Here’s a great post from our friends over at www.thebassplace.com that we thought you might like.
Matching speaker impedance to an amplifier can be confusing, but it is important to make sure that your amp and speaker cab are properly matched to avoid damaging equipment and to get the best possible sound from your gear.
Every speaker and speaker cab has an impedance rating. Most speakers are rated at 4, 8, or 16 ohms. Here is a simple way to understand impedance. Your amplifier is a source of energy, but you can think of it as a tank full of water. attaching a speaker to your amp is like attaching a spigot to that tank of water. The energy from the amp can’t flow out without a speaker attached to it, and water can’t flow without a spigot attached. If you attach a small spigot you can get a certain amount of water, but if you attach a larger spigot you can get more water in the same amount of time. The same is true with impedance. Impedance is a measure of the amount of energy that is kept (impeded) from flowing out of your amp. A speaker with high impedance is like a small spigot, because less energy can flow. A speaker with low impedance is like a larger spigot, allowing more of the potential power of the amp to flow out. So if you have a head that puts out 100 watts at 2 ohms, it will put out less power (a bit more than half) at 4 ohms, and even less at 8 ohms. To get the full 100 watts you need a cabinet that is wired at 2 ohms. Incidentally, you might not notice a big difference in volume level at the lower wattages because there are other factors that affect perceived volume.
You must match your speakers to your power amp. If you try to power a 2 ohm cabinet with an amp that is rated for a 4 ohm minimum, you will likely overheat your amp head and possibly cause damage. Most modern amps have a thermal shutdown, but it’s not easy to be heard over your way too loud lead guitar player when your amp overheats and turns itself off. If you are running a tube power amp then it’s even more important to match impedance. You can damage the output transformer with mismatched impedance on a tube head, and that’s not an easy thing to fix.
Keep in mind that there are lots of ways speakers can be wired that will affect the total impedance of your cabinet. If you have two 4 ohm speakers wired in parallel – positive to positive and negative to negative – the total resistance of your cab is 2 ohms. Wire those same two speakers in series and you have 8 ohms of resistance. Four 8 ohm speakers wired in parallel equals 2 ohms, but you can also wire four 8 ohm speakers in series/parallel to get 8 ohms – wire two speakers in series for 16 ohms, and the other 2 speakers in series for 16 ohms, then wire each PAIR of speakers in parallel for a total impedance of 8 ohms. As you can see, it is confusing, but remember that series adds while parallel divides.
There are other factors to consider when matching speakers with amp heads including the wattage of the speakers, the wattage of the amp, the frequency response of the speakers, the size of the speakers, etcetera. All of these things will affect your sound, but regardless of all of those other options, it is critical that the total speaker impedance is correct for the amp that you are using.