As residents of Chicago learned last week, sonic booms can cause damage such as broken windows, minor structural damage, and psychological unsettlement.
Last week, the US Air Force’s Thunderbirds demonstration team, flying the supersonic General Dynamics F-16 Fighting Falcon, made news in Chicago for allegedly causing a sonic boom that broke windows across the city.
While a sonic boom can be awe-inspiring—and indeed can have a significant psychological effect against enemy combatants—creating one is generally discouraged in domestic airspace, and particularly near one of the most densely populated cities on the continent. In spite of technological advances in air travel, the loud noise associated with supersonic aircraft has largely kept them out of regular use around the world.
Understanding the Physics Behind Sonic Booms
A sonic boom is a rather dramatic physical effect created when a fighter jet exceeds the speed of sound. The fighter jet’s engine does not cause the sonic boom; rather, the boom is a phenomenon of aerodynamics and shockwaves.
Sound is pressure waves moving through the air. At sea level, and under standard conditions, sound travels at about 767 miles per hour, a speed known as Mach 1. When a fighter jet moves below the speed of sound, the pressure disturbances it creates from the nose, wings, and fuselage moving through the air cause wake ripples that move faster than the aircraft itself is moving, which causes air molecules to disperse. However, as the aircraft moves faster and faster, approaching Mach 1, those pressure disturbances will stack upon one another, because the aircraft is moving as fast as the sound waves being created. This creates a buildup of compressed air that cannot escape the aircraft. As a plane approaches the sound barrier, its traditional aerodynamics—based on Bernoulli’s Principle, the physics principle that gives airplane wings lift—cease to apply.
Once the aircraft crosses into supersonic speed, the stacked pressure waves finally coalesce into shock waves, which form a cone (known as the Mach cone) that trails the aircraft. At the edges of the cone exist abrupt changes in air pressure, temperature, and density. As the cone sweeps across the ground, observers will recognize a sudden, explosive change in pressure, or the sound of a sonic boom. On the ground, observers will also note a distinct double-boom—one boom for the show wave at the nose of the cone, and another from the trailing shock wave at the tail.
A “Sonic Boom” Is Really a Sonic Trail
A sonic boom is not a single, isolated bang, occurring at the moment the Mach threshold is passed. Instead, the cone of shock waves, which creates the sonic boom effect, persists for as long as the aircraft continues to fly at supersonic speeds.
The intensity of the boom depends on the aircraft’s size and shape, altitude, speed, and atmospheric conditions. For obvious reasons, aircraft closer to the ground create louder booms—and as residents of Chicago learned last week, these can cause damage such as broken windows, minor structural damage, and psychological unsettlement.
For this reason, supersonic travel is sharply restricted within the United States, as it is in most countries around densely populated areas. During the heyday of the Concorde supersonic jet, the plane’s only consistent flight itinerary was between New York and London—a route that not coincidentally passed almost entirely above the Atlantic Ocean, where no one below could complain about the noise.
To mitigate the effects of the sonic boom, both military and commercial aerospace firms are working on “low-boom” aircraft, which are designed to shape shock waves that can diffuse or cancel their intensity, in the hopes of allowing for future supersonic travel with less disturbance to those in the path of the shock waves.
About the Author: Harrison Kass
Harrison Kass is a senior defense and national security writer at The National Interest. Kass is an attorney and former political candidate who joined the US Air Force as a pilot trainee before being medically discharged. He focuses on military strategy, aerospace, and global security affairs. He holds a JD from the University of Oregon and a master’s in Global Journalism and International Relations from NYU.
Image: Wikimedia Commons.
