4 Ohm vs 8 Ohm Sub Drivers
In the 4 ohm vs 8 ohm choice, neither impedance is better. A 4 Ω load draws twice the power from the same amplifier voltage; an 8 Ω driver lets you parallel twice as many cabs before the load hits the amp's minimum. Decide by counting cabs per channel, not by the number on the magnet.
The maths
An amplifier behaves close to a voltage source, so P = V² / R. To put 1800 W into an 8 Ω driver takes √(1800 × 8) = 120 V RMS. The same 120 V into 4 Ω is 3600 W, at 30 A instead of 15 A. Halving the load doubles the demand on the amp's current and power supply, which is why real amps rarely deliver a full doubling: use the maker's published figure for each load.
Nominal impedance is a rating near the minimum of the impedance curve in the working band; the real load varies with frequency and peaks around the box tuning. See impedance and wiring.
Parallel and series
- Parallel (Speakon link-through): 1/R = 1/R₁ + 1/R₂ … Identical cabs divide: n × 8 Ω cabs give 8/n Ω.
- Series: loads add. Two 8 Ω drivers give 16 Ω.
| Cabs per channel (parallel) | 8 Ω cabs | 4 Ω cabs |
|---|---|---|
| 1 | 8 Ω | 4 Ω |
| 2 | 4 Ω | 2 Ω |
| 3 | 2.67 Ω | 1.33 Ω |
| 4 | 2 Ω | 1 Ω |
Below the amp's rated minimum load it will current-limit, run hot, go into protection or fail. Check the spec sheet before you add a box. Two 8 Ω cabs per channel (4 Ω) is within the rating of most professional amps; four (2 Ω) needs an amp rated for 2 Ω.
Cable loss
Cable resistance sits in series with the load, so the lower the load, the bigger the share it takes. A 20 m run of 2.5 mm² copper is 40 m of conductor, about 0.28 Ω:
- Into 8 Ω: 20·log₁₀(8 / 8.28) = −0.3 dB
- Into 4 Ω: −0.6 dB
- Into 2 Ω: −1.1 dB, plus reduced damping of the driver
Long runs favour higher load impedance or heavier cable.
Choosing
- 8 Ω when you run two cabs per channel at 4 Ω, build double cabs from two drivers in parallel (two 8 Ω make 4 Ω), expect to add cabs later, or run long cables.
- 4 Ω when each cab gets its own channel and the amp makes its best power into 4 Ω, or when two 4 Ω drivers go in series inside one cab to make 8 Ω.
- Never mix impedances on one channel without working out the combined load: the lower-impedance driver takes the larger share of power.
Bridging
Bridging drives the load from two channels in opposite polarity, doubling the voltage swing. Each channel then sees half the load, so a bridged minimum is typically double the per-channel minimum: a 2 Ω-stable amp is usually 4 Ω minimum bridged. Confirm in the manual. See amp matching.
Wavemark options
The WM1850/8 is 8 Ω: 1800 W AES needs 120 V RMS, and two on one channel present 4 Ω. The WM18X451 is 8 Ω with a 4 Ω option, so it can be matched to one-cab-per-channel or paralleled rigs. Both are rated 1800 W continuous; at £275 and £345 that is 15.3p and 19.2p per rated watt.
Pin layouts: Speakon wiring.
Sources: Ohm's law and P = V²/R; copper resistivity 1.72 × 10⁻⁸ Ω·m at 20 °C. Wavemark WM1850/8 spec sheet and WM18X451 product data. Calculated values rounded.
Questions
Is a 4 ohm speaker louder than an 8 ohm speaker?
From the same amplifier voltage, a 4 ohm load draws twice the power, which is up to 3 dB more if the amp can supply the current and the driver's sensitivity is the same. In practice amps rarely deliver a full doubling, so check the amp's published 4 ohm figure.
Can I use 8 ohm speakers on an amp rated for 4 ohms?
Yes. Any load at or above the amp's minimum is safe. The amp will deliver less power into 8 ohms than into 4.
How many 8 ohm subs can I run on one amp channel?
Two in parallel make 4 ohms, within most professional amps' ratings. Three make 2.67 ohms and four make 2 ohms, which need an amp rated for 2 ohm loads.
How much voltage does an 1800 W 8 ohm sub need?
From P = V²/R, 1800 W into 8 ohms is 120 V RMS, at 15 A. The same voltage into 4 ohms would be 3600 W at 30 A.