A sonic boom is the sound produced when an object, typically an aircraft, travels through the atmosphere faster than the speed of sound. It is a physical phenomenon that releases a tremendous amount of sound energy, often described by observers on the ground as an explosive clap or the distant rumble of thunder. This dramatic noise is the audible manifestation of shock waves continuously generated by the object during supersonic flight. These shock waves are pressure disturbances that are normally imperceptible until they rapidly pass over an observer.
Understanding Mach 1 and the Sound Barrier
The speed of sound, known as Mach 1, is the baseline that determines whether a sonic boom will occur. This speed is not a fixed number, but varies significantly depending almost entirely on the temperature of the air. The speed of sound decreases as an aircraft gains altitude because the temperature of the atmosphere generally decreases. While pressure and density play a role, temperature remains the primary variable affecting Mach 1.
An object moving near Mach 1 begins to outrun the pressure waves it constantly creates. These waves cannot propagate away fast enough and begin to compress or “pile up” ahead of the airframe. This creates a zone of highly compressed air, historically referred to as the “sound barrier,” which an aircraft must overcome to achieve supersonic flight. Once the object’s speed exceeds Mach 1, it leaves all its newly created sound waves behind it.
The Physics of Shockwave Formation
When an object travels faster than sound, the continuous pressure waves it generates cannot move forward and instead coalesce into powerful shock waves. These shock waves originate at every point on the airframe where the air flow is abruptly disturbed, such as the nose, canopy, wings, and tail. The waves combine and propagate away from the aircraft in a distinct cone shape, known as the Mach cone.
The angle of this cone is directly related to the aircraft’s speed; the faster the object flies, the narrower and more pointed the cone becomes. The boom is generated by the rapid compression and decompression of air that occurs within this structure. This system of shock waves continuously trails the aircraft throughout the entire duration of supersonic flight.
The pressure waves that form the Mach cone spread out and eventually reach the ground, affecting a continuous area beneath the flight path. This area on the ground where the boom is heard is often called the sonic boom “carpet.” The intensity and exact shape of the shock wave are influenced by the aircraft’s size, weight, altitude, and atmospheric conditions.
What the Sonic Boom Sounds Like on the Ground
The sound of the sonic boom results from a specific pressure change known as the N-wave signature. When graphed, the air pressure profile resembles the letter ‘N’ due to an instantaneous rise in pressure, followed by a gradual decrease to negative pressure, and then a sudden return to ambient atmospheric pressure. The “boom” sensation is caused by the ear responding to these two sudden and rapid pressure changes.
This N-wave structure means the boom is heard as two distinct sounds, or a “double boom,” corresponding to the shock wave created at the front (the bow wave) and the one created at the tail (the tail wave). These two impulses are often so close together in time that the human ear perceives them as one singular, loud event, similar to an explosion. The sound is heard only briefly as the shock wave cone sweeps over the observer. For a supersonic aircraft flying at normal operating conditions, the overpressure associated with an N-wave boom is very small, often less than 10 pounds per square foot.
Modern Context and Sources of Sonic Booms
While military jets are the most common source, any object that exceeds the speed of sound can generate a shock wave, including a bullwhip, a rifle bullet, or a spacecraft re-entering the atmosphere. However, large, sustained sonic booms are rarely heard by the general public over land because of civil aviation regulations. In the United States, civil aircraft are prohibited from operating at speeds greater than Mach 1 over land.
These regulations, put in place in 1973, were designed to prevent the noise and potential property damage associated with high-intensity sonic booms. Due to these restrictions, commercial supersonic flight paths are limited to high altitudes over water or sparsely populated areas. New research focuses on shaping the airframe to produce a quieter “thump” instead of a disruptive boom, potentially allowing for future changes to these overland flight rules.