Nothing draws complaints faster than a public-address system that crackles during an evacuation drill. For buildings equipped with Voice-Evacuation and Emergency Warning & Intercommunication Systems (EWIS), speech must remain intelligible even when the mains are loaded, lifts are running and fluorescent ballasts are buzzing. Choosing the right speaker cable is one of the simplest ways to stop that background hum before it ever reaches an amplifier channel.
Two common cable families
Most Australian installers reach first for flat twin TPS (thermoplastic-sheath) cable because it sits on every wholesaler’s shelf. It is inexpensive, easy to strip and—when ordered in the fire-resistant red jacket with a white stripe—meets the visibility and low-smoke rules in AS 1670 for speaker circuits classified as low-voltage (LV).
Twisted pair speaker cable looks more like data wiring. The conductors spiral along the run, locking them together. Variants rated for 100 V line work now arrive in the same red PVC jackets as TPS, so they drop neatly alongside the rest of the EWIS loom.
Why twisting matters
Every alternating current produces a magnetic field. When two conductors carry equal and opposite currents—and sit right beside each other—the fields largely cancel. Twisting tightens that coupling, shrinking the loop area and pushing unwanted induction down by as much as 30 dB. Audio University offers a clear plain-language walk-through of the physics, complete with audible examples.
TOA engineers frame it in practical terms: twist the pair correctly and any external “noise” lands equally on both wires, allowing the amplifier’s differential input to push it into the background. Miss the colour coding or untwist too much at terminations and the benefit disappears.
Hum in the real world
Fifty-hertz hum rarely comes from the loudspeaker itself—more often it rides in on the cable after running parallel with power for a few dozen metres. Installers swapping Flat red TPS cables for twisted pair in interference-prone classrooms have reported the buzz vanishing instantly at the same gain setting. Klipsch forum contributors note that even unbalanced speaker lines can pick up mains hash when routed near switch-mode supplies.
Large sites exaggerate the effect. A shopping-centre Voice-Evac loop can exceed 300 metres. West Penn’s cable guide charts how resistance and inductance accumulate over long runs, shaving amplifier headroom and exaggerating any induced artefacts.
Cost, labour and inspection
TPS remains cheaper per metre and pulls smoothly through conduit, especially on straight drops. Terminations are quick because the jacket peels cleanly and the inner cores sit flat under screw terminals. These points matter on budget-sensitive retrofits.
Twisted TPS cable asks for a little more care: keep the lay intact right up to the ferrule, and remember that the cable has a memory—it wants to coil. Yet installation time tends to balance out once fault-finding is included. A hum-free loop that sails through the final commissioning saves return visits.
Compliance considerations
AS 1670 does not force designers to choose one type over the other; it speaks in functional terms—fire performance, monitoring, segregation from extra-low-voltage data lines and clear identification. The white stripe on LV speaker circuits is mandatory, and both TPS and twisted pair are supplied with it. What the Standard leaves open is mitigation of electromagnetic interference. On sites where speaker lines must share a ladder tray with 415 V feeders, twisted pair is the simplest path to compliance with intelligibility targets in AS 60849 and ISO 7240-24.
When TPS is still fine
Flat red with white stripe TPS cables is adequate where cable trays remain well away from mains, run lengths are short, and budget is under pressure. Residential open-plan speakers on 100 V lines, for instance, rarely justify the premium for twisting. Keep TPS perpendicular to any unavoidable power crossing, observe the 50 mm separation rule, and the line should stay quiet.
Choosing wisely
A speaker line lives or dies on intelligibility, not on catalogue price. Twisted pair suppresses induction at the source, easing the load on balanced amplifiers and allowing headroom for voice messages that must be heard. TPS delivers convenience and cost savings when interference is unlikely. Weigh the environment, route, and compliance matrix: in electrically busy plant rooms, a twist in the copper can be the difference between a calm evacuation announcement and a garbled din.
Frequently Asked Questions
1. Why does twisting speaker cable reduce hum in Voice-Evac systems?
The spiral keeps the conductors pressed together, so their magnetic fields cancel. Any interference, such as 50-hertz mains, lands evenly on both wires and is rejected by the balanced amplifier input. Less induced noise equals clearer, intelligible evacuation messages.
2. Is conventional flat TPS acceptable for EWIS speaker lines in Australian buildings?
Provided the run is short, kept clear of power, and marked with the mandatory red and white jacket. It remains cost-effective and easy to terminate, though you may sacrifice some interference immunity compared with properly twisted conductors.
3. How do I choose between twisted pair and TPS for a multi-purpose building?
Map speaker runs alongside nearby mains or data cabling. Where exposure to switch-mode supplies, drives, or extended parallel paths exceeds twenty metres, the price premium for twisted pair usually buys assured intelligibility and fewer future call-backs.
