91av

Why do heavy thunderstorms and fireworks set off car alarms?

• The alarm systems of modern cars have a variety of sensors, but the ones that are most likely to react to thunder or fireworks are shock and pressure detectors. A shock detector senses if something bumps your car in some way. A pressure sensor detects a change in air pressure inside the car if a door is opened or a window is knocked in.

The speakers for a car stereo can act as pressure detectors. A speaker normally works by vibrating, according to the input of electrical signals, to produce sound. When used as a detector, the opposite happens. A loud sound or change in air pressure moves the speaker, producing an electrical signal that can trigger the alarm.

Lightning, or an explosion from a firework, produces a shock wave that propagates radially outwards from its source. A shock wave is a compression wave, characterised by a sharp increase in air pressure followed by a sudden drop. If the initial detonation is energetic enough, the shock wave expands supersonically and is heard as a loud, truncated clap or click. When such a wave hits a solid body, some of its energy is absorbed, deforming that body, so it can easily set off a car alarm.

“Lightning or explosions produce a shock wave that propagates radially outwards from its source”

Shock waves can do more than set off car alarms – they can kill. Waves from high explosives can make a skull distort, causing concussion and brain damage. Many soldiers in the first world war who were close to shell impacts were killed, even though they showed little outward sign of damage. And the shock waves from roadside bombs can cause traumatic brain injury and subsequent mental health problems. This is a hidden legacy of the recent campaigns in Iraq and Afghanistan.

David Muir, Edinburgh, UK

• A comprehensive modern car alarm system usually has a wide range of sensors. These include devices that monitor electrical activity within the car’s own circuitry, acoustic and electromagnetic phenomena in the immediate vicinity, and perimeter radars and tilt-sensors. These sensors check for indicators that, individually or collectively, may suggest an attempt to steal, break into or damage the car. A central processing unit then cross-references the data to decide whether to activate the alarm.

The oldest types of sensors, essentially microphones (although some acoustic monitors now use the speakers on a car’s sound system), listen for the distinctive frequency of glass cracking, which might indicate a direct or clumsy method of entry to the car. Electrical sensors detect anomalous voltage drops, such as those caused if a light comes on because the doors have been opened even though they’ve not been unlocked correctly.

Perimeter radar detects approaching objects, and the central processor determines according to set criteria whether they pose a threat. Then there are electromagnetic sensors that monitor sudden variations in air pressure, such as if a window had been shattered or a door forced. Last, the tilt-sensor, usually a float suspended in a mercury-filled capsule, monitors the inclination of the car, in the same way the fluid in a human inner ear regulates our sense of balance. This alerts the car to attempts to tow it or load it onto a trailer.

So in a thunderstorm, it might be possible for fluctuations in atmospheric pressure to trip the sensors, and for lightning to fool the proximity radar or cause a power surge in the car’s circuitry. It is not obvious how fireworks or a crack of thunder might do this, however, and the likely reason is that the noise fools the acoustic pick-ups into thinking that the shock wave they are detecting is the sound of breaking glass.

“The crack of thunder might fool the car alarm’s sensor array into thinking that glass is breaking”

That said, it is by no means impossible for sophisticated car alarm sensors to be duped, albeit under bizarre circumstances. When the first entered service with the UK Royal Air Force, and began attending air displays, commentators would warn the public that, when the aircraft made a low pass, its electronic countermeasures system could set off nearby car alarms.

Hadrian Jeffs, Norwich, UK