Sonic booms occur when objects travel through the air at extremely high speeds, surpassing the speed at which sound waves propagate. These acoustic events result from an object’s interaction with the medium it moves through.
Defining the Speed of Sound (Mach 1)
A sonic boom is a consequence of exceeding Mach 1, the local speed of sound. This speed is not constant; it changes based primarily on air temperature and, to a lesser extent, on humidity and air pressure.
In colder temperatures, air molecules move more slowly, reducing the speed at which sound vibrations propagate. Warmer air allows sound to travel faster due to increased molecular kinetic energy. At sea level, under standard atmospheric conditions (around 15°C), the speed of sound is approximately 343 meters per second (1,225 kilometers per hour). As altitude increases, air temperature generally decreases, causing the speed of sound to drop. For instance, at 11,000 meters (36,089 feet), where temperatures can fall to -56.5°C, Mach 1 is reduced to about 295 meters per second.
How Sonic Booms Form
When an object, such as an aircraft, travels at subsonic speeds, it continuously creates pressure waves that spread out in all directions, similar to ripples from a stone dropped in water. As the object accelerates and approaches the speed of sound, these pressure waves begin to compress and pile up in front of it.
Once the object reaches or exceeds Mach 1, it outruns these pressure waves. The accumulated waves merge into a single, powerful disturbance called a shock wave. This shock wave forms a conical shape behind the object, known as a Mach cone. Within this Mach cone, air properties like pressure, density, and temperature undergo abrupt changes. The angle of the Mach cone becomes narrower as the object’s speed increases beyond Mach 1.
Characteristics of a Sonic Boom
The audible “boom” associated with this phenomenon is the result of the sudden pressure change caused by the shock wave reaching an observer’s ear. It sounds much like an explosion or a clap of thunder. A common misconception is that a sonic boom only occurs at the moment an object “breaks” the sound barrier; in reality, the boom is a continuous effect that trails the object as long as it maintains supersonic speed. The shock waves spread outward and downward from the aircraft, creating a “boom carpet” on the ground along the flight path.
Many sonic booms are perceived as a distinct double boom. This occurs because supersonic aircraft typically generate two primary shock waves: one from the nose and another from the tail. For smaller aircraft, these two booms often merge into a single perceived sound, but for larger vehicles like the Space Shuttle, the separation was noticeable.
The intensity of a sonic boom is influenced by the aircraft’s size, shape, altitude, and atmospheric conditions, with higher altitudes generally resulting in lower overpressure on the ground. A visual phenomenon, the vapor cone (sometimes called a Prandtl-Glauert singularity), can sometimes be seen around aircraft near the speed of sound. This visible cloud is condensed water vapor caused by rapid pressure drops and temperature changes in the air, not the sonic boom itself, but it often accompanies the conditions that lead to a boom.