Getting impedance right protects your amplifier and gets the power you paid for. Impedance, measured in ohms, is the electrical load a speaker presents to an amplifier. Amplifiers produce more power into lower loads, up to a limit. Wire your subwoofers to a load below that limit and the amplifier overheats, shuts down or fails. Wire them too high and you leave power on the table. This guide explains the math, the difference between single and dual voice coil subs, and the most common wiring setups.
Series: impedances add. Two 4-ohm coils in series = 8 ohms. In series, positive from the amplifier goes to the first coil's positive; that coil's negative connects to the second coil's positive; the second coil's negative returns to the amplifier.
Parallel: equal impedances divide by the count. Two 4-ohm coils in parallel = 2 ohms. In parallel, all positives connect together to the amplifier's positive and all negatives to the amplifier's negative.
For unequal values in parallel, use the formula 1/Total = 1/R1 + 1/R2 + … However, it is best to avoid mixing impedances because the coils will receive unequal power.
A single voice coil (SVC) sub has one pair of terminals. A dual voice coil (DVC) sub has two coils on the same former, each with its own terminals, giving more wiring options: a DVC 4-ohm sub can be wired to 8 ohms (series) or 2 ohms (parallel). A DVC 2-ohm sub can be wired to 4 ohms (series) or 1 ohm (parallel).
Always use both coils of a DVC sub. Leaving one coil disconnected makes the sub work harder, with less output and more risk of damage. Both coils should also be wired the same way and receive the same signal; do not connect one coil to the left channel and the other to the right unless both channels carry identical mono bass.
| Subwoofers | Wiring | Final load |
|---|---|---|
| 1 × SVC 4 Ω | Direct | 4 Ω |
| 1 × DVC 4 Ω | Coils in series | 8 Ω |
| 1 × DVC 4 Ω | Coils in parallel | 2 Ω |
| 1 × DVC 2 Ω | Coils in series | 4 Ω |
| 2 × SVC 4 Ω | Subs in parallel | 2 Ω |
| 2 × DVC 4 Ω | Each sub series (8 Ω), then subs in parallel | 4 Ω |
| 2 × DVC 4 Ω | Each sub parallel (2 Ω), then subs in parallel | 1 Ω (only for 1-ohm-stable amps) |
| 2 × DVC 2 Ω | Each sub series (4 Ω), then subs in parallel | 2 Ω |
Check your amplifier's manual for the lowest impedance it is stable at (for example "2-ohm stable"). Never wire below that number. Running a sub at a higher impedance is safe but gives less output. Many monoblock amplifiers are rated at several loads, for example 250 W at 4 ohms and 500 W at 2 ohms; choose a wiring plan that lands on the load where the amp's power best matches your subs' combined RMS rating.
If you bridge a two-channel amplifier, the amplifier sees roughly half the load on each channel. A bridged amplifier connected to 4 ohms behaves like each channel driving 2 ohms, so most two-channel amps should be bridged only into 4 ohms or more. Check the manual.
Keep polarity consistent: positive to positive throughout. If one sub or coil is reversed, its cone moves inward while the other moves outward, and the bass cancels. A quick check is to briefly touch a 1.5 V battery to the sub's leads: the cone should move outward when battery positive touches the sub's positive terminal.
A multimeter's resistance (Ω) setting measures DC resistance, which is a little lower than the rated impedance: a 4-ohm coil typically reads around 3–3.6 ohms. Measuring at the amplifier end with the wiring connected (and the amplifier off) confirms you wired what you intended. A reading near zero means a short; an open (no reading) means a broken connection or coil.
Follow the enclosure volume the manufacturer recommends, and compare RMS (not peak) power between sub and amplifier. A sealed box gives tight, accurate bass in a smaller space; a ported box gives more output around its tuning frequency but is larger. Two subs in one box need roughly double the volume of one. Once the wiring is done, set the amplifier gain carefully; our guide on how to set car amplifier gain explains how to avoid clipping, the most common cause of blown subwoofers.
Subwoofers draw more current than door speakers, especially at low impedance. For most car installs, 12 or 14 AWG speaker wire between amplifier and subs is a good choice, with heavier wire for long runs or very high power. See our speaker wire gauge guide.
Subwoofers to consider: the 8" dual voice coil subwoofer, dual 2 ohm (wire to 4 ohms or 1 ohm), the 8" dual voice coil subwoofer, dual 4 ohm (wire to 8 ohms or 2 ohms), and the 12" single voice coil subwoofer, 4 ohm.
Related guides: Speaker wire gauge guide