Testing and fault-finding · Guide

7 loudspeaker circuit testing tips that turn one impedance reading into a diagnosis

British Standard BS 5839-8:2023 now recommends measuring every loudspeaker circuit every year and comparing it with the last. Here is how to make that one reading tell you something.

By Nigel Williams · Published 8 November 2026

Reviewed on 16 October 2026

A loudspeaker circuit test is only as useful as the number you expected before you made it. An impedance reading on its own is a figure on a screen. Set beside the load the circuit should carry, and beside last year's reading, it tells you whether the circuit is healthy, shorted, broken or quietly losing loudspeakers.

Since the 2023 edition, the code of practice for voice alarm systems recommends measuring the impedance of every loudspeaker circuit once a year and comparing it with the previous year's result (BS 5839-8:2023, Clause 40.2 i). These seven tips are the method behind that one number.

Tip 1: work out what the meter should read before you connect it

Add up the power taps of every loudspeaker on the circuit. The installer records the tap settings (Clause 31.5), so the figures should be in the handover documents.

Two examples on a 100 V line:

Then turn the expected load into the impedance you expect to see: impedance = voltage² ÷ power.

• 50 W on 100 V: 10 000 ÷ 50 = 200 ohms.
• 170 W on 100 V: 10 000 ÷ 170 = 58.8 ohms.

Treat this as an estimate. Transformers, cable and the loudspeakers themselves all move the real figure, which is why the reading taken at installation, not the sum, becomes the reference (Tip 7).

Tip 2: disconnect the circuit, then measure at 1 kHz

Disconnect the loudspeaker circuit from the amplifier first. A connected amplifier, powered or not, changes the reading, and a powered one can damage your meter.

Connect the impedance meter at the start of the circuit. Polarity does not matter, but test every circuit with the leads the same way round. If your meter asks for a frequency, use 1 kHz: that is the frequency at which the installer measures each circuit once all the loudspeakers are connected (Clause 33.4). Change the meter battery if it is low, because a weak battery gives readings you cannot trust.

Tip 3: turn ohms into watts

Amplifiers and loudspeakers are rated in watts, so convert the reading:

Power (W) = Voltage² ÷ Impedance (ohms)

A reading of 25 ohms on a 100 V circuit is 10 000 ÷ 25 = 400 W. If that circuit is the 25-loudspeaker example from Tip 1, the meter is seeing eight times the load those loudspeakers can draw.

The conversion also guards against a trap that catches experienced technicians: ohms go down as loudspeakers are added, while watts go up. A reading of 75 ohms is a load of 133.3 W, not 75 W. Thinking in watts keeps the reading pointing the same way as the design.

Tip 4: read the gap between expected and measured

If your sum and the meter disagree by much, and you trust both, the direction of the gap points to the fault:

• Measured power too high: look for a short circuit, an extra load, or a loudspeaker tapped higher than recorded.
• Measured power too low: look for a break, or loudspeakers that are no longer connected.

If the meter's test tone is audible through the loudspeakers, leave it connected and walk the circuit. Where the tone stops is usually where the problem starts.

Four causes account for many confusing readings:

1. A loudspeaker tapped for a different line voltage from the circuit's.
2. A loudspeaker with no transformer. It is a low-impedance load and drags the whole reading down.
3. An amplifier still connected to the circuit.
4. On a circuit monitored with end-of-line resistors, a missing resistor or one too many.

Before chasing any of them, check you are on the right circuit and that you squared the voltage before dividing.

Tip 5: use the multimeter for the test it can do

A multimeter tells you little about a loudspeaker circuit's load. Every 100 V loudspeaker has a transformer, and the meter's DC test current passes straight through it, so healthy and faulty circuits can read almost the same.

What the multimeter does well is the check between each conductor and earth. Touch one probe on metalwork you know is earthed and the other on each leg in turn. A healthy leg reads open circuit.

Do not accept tens of kilohms as good enough. The code's own note gives 1 MΩ as an example setting for the voice alarm equipment's earth-fault detection (Clause 33.1, Note), and at that setting a leg reading 50 000 ohms is well inside fault territory. A reading that keeps changing usually means water: check junction boxes and anything outdoors. A reading near zero usually means a conductor has worked loose and is touching metal, which ceiling loudspeakers are prone to.

The control equipment is expected to report an open circuit, short circuit or earth fault on any loudspeaker circuit within 100 s (Clause 10.1.1 a). The multimeter tells you which leg; it does not replace the fault monitoring.

Tip 6: on DC-blocked circuits, count the resistors

Some loudspeakers have a capacitor in series with the transformer, which blocks DC. On those circuits the multimeter sees only the end-of-line resistors, and the reading tells you how many are fitted.

Say the manufacturer specifies 10 000 ohm resistors and the circuit reads 2500 ohms at the rack. Resistors in parallel divide: 10 000 ÷ 2500 = 4. That suggests a main run and three spurs.

It is a suggestion, not a proof. It does not tell you whether a spur has come adrift, whether a spur has no resistor, whether two resistors share one spur, or whether the resistors are really at the ends. Their locations should be on the as-fitted drawings (Clause 31.6).

Split the circuit halfway and measure both ways. Reading 3333 ohms towards the amplifier and 10 000 ohms towards the end means three resistors on one side and one on the other. Keep splitting and you close in on the fault much faster than by walking every ceiling tile.

On a DC-blocked circuit, a short circuit on the multimeter is always a fault. Without the capacitors, every healthy circuit reads as a near short, so the multimeter cannot help.

Tip 7: keep the number, and compare it next year

Fault monitoring watches the cable, not each loudspeaker. Take the 25-loudspeaker circuit from Tip 1 and lose one loudspeaker: the load falls from 50 W to 48 W and the impedance rises from 200 ohms to 208.3 ohms, a change of about 4 %. Depending on the monitoring method, that can go unreported between visits.

That is what the annual comparison is for. Record every circuit's impedance and watts, with the meter and the date. The readings taken at installation (Clause 33.4), passed on to the commissioning organisation (Clause 33.5), are the first baseline. After that, each year's reading is compared with the last, ideally on the same calibrated meter, and any deviation is investigated (Clause 40.2 i).

The same reading answers a design question too. The amplifier should be rated at least 20 % above the design loudspeaker load (Clause 15.3). The 170 W circuit in Tip 1 needs at least 204 W of amplifier behind it; a measured load creeping up towards that figure is worth knowing about before the next fit-out adds to it.

In short

• Work out the expected load and impedance before you connect the meter.
• Disconnect the amplifier and measure at 1 kHz.
• Convert ohms to watts, then read the direction of the gap.
• Use the multimeter for the earth check, and on DC-blocked circuits for counting resistors.
• Record every reading and compare it with last year's (Clause 40.2 i).

The reading is the same number it always was. What turns it into a diagnosis is the number you wrote down before it, and the one you wrote down a year ago.

Next step. Find last year's maintenance report and look for a table of loudspeaker circuit impedances. If there is no such table, there is nothing for this year's readings to be compared with. The Maintenance Navigator shows what a visit covers for your system and the standards that apply to it, item by item, and the yearly comparison of every loudspeaker circuit's impedance with its recorded baseline is one of those items.