A sonic boom happens when an object travels faster than the speed of sound, creating a shock wave that reaches the ground as a sudden, explosive noise. It’s not a one-time event that occurs only when the object “breaks” the sound barrier. The boom is continuous, following the aircraft the entire time it flies at supersonic speed, like a wake trailing behind a boat.
How Sound Waves Pile Up
Sound travels as pressure waves that spread outward in all directions from a moving source. When a plane flies slower than sound, these waves radiate ahead of it and dissipate normally. But as the plane approaches the speed of sound (about 767 mph at sea level), it begins catching up to the pressure waves it already created. The waves can no longer outrun the plane, so they stack together, compressing into a single, powerful wall of pressure.
Once the plane crosses Mach 1, it’s outrunning its own sound. All those pressure waves merge behind the aircraft into a cone-shaped shock wave. The process is fundamentally nonlinear: as compressions pile up, they steepen dramatically, transforming gradual pressure changes into nearly instantaneous jumps. This is the shock wave, and where it intersects the ground, you hear it as a boom.
The Shape of the Shock Wave
The shock wave doesn’t radiate in a sphere. It forms a cone that trails behind the aircraft, much like the V-shaped wake behind a speedboat. The angle of this cone depends directly on how fast the aircraft is moving. The relationship is simple: the sine of the cone’s half-angle equals one divided by the Mach number. At Mach 1, the cone is nearly flat, spread at 90 degrees. At Mach 2, it narrows to about 30 degrees. The faster the aircraft, the tighter the cone.
As this cone sweeps across the ground, it traces a long strip called the “boom carpet.” Everyone within that strip hears the boom as the cone passes over them. According to the U.S. Air Force, the width of this carpet is roughly one mile for every 1,000 feet of altitude. An aircraft cruising supersonic at 30,000 feet creates a boom carpet about 30 miles wide.
What You Actually Hear
Most people describe a sonic boom as a sharp double bang, like two gunshots close together. That’s because a supersonic aircraft creates two main shock waves: one at the nose and one at the tail. These two pressure spikes, separated by a brief drop in pressure between them, produce what scientists call an N-wave, named for its shape on a pressure graph. The first bang is the leading shock from the nose, and the second is the trailing shock from the tail. On a small, fast-moving object like a bullet, these merge into a single crack. On a large aircraft, the gap between them is more noticeable.
The intensity of the boom depends on several factors: the aircraft’s size, speed, altitude, and flight path. A typical supersonic bomber or supersonic transport at 60,000 feet and Mach 2 produces an overpressure of about 100 newtons per square meter along the center of the boom carpet. That’s a firm thump, clearly startling but well below levels that cause structural damage. Controlled testing by NASA and the EPA found that standard residential window glass withstands repeated sonic booms at normal overpressures without any observable damage. Glass shattering requires overpressures above roughly 1,000 newtons per square meter, about ten times what a high-altitude supersonic cruiser generates.
Why Altitude and Weather Matter
A sonic boom doesn’t travel in a straight line from the aircraft to your ears. The atmosphere bends it. Air temperature decreases with altitude under standard conditions, and since the speed of sound depends on temperature, sound waves naturally curve upward as they descend through warmer, lower air. This refraction effect means the boom can bend away from the ground in certain conditions, or focus more intensely in others.
Higher altitude spreads the boom over a wider area but also weakens it, since the shock wave has more distance to dissipate before reaching the ground. Lower altitude concentrates the energy into a narrower carpet and produces a louder boom. Atmospheric turbulence also affects the experience. It can distort the clean N-wave shape, creating irregular spikes and ripples in the pressure signature. On a turbulent day, a boom might sound rougher or more rumbling than the crisp double-bang heard in calm conditions. The rise time of the pressure wave (how quickly the shock hits) can stretch to 40 milliseconds or more in weaker signatures, softening the perceived sharpness of the boom.
Supersonic Flight Over Land Is Banned
The disruptive nature of sonic booms led to strict regulations. The FAA currently prohibits all civil aircraft from operating above Mach 1 over land in the United States. This ban has been in place since the early 1970s, largely in response to public complaints during supersonic test flights over cities in the 1960s, when thousands of noise complaints and minor property damage claims piled up. The Concorde, which flew commercially from 1976 to 2003, was restricted to supersonic speeds only over open ocean.
Companies developing new supersonic aircraft can apply for a special flight authorization under federal regulations to test above Mach 1 over land, but these are granted on a case-by-case basis. Much of the current engineering effort in supersonic aviation focuses on “low-boom” designs that reshape the aircraft to produce a softer, more spread-out pressure wave instead of the sharp N-wave. The goal is to reduce the boom to something closer to a distant thump or car door closing, quiet enough that regulators might eventually allow supersonic commercial flights over populated areas.