What is impedance? Well, if you remember your high school physics class, maybe you took an electrical course in school, you may be familiar with the term resistance. Resistance as the name suggests, is a force or a factor that resists the flow of current. The flow of current when we're talking resistance is normally reserved to DC or direct current. You can also think of direct current as an AC or alternating current at a frequency of zero Hertz. So there is no alternating, it's a steady state constant direct current. So when we say resistance, that's usually reserved for talking about resistance to the flow of current when we're talking about a DC current or a DC voltage.
Impedance is just the partner of resistance but impedance is the term that we use when we're talking about an alternating current, or a signal that alternates. Impedance, as the name suggests, impedes flow of alternating current through a circuit, through a device, through a load.
The best analogy is water flowing through a hose. If I pinch the hose, the amount of water coming out of the end is going to be reduced. That pinch is essentially resistance or impedance in an alternating current.
Now when it comes to loudspeakers, we talk about impedance because we drive them with an alternating current. Music signals, amplifiers amplify alternating current. A positive current makes the woofer or the drive unit go out and a negative one makes it go in. So that's how we produce frequency vibrations by that in and out motion of the drive unit. It is powered by an alternating current and we use the term impedance there.
One thing that's important to understand: resistance is a constant value because we measure it as DC. Impedance is not a constant value. It varies with frequency. And how much variation and what the values are depend on the coil of wire, the drive units, and the crossover and cabinet combination all coming together to give you what's called a loudspeaker impedance curve. Many manufacturers actually publish that curve so you can see what the impedance is at different frequencies. It's defining how much work an amplifier or the amplifier section in the receiver has to do to get those parts moving and generate an audio or acoustic signal. Whether it's music or text tone, it really doesn't matter.
So you might have seen on spec sheets for loud speakers the term nominal impedance. What is nominal impedance? It's usually eight ohms or four ohms, sometimes you might even see six ohms. I mentioned that the impedance value will vary with frequency. And you can think of the nominal impedance as an average over the operating range of the speaker so that you get one number out of it. Eight ohms or four ohms, let's say. And that's really just to give you a gauge and a guide as to how much of a load that loudspeaker is going to present to your amplifier or receiver. So just understand that nominal impedance doesn't mean that at 100 Hertz or at 1,000 Hertz that that speaker is going to be eight ohms. It just means it's average. You may also have seen a spec that some manufacturers include called minimum impedance. That’s the operating range of the speaker. It's almost never at DC or at zero Hertz. Again, remember, you're measuring now at DC. And the DC resistance can be far lower than the nominal impedance of the speaker.
Now that you understand the basics of what impedance of a loud speaker means, but now why are people always concerned about, "Can I use this speaker with my receiver or my amplifier?". Many manufacturers will only rate the output power capability of a receiver or amplifier at eight ohms. The question then comes up, "Well, what if I were to use a four ohm speaker with that receiver or amplifier, is it okay?" The concern is because a lot of people will do Google searches on if it is “okay to put a four ohm speaker on an amp or a receiver that's only rated for eight ohms?”. People will say, "No, no, no, don't do it. You'll destroy it. You'll void your warranty."
Well, they may be right but let's explain for a second. Why does a four ohm speaker present a difficult load for an amplifier that's only rated for eight ohms? Actually a four ohm speaker will always present a more difficult load or a more impeding load than an eight ohm speaker. Why is that? Well, if you remember Ohm's Law, resistance equals voltage divided by current. You probably ran into it at some point in your life. Power equals voltage times current. If we take those two equations and we do a little rearranging, we can get an equation that says power equals voltage squared or voltage times voltage divided by resistance, or impedance in our case.
So why does this matter? The RMS voltage at a particular signal level from an amplifier is divided by resistance to give you the power. Example: Let’s say I've got 20 volts, 20 volts RMS. 20 x 20 is 400. With an eight ohm speaker, 400 / 8 is 50 Watts. But what if that's a four ohm speaker. 400 divided by 4 is now 100 Watts. That means that for the same voltage or same output level from the amplifier, I'm asking it to deliver 2x the power into a 4 ohm loudspeaker than an 8 ohm loudspeaker. Why does that matter? If the amplifier is rated at 100 Watts and I'm only driving the eight ohm speaker at 50 Watts, then I should be able to put 100 Watts into the four ohm speaker. Right? Well, not necessarily. Let's go back to our Ohm's Law equation. Resistance equals voltage divided by current. If we rearrange those terms, current equals voltage divided by resistance.
Now I think you can see what might be going on here. If we take that 20 volts and we have a four ohms speaker connected to the amplifier, we're drawing five amps from that amplifier output. In the case of the eight ohm speaker, it's only two and a half amps. Amplifiers are built with a power supply and output devices that are rated for a certain amount of current. Even though the amplifier might be capable of producing, let's say if it's specified 100 Watts in eight ohms, it doesn't necessarily mean that it can support four ohms because you may be over the current capability of the amplifier and power supply.
Manufacturers that do provide a four ohm power rating or sometimes even a two ohm power output rating, that's telling you something. That's telling you that the amp section of the receiver and the power supply is designed to provide a lot of current. If you only see an eight ohm rating, or if the receiver manufacturer says, don't use less than eight ohms or six ohms, stay away from a four ohm speaker, you can judge that it’s possibly not a very over-designed amplifier or power supply.
What's the harm and what can happen? If I have a receiver that's rated at only eight ohms to drive eight ohm loads and I put a four ohm speaker, what's the consequence? These days you would hope that well-designed electronics will never actually self-destruct so that you can damage them. Most receivers and power amplifiers have protection circuitry that if it's unhappy with the current that it's being asked to deliver to the speaker, that it will cut out or it'll shut down or something will intervene to prevent any damage.
Now I'm not saying that it’s 100% the case. I'm sure there's some equipment that you can actually blow up or damage. Like a four ohm speaker if it's not rated for it. More likely other than the protection circuit kicking in is if you're listening to a movie and you've now got a pair of four ohm front speakers, the front left and right that are doing a lot of the work and you know that receiver is only rated for eight ohms and you've got the movie cranked up and you watch it.
Well, over time the power that the amplifiers are going to be asked to deliver is going to vary depending on the content in the film. But the thing is, is that if it has a lot of very loud content and you're drawing a lot of power and that movie may last an hour and a half or two, is that what might happen if the amplifier or receiver doesn't actually shut down or go into a protection mode, is that it may overheat. You might get a flashing light on the front or something that your manual tells you that there's some thermal protection that's kicked in. Those are really the things that can happen if you take an eight ohm only rated amplifier or receiver and connect the four ohm speaker to it.
Unless you actually download the manual and go deep into the specifications, sometimes they'll leave out the fact that it has a power rating for six ohms or four ohms. So if you're concerned or in doubt, before you buy that shiny brand new pair of speakers that you've always had your eye on and it happens to be four ohms and your receiver manual says a power rating in eight ohms, give the manufacturer a call or send them an email. Maybe also look at upgrading your amp or receiver or choosing a different pair of speakers.
Hopefully that will alleviate a little bit of confusion. I know it's a long and detailed topic and I've glossed over a lot, but the fact of the matter is that people that are concerned and ask the question about, "Can I run a four ohm speaker on an eight ohm receiver or amplifier if it's only rated for eight ohms?" In most cases it's actually going to be okay, but double-check with the manufacturer. The last thing you want to do is damage something because you ignored the instructions that are in the manual. One more thing. If the manual specifically forbids and says, minimum allowable impedance, six ohms, don't connect the four ohm speaker to it. You could be in trouble.
