Impedance is one of those tech terms whose meaning is just close enough to its general definition to cause some confusion. In everyday language, an impedance is a hindrance — an obstacle to be overcome. Electrically, impedance is a neutral term — part of the physical properties of electrical components.

In a simple direct current (DC) circuit, the signal moves in one direction. There’s only one force at play – resistance, and resistance is simply how well — or how poorly — the system transfers the current. But the connection between home or car amplifiers/receivers and speakers uses alternating current (AC). Impedance describes the resistance to the flow of energy through an AC circuit, plus the other factors that affect current transfer when power continually alternates direction.

Make sure to match receiver/amp impedance to speaker impedance
Impedance ratings (measured in ohms) help you match home or car speakers to a receiver or amplifier. For speakers, this spec tells you the resistance to the flow of power from the amplifier or receiver. (All amplifiers or receivers must encounter some resistance to function properly.) The impedance spec for amplifiers and receivers tells you what the minimum recommended speaker impedance needs to be for the component to operate within a safe range. When you match the impedance spec of your amplifier or receiver with your speakers, you ensure that they’ll operate in harmony with each other.

Why the impedance spec matters
Using speakers with a higher impedance rating than the receiver or amplifier will result in an underutilization of the component’s potential power output. For example, if you connected speakers with an impedance of 16 ohms to a receiver rated at 8 ohms, then the speakers would get about the half the power the receiver could potentially deliver. While the amp wouldn’t be producing its full output, the difference in maximum potential volume would be relatively small.

But the consequences are much more serious when using speakers with a lower impedance rating than the amplifier or receiver. Connecting 4 ohm speakers to an 8 ohm amp or receiver may seem like a good thing. After all, if mismatching one way halves the power to the speakers, then mismatching the other should double it, right?

Yes, but that’s a bad thing. When your receiver or amp doesn’t encounter sufficient impedance, it pushes out more power than it was designed to safely provide. The component’s heat sinks get overwhelmed, causing the temperature inside the unit to get dangerously high. At best, the receiver or amp will simply shut itself off. At worst, if the heat builds up faster than the component’s safeguards can react, it can seriously damage the unit’s circuitry.

Remember: A good rule of thumb is to try to match the impedance ratings of your speakers and components.