is it important?  Should I care?  They’re sensible questions, particularly if you’re trying to make sense of loudspeaker specifications. 

So we’ll answer them – but maybe not in the way you think. 

Let’s start with an example.  You’re in a room.  There’s an 800 lb gorilla in the same room.  Naturally, you want to leave the room as quickly as possible but the gorilla is sitting in front of the only door and making threatening noises.  In short, the gorilla is impeding your exit from the room. 

Here’s another example.  You’re back in that same room again.  But this time the door is blocked by a five year old kid sitting exactly where the gorilla was.  You still want to leave the room and know that the kid, all 30-some-odd pounds of him, won’t offer much resistance. 

Eureka!  You now understand “impedance”.  Huh?  Well, think of “impedance” as just another word for “resistance”,  The gorilla resists or impedes your exit from the room.  That’s high impedance.  The kid isn’t going to offer much resistance as you walk through the door, is he?  That’s low impedance. 

In electrical terms, impedance simply means resistance to the flow of electrical current.  Impedance is measured in ohms, a unit named after George Simon Ohm, a German physicist who died in the mid-1850s.   He was the first to codify the relationship between voltage, current, power, and resistance into something called Ohm’s Law. 

There are two things you need to consider when using impedance specifications to help choose a speaker:

• A speaker’s impedance changes depending on the frequency of the signal coming from the amplifier.  That’s why almost all impedance ratings are “nominal” rather than exact.  For instance, an “8 ohm” speaker may exhibit impedance measurements ranging from 3 ohms (or less) to 40 ohms (or more) at different frequencies.  Although the relationship isn’t perfect, think of “nominal” as “average” and you’ll be OK. 
• Although a common misunderstanding states that a low impedance speaker (one with a nominal rating of 4 ohms or less) draws more current from an amp, the truth is that the low impedance speaker doesn’t resist current flow as much as a high impedance speaker does.  This can mean that a low impedance speaker might cause some amplifiers to try to deliver more current than they are designed to.  This condition is analogous to driving a stick shift car with a heavy foot on the gas while the clutch is depressed.  Sooner or later, something’s gonna break. 

Our advice about impedance?   Don’t worry too much about it unless you’re using a smaller, low-current amplifier or receiver.  Then, if all else is equal, a speaker with a higher nominal impedance rating – 8 or even 16 ohms – is probably better. 

Last, but certainly not least, remember that impedance ratings have almost nothing to do with sound quality.  There are some great-sounding low impedance speakers out there, just as there are great-sounding speakers with higher impedance ratings.  Your ears rule!