A sonic boom sounds like an explosion, but no explosion happens. When an aircraft flies faster than the speed of sound, it pushes air aside faster than sound waves can move away. Those sound waves pile up into a cone-shaped shock wave that trails behind the aircraft. When that shock wave reaches the ground, the sudden pressure change hits your ears as a loud boom, sometimes described as a double crack.

The double sound has a clear cause. An aircraft creates shock waves at its nose and its tail, and at supersonic speed those waves travel together to the ground. The first crack is the pressure rising sharply as the nose shock passes, and the second is the pressure dropping back as the tail shock passes. Depending on the aircraft size, altitude, and your distance, the two cracks may merge into one boom or arrive as a distinct boom-boom.

Speed of sound is not fixed. It changes with temperature and altitude, roughly 1,225 kilometres per hour at sea level but slower in the cold air high above the ground. That is why pilots talk about Mach number, which is speed relative to the local speed of sound, rather than a single speed in kilometres per hour. An aircraft at Mach 1 is travelling at the speed of sound for its altitude, and above Mach 1 it is supersonic.

A common misconception is that a sonic boom happens only when an aircraft breaks the sound barrier. In reality, a supersonic aircraft produces a continuous boom along its entire supersonic flight path, like a boat producing a wake as long as it moves. People below hear it only when the shock cone sweeps past them, which is why the boom seems sudden even though the aircraft has been supersonic for minutes.

This continuous boom is the main reason supersonic flight is restricted over land in many countries. A loud boom can startle people, rattle windows, and disturb wildlife, even though it does not normally cause structural damage at normal cruising altitudes. Modern research into quiet supersonic flight focuses on shaping the aircraft so the shock waves are gentler and spread out, turning a sharp crack into a softer thump.

So a sonic boom is best understood as sound piling up and arriving all at once. The aircraft outruns its own noise, the pressure waves merge into a shock cone, and anyone under that cone hears the result as a boom. It is a striking reminder that sound, like water, can form waves that behave very differently when something moves through them faster than they can spread.

At normal supersonic cruising altitudes, a sonic boom is startling but rarely damaging. The pressure change is brief, and most buildings easily absorb it, much as they absorb a sudden gust of wind or a nearby thunderclap. Damage becomes a concern only at very low altitude, close to the aircraft, or with unusually large aircraft, which is why military supersonic training is kept away from populated areas and why civil supersonic flight over land remains restricted.

That restriction is also why new supersonic projects invest so much in shaping. By carefully designing the nose, wings, and tail, engineers can spread the shock waves over time, so the pressure rises and falls more gradually. The result is a softer sound at ground level, sometimes described as a distant thump rather than a crack. If that approach proves reliable, it could open the door to faster civil flights without the disruptive boom that grounded earlier ambitions.