Sonic boom

A sonic boom is a powerful sound associated with shock waves. These waves occur when an object travels through the air faster than the speed of sound. This critical speed is known as Mach 1. When an object exceeds this speed, it is traveling at supersonic speeds. Supersonic flight can include speeds up to Mach 5.

While we often associate them with jets, small sonic booms occur in daily life. You might hear the crack of a bullwhip or the snap of a rolled-up towel.

Large aircraft produce much more significant booms that can be quite startling.

To understand the mechanism, imagine a boat moving through water. As an aircraft passes through the air, it creates pressure waves in front of and behind it. These waves travel at the speed of sound. As the aircraft's speed increases, these waves are forced together. They cannot get out of each other's way quickly enough, so they become compressed. Eventually, they merge into a single shock wave that travels at the speed of sound. In smooth flight, this shock wave starts at the nose and ends at the tail. This forms a geometrical shape called a Mach cone.

The faster the plane flies, the more pointed this cone becomes.

Observers do not hear a boom in every direction from the aircraft. Instead, the boom is a continuous effect that occurs while the object is supersonic. It affects only observers positioned where the edge of the Mach cone intersects them. As the object moves, this conical region moves behind it. This creates a narrow path on the ground called a boom carpet. The width of this carpet depends on the altitude of the aircraft.

The boom is experienced when there is a sudden change in pressure. This specific pressure profile is known as an N-wave because of its shape. In an N-wave, pressure rises at the nose, decreases to a negative pressure at the tail, and then returns to normal. This causes a distinctive double boom: one from the initial rise and one from the return to normal pressure.

History shows that managing these sounds is a major challenge for engineers. In the late 1950s, designers thought flying at very high altitudes would prevent booms. However, the North American XB-70 Valkyrie proved this wrong. It was found that booms still reach the ground from 70,000 feet. During these tests, the N-wave was first characterized. Later, researchers Richard Seebass and Albert George studied how to measure these levels. They created a "figure of merit" (FM) to describe boom intensity. This value is a function of an aircraft's weight and length. A lower FM means a quieter boom. For example, the Concorde had an FM of about 1.4, while the Boeing 2707 was 1.9.

Sonic booms carry enormous amounts of sound energy. This energy can wake people or even cause minor structural damage. For instance, booms can shatter glass. However, buildings in good condition should not suffer damage from pressures of 530 Pa or less. Most community exposure stays below 100 Pa. The strongest sonic boom ever recorded was 7,000 Pa. This occurred when an F-4 jet flew just above Mach 1 at an altitude of 15,000 meters.

Different aircraft produce different peak overpressures. The Concorde produced 93 Pa at Mach 2. In contrast, the SR-71 Blackbird produced only 43 Pa at Mach 3 plus.

The power of the wave also depends on the size and shape of the aircraft.

Engineers are now using advanced research to reduce these sounds. Because of the noise, routine supersonic flight is currently prohibited over land. To fix this, researchers are looking at the Jones-Seebass-George-Darden theory. This theory suggests shaping the vehicle to spread out the N-wave. One method involves using a wide nose cone to create a downward-focused shock. Another method is the area rule, which involves shaping the fuselage below the wing. NASA has also tested the "Quiet Spike," which is a telescoping boom on the nose.

These tests aim to make supersonic travel a feasible option for overland flight.

Recent experiments have shown real progress in noise reduction. The Shaped Sonic Boom Demonstration (SSBD) used a modified F-5E aircraft. This plane had a very long nose to help change the shock waves. After 21 flights and 1,300 recordings, the SSBD reduced the boom by about one-third. While this is not a total elimination, it is a significant step.

If the boom were reduced enough, it could reach an acceptable level for overland flight. Even though creating a shock wave is nearly inescapable for aircraft that generate lift, science continues to find ways to make the experience much quieter.