It becomes slightly less perceptibly intense the higher the aircraft travels, but not by much. Surprisingly, it doesn’t matter how high the plane flies – a shock wave will still be heard and felt on the ground. Observers on the ground, along the plane’s entire flight path, will hear the sonic boom one to 60 seconds after the aircraft passes overhead. The effect doesn’t just occur once as the plane reaches and passes the speed of sound. This shock wave will follow the plane as long as it flies at supersonic speeds. When that shock wave reaches the ground, any person within earshot will hear a double thunder clap. The shock wave generated stays mostly behind the aircraft, and radiates out in a cone. Actually, this creates two shocks, one forming as the aircraft passes the front of the wave and then another as it leaves the wave. These waves get in the way of the airplane, causing compression which results in a shock wave. The nose of a supersonic aircraft pushes ahead of its forward waves. Things get interesting, and complicated, when you fly faster than the speed of sound – supersonic flight. When an aircraft flies at subsonic speeds, the nose of the aircraft is always behind the waves in front of the plane – much as the front of a boat is always behind the waves ahead of its bow. Behind the plane, the air creates waves that radiate out and away in the shape of a cone – three-dimensionally. In front of the nose of a plane, air is pushed together and compressed as the aircraft flies forward. Similar principles are at play with aircraft. In this case, you only see the waves on the surface of the water, and it appears two-dimensional. Behind, the waves spread out away from the boat. In front, the waves are compressed together as the boat sails forward. Think of the waves that a boat creates at its bow and stern. When any object moves, it creates waves in front of and behind it. First, what causes a sonic boom? Second, how do we eliminate it? Unfortunately, it was thus not feasible to fly at supersonic speed over water and then at subsonic speed over land. The Concorde was specifically designed for supersonic flight (specifically, Mach 2) and was very fuel-inefficient at subsonic speeds (less than Mach 1). Only the US, Great Britain, and France allowed the Concorde to enter their airspace, and then only to cities in close proximity to the ocean – NYC, London, Paris, and Washington, DC. But as awareness of the sonic boom effect grew, almost every country banned the aircraft. The plane did an around-the-world publicity trip, and was well-received. When the Concorde was originally designed, in the early 1960s, governments and airlines around the world lined up to place orders. 15,000 complaints and a class action lawsuit were filed. The experiment consisted of eight sonic booms, every day, for six months. In 1964, the FAA and NASA conducted a six-month sonic boom research project in Oklahoma City – without warning residents beforehand. Being caught by surprise in certain situations is rather annoying, and in others, potentially dangerous. Your immediate responses are most likely surprise, shock, and an instinctive search for the source. Imagine that you are driving on your way to work, and with clear skies overhead, you suddenly hear the sound of thunder. But even when the sonic boom sounded like a “softer” distant thunder clap, it was distracting to people and caused disruption of sleep and interruptions in activity. The force of the boom rattled windows and loosened roof tiles. The sonic boom, at its worst, would be heard as a very loud thunder clap that was right overhead. As incredible as the Concorde was, the sonic booms created by its supersonic flights were so disruptive that most countries restricted or completely prohibited the aircraft from flying over land.
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