How to Test a Speaker With a Multimeter

To test a speaker with a multimeter, measure DC resistance across the driver's terminals with it disconnected. A healthy 8 Ω driver reads below 8 Ω, typically around 5 to 7 Ω: B&C's 18SW115 8 Ω, for example, is specified at Re = 5.3 Ω. Open circuit or near zero means a failed coil or lead.

Setup

• Amp off and disconnected. Never measure a driver connected to a powered amplifier.
• Driver disconnected from the cab socket and any crossover, so you measure the driver alone.
• A digital meter on its lowest resistance range (often 200 Ω on a manual-range meter).

Method

1. Short the probes together and note the reading. That is lead resistance, usually a fraction of an ohm. Subtract it.
2. Hold one probe firmly on each terminal.
3. Read the value. A slight cone movement or click as the meter's test current flows is normal.

Re vs nominal impedance

The meter reads Re, the DC resistance of the coil wire. Nominal impedance (4 Ω, 8 Ω) describes the load the driver presents with music. Real impedance varies with frequency: a peak at resonance, a rise at high frequency from coil inductance, and a minimum just above resonance. The rating sits near that minimum, and Re is always below it.

Your reference is the Re figure on the driver's own datasheet, or an undamaged twin from the same stack measured with the same meter. As a rough guide:

Re rises with temperature, so measure a driver that has cooled. Background in impedance and wiring.

Reading the result

Open circuit

"OL", a "1" at the left of the display, or infinity: the circuit is broken. Burnt-through coil, snapped lead-out wire or fatigued tinsel lead. Check the terminals and lead-outs before condemning the driver: a broken lead-out can sometimes be repaired; a burnt coil means a recone or a new driver.

Low or near zero

Heat has melted the insulation and turns are touching. A partly shorted coil may still play, weak and distorted, and presents a lower load to the amp.

Correct reading, still faulty

The meter proves only that the coil conducts. A rubbing coil, torn cone or damaged spider reads normal. Follow up with the push test and low-level sweep in is my speaker blown.

Reading jumps

Unstable readings while you flex the leads or press the cone point to a broken tinsel lead or poor joint, often heard as a driver cutting in and out at level.

Checking a whole cab

Measure at the input socket to check the wiring. Two identical drivers in parallel read half one driver's Re; in series, double. Two 18SW115 8 Ω in parallel would read about 2.65 Ω. Anything else points to a wiring fault or a failed driver. Pin conventions are in Speakon wiring.

Polarity with a battery

A 1.5 V cell briefly across the terminals, + to +, pushes the cone outward on a correctly marked driver. Keep the contact brief. On large subs the movement is small, and terminal markings or a polarity tester are easier. See phase and polarity.

Replacing a failed 18

If the coil is gone and a recone is not worth it, the WM1850/8 (£275, 1800 W AES, 125 mm coil, full T/S published) and the WM18X451 (£345, 1800 W RMS, 115 mm copper coil, for scoops and horns) are the Wavemark options. Check fit first: will a new driver fit my cabinet.

Sources: B&C Speakers published data for the 18SW115 8 Ω and 4 Ω versions, checked October 2026. Typical Re ranges are a guide only; the driver's own datasheet is the reference.