What is a Sonic Boom?
When an aircraft exceeds the speed of sound the air it displaces compresses into a shockwave. We know it as a "sonic boom" for the explosive noise that can be heard when it reaches the ground. In 1973 the U.S. outlawed civil supersonic flights over land for fear that sonic booms posed too great of a public nuisance. Today, technology has advanced to all but eliminate the "boom" from sonic booms. With the aid of new light weight materials and sophisticated aircraft designs, a low-boom jet is capable of producing a boom measuring less than 80 decibels. This is 100 times quieter than the Concorde, or about as loud as a motorcycle driving by. This technology could in principle be deployed today, however the ban remains in place due to regulatory inertia.
Myth: Sonic booms are discrete events
Many people mistakenly believe a sonic boom is a discrete, one-off event that occurs the moment an aircraft reaches the speed of sound. In fact, a sonic boom is really two booms, but they happen so close together we only hear it as one. The pocket of over-pressure at the front of a supersonic aircraft creates a corresponding pocket of under-pressure near its rear. By the time the shockwave reaches the ground it coalesces into an N shape, with air pressure rising sharply only to dip before sharply returning to normal. The human ear perceives the loudness of a noise based on its suddenness, so the sharp peak and trough of the N-wave is what puts the "boom" (or booms) in "sonic boom."
Once an aircraft is supersonic the shockwave is continuous in its wake not unlike the waves made in the wake of a boat, only these waves propagate out in a three-dimensional cone. Where the edges of the cone reflect off the ground defines an area known as the boom carpet. The width of the boom carpet grows approximately one mile with every 1000 feet of altitude. Thus if a supersonic aircraft is flying at 50,000 feet its sonic boom reaches a 50 mile width below its flight path.
Focused booms, on the other hand, are discrete events at particular points in the flight path. Focused sonic booms occur when the aircraft makes a maneuver that amplifies the shockwave, but are easily mitigated with an offsetting maneuver, such as decelerating into a turn.
MYTH: Sonic Booms Will Always "Boom"
The Concorde's sonic boom reached 100-110 decibels, and sounded like an explosion going off. In contrast, an overland noise rule would require supersonic aircraft to make no more noise than what we already find reasonable. Since decibels are on a logarithmic scale, a noise limit of 80 decibels, say, would allow supersonic jets overland that are roughly 100 times quieter than the Concorde. It would sound about as loud as a lawn mower or motorcycle, and only last about half a second.
How is this possible? Before, testing the aerodynamics of a new aircraft was painstaking, requiring the costly use of real models and wind tunnels. Aircraft designers have since taken advantage of computer simulations to discover wing and body shapes that alter the way the aircraft displaces air. Shapes that blunt the points of the N-wave decrease perceived loudness substantially. In the limit, the N-wave can be shaped into something close to a sine-wave, making the "boom" nearly inaudible to people on the ground. Such low- and no-boom designs have already been tested in the field, and NASA's Langley Research Center maintains a demonstrator for visitors to experience the amazing contrast between an explosive boom and the soft "thump" of the most advanced designs.
Modern designs use a number of other strategies to reduce boom intensity. This includes use of carbon fiber bodies to reduce the aircraft's mass, flying at high altitudes where the atmosphere is thinner, and shaping the aircraft to displace the air as if it were more elongated than in actuality.
MYTH: SONIC BOOMS INEVITABLY CRACK WINDOWS, DAMAGE BUILDINGS
Unmitigated sonic booms can cause windows or old plaster to crack, but for civil applications this is extremely rare. The intensity of a sonic boom can be measured in pounds per square foot (psf) of air pressure. Buildings in good condition can withstand shockwaves up to 11 psf without experiencing damage. The Concorde's boom produced less than 2 psf, which posed a minor risk to historical structures and structures with existing compromises, but not more than natural weather events. A low boom SST would, by definition, produce a significantly softer shockwave than even this.
The myth that sonic booms easily cause building damage likely dates back to an infamous series of tests conducted in 1964 under FAA supervision wherein supersonic fighter jets flew over Oklahoma City eight times a day for six months. In the end 73% of surveyed residents said they could tolerate the booms indefinitely. Nonetheless, the U.S. government received over 15,000 complaints and nearly 5000 damage claims. When most were rejected as spurious it fueled a public-relations disaster for the FAA and supersonic overland more generally.