Short answer: "8 ohms" is a nominal figure for a speaker's impedance, which actually varies with frequency. Wiring drivers in series adds their impedance (8 + 8 = 16 ohms). Wiring them in parallel divides it (two 8-ohm drivers = 4 ohms). Keep the total load within what your amp is rated for, wire positive to positive, and pick cable thick enough that its resistance is small next to the speaker's. Beyond that, expensive cable doesn't buy you anything you can hear.
What impedance actually is
Impedance is the speaker's resistance to the amplifier's AC signal, in ohms (Ω). Unlike a plain resistor, it changes with frequency:
- It peaks at the driver's resonance. In a ported box you get two peaks instead, with the port tuning at the dip between them.
- It rises at high frequencies (voice-coil inductance).
- In a multi-way speaker, the crossover adds its own peaks and dips.
Nominal impedance (4, 6, 8 ohms) is a rounded, representative figure. The minimum impedance is what really matters to the amp, and it can sit noticeably below the nominal figure.
Your multimeter reads Re, the voice coil's DC resistance. It's always lower than the nominal figure: a 4-ohm driver might read about 3.4 ohms, an 8-ohm one about 6 ohms. That's normal.
Why it matters to your amp
An amp delivers more power into a lower impedance, up to its limits. A typical rating might be "100 W into 8 ohms, 160 W into 4 ohms". Go below what it's rated for and it'll run hot, cut out, or fail.
- Check the amp's minimum rated impedance. Many home amps are happy at 4 ohms; some aren't. Many pro amps handle 2 ohms per channel.
- Bridged mode (two channels combined) usually needs a higher minimum load, often double the per-channel minimum. Check the manual.
Series and parallel
Series: positive of the amp to driver 1 +, driver 1 − to driver 2 +, driver 2 − back to the amp. Impedances add.
Parallel: both drivers connected straight across the amp terminals, + to + and − to −. Impedance divides.
| Drivers | Series | Parallel |
|---|---|---|
| 2 × 8 Ω | 16 Ω | 4 Ω |
| 2 × 4 Ω | 8 Ω | 2 Ω |
| 2 × 16 Ω | 32 Ω | 8 Ω |
| 4 × 8 Ω (series-parallel: two series pairs in parallel) | 8 Ω | |
| 4 × 4 Ω (series-parallel) | 4 Ω |
For two different impedances in parallel: (A × B) ÷ (A + B). An 8 Ω and a 4 Ω in parallel come to 32 ÷ 12 = 2.67 Ω.
What happens to output? At low frequencies, where two drivers sit close together: - Two in parallel on the same amp: each driver gets the full voltage, the amp delivers twice the power, and the two cones add together. You get about +6 dB over one driver (if the amp can supply the current into the lower load). That's why our Sealed 18SW500 build pairs two 8-ohm subs in parallel for a 4-ohm load. - Two in series: each gets half the voltage, so the total output ends up about the same as one driver on its own, but with more cone area sharing the work (less excursion each, more headroom before bottoming out).
Dual voice coil (DVC) subwoofers
A DVC sub has two separate voice coils on one cone, each with its own terminals. A "dual 4 ohm" (4+4) driver can be wired:
- Coils in series: 8 Ω.
- Coils in parallel: 2 Ω.
A "dual 2 ohm" (2+2) driver gives 4 Ω in series, 1 Ω in parallel. Always use both coils (don't leave one disconnected), and wire them the same polarity.
With two DVC subs you've got lots of combinations, which is the point: pick the one that lands on a load your amp likes.
Polarity: + to +
- Between speakers: both speakers in a stereo pair must be wired the same way. If one's reversed, the bass cancels and the stereo image goes vague.
- Quick test: briefly touch a 1.5 V AA battery to a woofer's terminals. If the cone moves out with battery + on the terminal marked +, the marking is right. (Woofers only. Never do this to a tweeter.)
- Within a speaker: follow the crossover schematic. Some designs deliberately connect the tweeter in reverse polarity. That's intentional, not a mistake.
How thick does speaker cable need to be?
The cable's resistance sits in series with the speaker. The goal is to keep it small compared with the speaker's impedance. A common guide is to keep the cable's round-trip resistance under about 5% of the speaker's nominal impedance.
Calculated from standard copper resistance figures (round trip = twice the run length):
| Cable | Resistance per metre (one conductor) | Max run for 8 Ω (5%) | Max run for 4 Ω (5%) |
|---|---|---|---|
| 18 AWG | about 0.021 Ω | about 9.5 m | about 4.8 m |
| 16 AWG | about 0.013 Ω | about 15 m | about 7.6 m |
| 15 AWG | about 0.010 Ω | about 19 m | about 9.6 m |
| 14 AWG | about 0.008 Ω | about 24 m | about 12 m |
| 13 AWG | about 0.0066 Ω | about 30 m | about 15 m |
| 12 AWG | about 0.0052 Ω | about 38 m | about 19 m |
For most living rooms, 15 or 16 AWG copper is plenty for hi-fi speakers. Go thicker for long runs, low-impedance loads and big subwoofer amps.
We started out as NB Speaker Cables, so we've heard every cable claim going. The no-bullsh*t version: gauge, a decent copper conductor and good terminations matter. Exotic geometry, "burn-in" and directional arrows don't. Spend the difference on drivers.
From our range: 15 AWG OFC cable, 13 AWG OFC cable, and for running two pairs (or a sub plus a top) down one cable, 14 AWG 4-core.
Connectors
- Bare wire: fine, if it's twisted tight and clamped well. Retrim it if it oxidises.
- Banana plugs: tidy and easy to swap. 4 mm banana plugs fit most binding posts.
- Spade terminals: a secure grip under a binding post nut.
- Speakon: the standard on pro amps. Locking, can't be shorted by accident, and a 4-pole one can carry two channels. A Speakon panel socket on a sub box makes it plug-and-play with a pro amp.
For terminals on the speaker itself, see our binding posts.
Quick questions
Can I run 4-ohm speakers on an amp rated for 8 ohms? Some amps cope, many will run hot or shut down at high volume. Check the manual. If it doesn't list 4 ohms, be careful.
Can I mix 4-ohm and 8-ohm speakers on one amp? On separate channels, yes. On the same channel in parallel, the 4-ohm one takes twice the power of the 8-ohm one, so they won't play at the same level.
Is thicker speaker cable always better? Up to the point where its resistance is negligible, yes. Past that, you're paying for copper you can't hear.
Why does my 8-ohm speaker measure 6 ohms? That's the DC resistance (Re). It's normal for it to be below the nominal figure.