This is a number that speaker manufacturers will give in ohms, usually eight ohms or four ohms, as something that's called the impedance or the nominal impedance. Now, why people get concerned about this is that you'll see lots of forums on the internet, people saying, "Oh, if it's a four-ohm speaker, you shouldn't connect it to a home theater receiver or a cheap home theater receiver because you'll blow it up, it will overheat, something will happen."
Now, we'll talk for a second about the basics of what does that number mean? What does it represent? And the reality is the number doesn't mean a lot. It's just a standard that we use to give an indication of how hard the speaker is to drive, or what kind of load it presents to the amplifier. Now, we're all familiar with the term load.
If I have a 10-pound load of bricks and 100-pound load of bricks, we know the 100-pound load is going to be a lot heavier and a lot harder to lift, right? So it requires more work, more energy to lift that 100-pound pile of bricks compared to a 10-pound pile. The same thing applies to loudspeakers. The lower the impedance number, so let's say a four-ohm speaker, is a harder load, requires more work to drive, than an eight-ohm loudspeaker.
Now, where this comes from is from Ohm's law. You may remember this equation from high school, and I'm not going to get into a lot of math, but basically, if we have the same voltage and the voltage signal is coming from your power amplifier, it follows the music signal. If we have a certain voltage across a certain resistance or a certain impedance, that equation says that voltage over resistance gives you the current.
So if we have a speaker that's eight ohms, it gives you a certain current. If we now put a speaker that's four ohms into that equation, now we get double the current which means double the power is required. So this is why people say that, you know, a lower impedance speaker is harder to drive and will say that some amplifiers or receivers are not capable of driving them.
Now, the reality is is that number is not a consistent number. Speakers are not resistors. They don't present a perfect four-ohm or six-ohm or eight-ohm load to the amplifier and that's because there are components in the speaker, the voice coil that's in the drive units, the crossover components that are made up of capacitors and inductors, all of these things change the resistance with frequency.
So if you've ever seen an impedance curve for a loud speaker, you'll note that it's a bunch of hills and valleys where the resistance is changing with frequency and that's because of these components that are not pure resistors. They don't provide a simple ohms measurement at all frequencies.
Those components are called reactive components and I'm certainly not going to get into reactants and phasing or difficult things like that. What, basically, you need to know is that a four-ohm speaker is going to present a harder load for an amplifier to drive. All you need to make sure of is that the manufacturer of the receiver or the amplifier that you're looking at purchasing or that you already own, states that it's capable of driving the load that you want to drive.
So if you have a four-ohm speaker, make sure it says it's four ohms compatible. It's that simple. Now, is that four-ohm speaker on a receiver that doesn't say it's four ohms compatible going to damage it or blow it up? No. Not likely. And it's probably in most cases going to be fine unless you listen at very, very loud levels.
A good receiver or amplifier, when presented with a load at a high level that it's not happy with, should just nicely shut down and go into protection and not cause any damage. So basically, that's the reality of loudspeaker impedance. Don't worry about it, don't be frightened about it. Just make sure that the equipment that's driving it is compatible for that load.