Why Electric Vehicles Play an Artificial Sound at Low Speeds

A petrol engine announces itself. Pistons fire, valves open and close, the exhaust pushes gas through a pipe, each of those actions makes noise, and together they produce the familiar rumble that tells a pedestrian a car is nearby. An electric motor has none of those parts. It spins a shaft using magnetic fields, and at low speeds, that spin is nearly silent. The only sounds are tyre contact with the road and a faint whine from the motor, neither loud enough to alert someone stepping off a footpath without looking.

The Physics of the Silent Motor

An electric motor works by alternating the polarity of electromagnets around a central rotor. The rotor chases the changing field and spins. There is no combustion, no exhaust stroke, no valve clatter. The mechanical losses, friction, air resistance, are tiny compared to a petrol engine, which is exactly why EVs are efficient. But efficiency and audibility trade off directly. At speeds above roughly 30 kilometres per hour, tyre and wind noise fill the gap and an EV sounds much like any other vehicle. Below that threshold, on a quiet street or in a covered car park, the vehicle is genuinely close to inaudible.

Who Gets Hurt by the Silence

Pedestrians who are blind or have low vision rely on sound to track moving vehicles. They listen for approach, gauge speed from pitch change, and time a crossing accordingly. A silent vehicle removes that information entirely. Studies from road-safety bodies in multiple countries documented a pattern, pedestrians and cyclists were struck at low speeds in car parks and at junctions at higher rates by electric and hybrid vehicles than by petrol ones of similar size. The risk was concentrated precisely where speed was lowest and the motor was quietest. Children, older pedestrians, and anyone wearing earphones faced the same gap.

How the Acoustic Vehicle Alerting System Works

The engineering response is called an Acoustic Vehicle Alerting System, or AVAS. A speaker, usually mounted low on the vehicle's exterior, facing outward, plays a synthesised sound that the onboard computer links to vehicle behaviour. The sound rises in pitch as the car accelerates, mimicking the relationship between engine note and speed that pedestrians have learned to interpret over decades. It cuts off automatically once the vehicle crosses the speed threshold where road noise takes over. The system runs continuously when the car moves forward or in reverse below that threshold, it cannot be switched off by the driver. Some manufacturers tune the sound to match their brand identity. Others use a more neutral tone designed purely for detectability across a range of urban noise environments.

Why the Sound Has to Be Designed, Not Borrowed

Playing a recorded petrol engine through a speaker sounds wrong because the relationship between the sound and the car's motion is artificial, the recording does not change the way a real engine does. Designers instead synthesise a tone that behaves the way pedestrians expect, it gets louder as the vehicle approaches, changes pitch with speed, and fades as the car moves away. The goal is not to fool anyone into thinking the vehicle has a petrol engine. The goal is to give the ear enough information to locate the vehicle and judge its movement. Some manufacturers have used entirely new sounds, tones that have no precedent in motoring, on the basis that a distinctive sound is easier to pick out from urban background noise than one that resembles a distant engine.

What This Means for the Road Around You

The next time an electric vehicle makes a soft hum as it rolls past a market or through a housing colony gate, that sound is not a mechanical byproduct. Someone decided what frequency it should be, how fast its pitch should climb, and at what speed it should stop. The silence that made the engineering problem necessary is the same silence that makes the motor efficient. The sound is the compromise between a technology that runs quietly and a street that depends on noise to keep people safe.