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The deafening and terrifying explosion that shook Paris and caused panic for a moment as the sensation of an explosion instinctively spread was a simple and ordinary sonic boom. Literally, the term “sonic boom” could be translated in Greek as “explosion of sound”. Actually, it is the sound that is produced when a moving medium exceeds or “breaks” the sound barrier. A very high intensity sound shock wave, but of very short duration.
In this case, the responsible for the sonic boom was a last generation fighter plane, the French Rafale. Its maximum speed is 2,223 km / h at high flying altitude and 1,390 at a lower altitude. The sound barrier at an altitude of about 1,239 km / h above sea level. Therefore, the Rafale has the ability to fly faster than sound (almost up to 2x faster) and is classified as a supersonic aircraft.
The sonic boom is caused by the breaking of the sound barrier, that is, when a moving medium reaches a speed of 1 Mach (Mach). A man on the surface of the Earth, relatively immobile, as there are said to be hundreds of thousands of people in Paris, perceives the sonic boom as the click of an explosion. However, although the result seems simple, the phenomenon by which it originates is quite complex, as well as fascinating.
Let’s say the airplane is a source of sound production. When the engines are started, like the Rafale, the sound is very loud, but to a listener at ground level it is perceived as continuous and, in a way, “normal”, even if it is unbearable without protection.
The sound produced by the Rafale property is transmitted through the atmospheric air in the form of waves, in the form of concentric circles (centered on the source) but of increasing diameter. The greater the distance between the listener and the object that produces the sound, the greater the diameter of the invisible circles of sound. And of course, the lower the volume.
But when the source is moving at high speeds, the listener stops perceiving the waves separately, but perceives them as identical, as if they touched each other. What we would abusively call “acoustic perspective” is lost the faster the sound source moves. And when the medium moves faster than the sound itself, these waves remain behind it, confused as if it were a single sound wave, enormous in size and intensity. And this is the sonic boom, the sound explosion, which includes all the individual sound waves produced by supersonic flight, p. of a Rafale.
Any source moving at Mach 1 causes a sonic boom. An aircraft like the Rafale is not entirely one sound source, but multiple: its main sources of sound, like any flying vehicle, are the mouth, the cockpit, the side wings and the tail.
Specific parts of the plane receive greater pressure from air particles, which they collide with during movement. The collisions cause oscillations, turning the Rafale’s parts into sources of sound, say, like the strings of a musical instrument or, better yet, like the membrane of a drum. And as in any audition, when the big drums are played, their sound is louder than that of any other instrument. The same is the case with the aircraft breaking the sound barrier: the noise produced by the wings, sides and tail is by far the most powerful, “disappearing” all the others.
The intensity of the sonic boom, how loud it can be heard, depends on the characteristics of the flying medium (shape, size), on the properties of the air (temperature, pressure, density) and also on the position of the listener in relation to the trajectory . the plane follows. In any case, the passengers of an ultrasonic ship are not aware of anything as they travel through the shock sound wave of the sonic boom.
Mach is a unit of measurement for ultrasonic speeds. It is the relationship between the speed of the source and the speed of sound in air. However, the speed of sound in air depends directly on its temperature, one of the parameters that make up the enigma of the sonic boom and, in general, on the behavior of bodies in motion at speeds beyond the sound barrier.
The French Rafale, of course, is not the only aircraft capable of exceeding the speed of sound. And if the Rafale doesn’t even hit Mach 2, there are several other jets that far exceed this price. The record, as the fastest manned spacecraft to move in Earth’s atmosphere and without a destination in space, was the North American X-15, an experimental spacecraft built by NASA in collaboration with the United States Air Force. . The X-15 reaches 7,274 km / h or 6.7 Mach.
Ultrasonic, however, was the famous Concorde, a passenger aircraft, with a top speed of approximately Mach 2 (2,180 km / h). The Concorde, like its Russian counterpart, the Tupolev TU-144, was withdrawn from active service for a variety of reasons. One of them was that the idea of ultrasonic travel is dangerous, since the sonic boom can cause serious damage to buildings, facilities, etc. This is why crossing the sound barrier in residential areas is prohibited, except in exceptional circumstances, such as in the case of Rafale, whose pilot received a special emergency permit to develop supersonic speed.
An interesting aspect of exploring the field of ultrasonic velocities has to do with paleontology and fauna on Earth hundreds of millions of years ago: scientists support the theory that some dinosaurs endowed with particularly long tails used it as a means of produce a sonic boom. Shaking it with unimaginable force and lightning, they swayed through the air. And causing a blast of sound similar to that of Rafale, they were intimidating their opponents in the unforgiving and harsh prehistoric environment of our planet.
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