You might not think of them this way, but every speaker you buy is an electronic component – a resistor – just like the little striped kind you see soldered onto circuit boards. Like their smaller cousins, they're rated in ohms, a measure of the resistance they provide. Your stereo might work with either 2-ohm or 4-ohm speakers, or some other rating, but you need to know what works best with your amplifier.
The speakers you connect to your stereo make up part of the amplification circuit. Your music goes out to the speakers and back to the amp as an electrical signal. The speaker wire itself adds some resistance or impedance to that circuit, and the speakers add more.
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Your stereo's amplifier is designed to work with speakers of a certain impedance or, in some cases, within a given impedance range. Traditionally home stereo speakers have usually been rated at 8 ohms, though 4 ohms and other ratings are not uncommon. Car speakers have traditionally been 4 ohms, although many are 2 ohms.
2-Ohm Speakers vs. 4 or 8 Ohm
The first and most important thing to know is what impedance your amplifier is rated for. That information should be printed somewhere near the speaker terminals or be mentioned in the instruction manual. If a car amp is rated for 2 to 4 ohms, you can use speakers of either rating. A home stereo might be rated for 4 to 8 ohms or only 8 ohms.
Have you ever leaned up against a piece of furniture, only to have it move and drop you to the floor? That's more or less what happens when you connect an amplifier to speakers with an impedance that's too low. In this case, your amp can overheat badly and might burn out. On the other hand, if the impedance is too high, it reduces the level of power to your speakers.
If you have a choice between 2-ohm and 4-ohm speakers, your amp can drive the 2-ohm speakers at a much higher power level. The other way to look at it is that at any given volume, your amp is only working half as hard. In a 2-ohm vs. 4-ohm subwoofer comparison, for example, the 2-ohm sub theoretically creates more powerful bass, but also cleaner bass at a given volume because the amp isn't pushing its limits.
Series vs. Parallel Connection
Unfortunately, getting it right goes beyond the speakers you choose: You also have to wire them correctly. There are two ways you can connect multiple speakers on the same circuit, in parallel or in series.
Connecting them in parallel means running the wire from the positive terminal of one speaker to the positive terminal of the next, and the same with the negative terminals. When you do that each speaker cuts the impedance in half, so two 4-ohm speakers only give 2 ohms impedance.
Connecting them in series means running speaker wire from the positive terminal of one speaker to the negative of the next, and so on. Wiring this way adds the impedances, so two 4-ohm speakers add up to 8 ohms or two 2-ohm speakers add up to 4 ohms.
Ohm's Law and Real-Life Wiring
This is easy enough to follow when you're only connecting a speaker or two. If you have a 2-ohm 6.5-inch mid-bass and a 2-ohm 6x9 woofer, and an amp rated for 4-ohm speakers, you hook them up in series and are done. For more complicated installations, balancing out multiple subwoofers and smaller drivers at different impedance levels, you need to fall back on Ohm's law and do some math.
Ohm's law describes the relationship between voltage, impedance, current and power. It's usually written as V = I x R, meaning voltage equals amperage (current) multiplied by resistance, but you can rearrange it to solve for any of those variables including resistance. You can do the math manually or rely on an online Ohm's law calculator.
Real-world audio systems can include dozens of speakers, so in practice, you might need to use a combination of different impedances and series and parallel wiring to arrive at the resistance you need. If your amp supports 2 to 4 ohms, for example, any end result within that range is acceptable.