NASA X-59 Explainer: Science of Sonic Booms

The most common misconception about sonic booms is that they're just the sound created at the moment when an aircraft "breaks the sound barrier." In reality, a sonic boom is continuous, the audible evidence of invisible pressure waves forming as an aircraft travels faster than sound. As these waves travel toward the ground, they can merge, creating an explosive bang heard by everyone the aircraft passes over.

Sonic booms have been a problem since aircraft first began exceeding the speed of sound, or Mach 1, so disturbing to people below that for decades U.S. and international regulations banned commercial supersonic flight over land. Overcoming this disturbance is one reason why NASA has conducted extensive research into how sonic booms form, how they travel through the atmosphere, and how to reduce their noise.

NASA's research led to the X-59, the agency's experimental aircraft with a unique shape that allows it to fly supersonic with a softer sonic thump. With the X-59, NASA's Quesst mission is working to help enable commercial supersonic flight worldwide. But the design required a thorough understanding of the boom the X-59 was created to quiet.

At lower speeds, pressure waves spread all around the aircraft, including ahead of it, allowing the air to "get out of the way". But once a plane flies supersonic, it moves faster than the waves can travel. Pressure changes around an aircraft flying supersonic are strong rapid jumps, or shock waves.

An aircraft creates shock waves around all its components: nose, cockpit, wings, engine inlets, tail and any bump the air must move around. As shock waves move toward the ground, they tend to pile up on each other, eventually leaving only two that reach the ground as a sonic boom.

The waves spread outward from the aircraft's flight path as they travel, covering a large "carpet" of ground with the sonic boom sound, affecting many people.

Supersonic aircraft fly fast and high, so by the time you hear a sonic boom, the plane may already be out of sight or hard to spot. A sonic boom made by a plane at 55,000 feet takes almost a minute to reach the ground, so if you're looking toward the spot where the sound originated, the plane has already moved miles ahead.

The type of sound that reaches the ground depends on the aircraft's shape, its flight path and atmospheric conditions, so people near one another may hear the same flight differently. A person on the ground might hear one sharp boom, two closely spaced booms or a softer rumble.

NASA's research into sonic booms includes how to design aircraft that are quiet, how people and structures respond to shock waves and noise, and how the atmosphere changes the boom sound. This research includes the use of computer models, sound laboratories, wind tunnels, microphones, sensors, aircraft, and some very clever researchers and engineers.

NASA's Quesst mission is the culmination of this research.

The X-59 will demonstrate a quiet sonic thump, but that's only part of Quesst. NASA will fly the X-59 over communities to learn how people react to the aircraft's sounds. The agency will provide that data to regulators to help develop future international standards for acceptable sound levels from supersonic flight over land - replacing the current "speed limit" with a "sound limit."

Learn more about the Quesst mission on NASA's site.