When an object, should it be an aircraft, a bullet, or a particle, reaches a speed of about 1235 kilometers per hour, it is said to have reached the speed at which sound travels. At that moment, an extraordinary phenomenon occurs during which an enormous amount of sound energy is released, generating a sound comparable to that of an explosion:
it is called a sonic boom and is the subject of this article. However, before delving into the particularities of this event, we will first uncover some of the basic concepts behind sonic booms, such as the propagation of sound waves and their properties.
The speed of sound explained
Like light, the sound is a wave, but it is a mechanical wave and not an electromagnetic one. A mechanical wave’s main property relies on the oscillation and collision of air particles (or particles of any other medium) in a longitudinal movement. Indeed, a disturbance is issued, which causes particles to oscillate and collide with neighboring ones, propagating the disturbance in all directions.
The speed of sound in dry air at 20°C is 343 m/s. It is the speed at which the mechanical wave propagates and creates a delay when a sound is emitted and perceived. We have all experienced it during thunderstorms. Lightning always precedes thunder. That’s because light travels at 300,000 km/s, whereas sound is traveling at around 343 m/s. This threshold of 343 meters per second is the fundamental principle behind sonic booms.
From the speed of sound to sonic booms
Typically, a sonic boom occurs when an object travels faster than the speed of sound and creates shock waves generating a large sound. However, it can only be perceived at a precise positioning relative to the moving object. We can picture the moving object forming an imaginary cone of rippling sound waves in its trail as it advances. Therefore, only observers positioned at a point in space intersecting the imaginary sound cone will be affected by the sonic boom.
Contrary to what one may expect (as an observer, we only experience the boom for a fraction of seconds), the boom is actually a continuous effect. However, the imaginary cone that we previously described moves along in the trail of the object. Therefore, the observed only experiences it as the sound ripples emanating from the object momentarily cross the observer.
But what are the more precise explanations behind it?
Let us consider an aircraft traveling at supersonic speeds or above Mach 1 (Mach number is the quantity representing the ratio of velocity past a boundary to the speed of sound). When it pushes through the air, it creates pressure waves in front and behind the aircraft. The waves generated travel away from the aircraft at the speed of sound.
However, as long as the aircraft travels beyond the speed of sound, the pressure waves are compressed closer and closer to each other because they cannot escape each other, being bound by the speed of sound threshold, and grow together gradually until they combine to form an enormous shockwave which grows and propagates, resulting into the large “boom” sound experienced.
Implications of and concerns over sonic booms
Generating loud sounds which exceed the normal decibel toleration, objects traveling beyond the speed of sound are now restricted in traveling routinely above land. In addition, regular sonic booms threaten humans’ hearing systems and structures that may be weakened by regular shock waves startling. Therefore, research is continuously being conducted in the fields of noise reduction solutions.
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