A sonic boom is a loud sound resembling thunder or an explosion. It occurs when an object travels through the air faster than the speed of sound, creating shock waves. While often perceived as a singular event, the science behind sonic booms reveals a more complex reality. The experience of a sonic boom is tied directly to how these pressure disturbances interact with an observer.
The Science of Sonic Booms
An object moving through the air generates pressure waves, similar to how a boat creates waves in water. When an aircraft accelerates to supersonic speeds, it outruns these pressure waves. The waves cannot move out of the way quickly enough and instead pile up, merging into distinct shock waves. These shock waves form a cone of pressurized air that trails behind the supersonic object.
This pressure profile is described as an “N-wave” due to its shape, characterized by a sudden rise in pressure, a gradual decrease to negative pressure, and then a rapid return to normal atmospheric pressure. The N-wave continuously radiates outward from the aircraft’s flight path. It creates a “boom carpet” on the ground, indicating the area where the shock wave impacts. The intensity and width of this carpet depend on factors like the aircraft’s size, speed, and altitude.
Why a Sonic Boom Seems Singular
Despite the continuous generation of shock waves by a supersonic aircraft, an observer on the ground typically hears only a single, brief “boom”. This occurs because the sound is not generated only at the moment the aircraft crosses the sound barrier. Instead, the boom is a continuous effect that travels with the aircraft along its supersonic flight path. A listener experiences the sound as the cone-shaped shock wave passes directly over their location.
The “boom” is the momentary perception of the rapid pressure change as the N-wave sweeps across a specific point. The duration of this perceived boom is very short. This brief, intense pressure fluctuation makes it feel like a distinct, isolated event to an observer at a fixed position. While the phenomenon is ongoing, the auditory experience for any single person is localized and temporary.
Factors Leading to Multiple Perceived Booms
While a single supersonic aircraft often produces one perceived boom, several factors can lead an observer to hear multiple distinct sounds. The N-wave itself, with its leading and trailing shock waves, can sometimes be perceived as two separate “thumps” or a “double boom.” These two pressure changes, one at the nose and one at the tail, can be distinct enough for the human ear to differentiate.
Sonic booms can also reflect off surfaces, causing additional perceived booms. Terrain features such as hills, valleys, or large buildings can reflect the shock waves, creating delayed echoes. Atmospheric layers can also act as reflective surfaces, bending the sound waves and causing them to reach the ground as multiple or delayed events.
Finally, the most straightforward reason for multiple perceived booms is the presence of multiple supersonic aircraft. Each aircraft flying faster than sound will generate its own set of shock waves, resulting in a distinct boom for each one as they pass overhead. Aircraft maneuvers, such as turns or accelerations, can also focus or amplify the shock waves, potentially leading to a stronger or more complex perceived sound signature.