Sonic Boom

When objects travel at speeds more than the speed of sound, this phenomenon is called a sonic boom. Some people get scared or amazed after witnessing a sonic boom sound, but it has become common these days with a large number of supersonic jets and rockets around. 

When an object travels at a speed greater than the speed of sound, shock waves are generated and a huge amount of energy is liberated. Large supersonic aircraft produce sonic waves that can be startling and may awaken you or cause a bit of damage, like cracks in your window glasses. 

A supersonic beam, although powerful cannot be heard in all directions. Imagine an object travelling at supersonic speed. A sonic boom can be experienced in the zone of an imaginary 3D cone at the back of that object. If you're in that zone, you will experience a sonic sound as the object passes you. As the object moves, the imaginary conical effect region also moves with it. As the object passes, the observer experiences the boom for a very brief period. 

Scientific Reason Behind a Sonic Beam

As per the sonic boom definition when the object moves more than the speed of sound a huge noise is created. But the exact sonic boom meaning was explained here scientifically. Similar to that of water, the air is also fluid with lesser density. Hence similar to the waves created on the water when an object travels on it, waves are created in the air as well. 

Unlike waves on water, these waves travel at the speed of sound. But with the increase in velocity of the object in the air to the speed of sound, the airwaves are compressed or forced together. This results in a wave merger, into a single shock wave. The power or intensity of the shockwave depends on the amount of air involved or in a way the size of the object being accelerated is called sonic boom speed. 

Sometimes several secondary smaller shock waves are formed at some convex points on the object. Let's say the wing head or engine inlet of an aircraft. The pressure generated through shock waves by an aircraft is generally a few pounds per square foot. 

Problem of Abating Sonic Booms

With the increase in the number of supersonic aircraft carriers, there arises a problem of sonic booms affecting general people on the ground. It was believed that by flying high the problem of sonic booms could be avoided but it was later proved to be untrue. Richard Seebass and one of his colleagues Albert George studied the problem carefully and finally went to introduce the "figure of merit". The figure of Merit or FM was a characteristic of the length of the aircraft and its length.  The lesser the FM an aircraft has, the lesser the boom it generates. In modern times, FM levels of 1 or lower are acceptable.

Sound of a Sonic Boom

The sound produced by the sonic booms to a great extent depends on the distance between the observer and the shape of the aircraft producing the sonic boom. The sonic booms sound like a deep double "boom" as the aircraft passes by. The sound is much similar to mortar bombs. The boom is continuous for the entire supersonic flight time and not only during the transition from subsonic to supersonic wave, which in general is a misconception among many.    

An obvious question might arise, " Do the on-borders hear the sonic boom?" Well, the answer is "No". The borders don't hear the sonic boom at all. A simple explanation of this is they don't fall in the hypothetical 3D conical figure on which the sonic boom has its effects. Atmospheric effects like temperature, humidity, pollution, winds can also affect the sonic boom felt on the ground. Even hard surfaces like concrete, tar, marble can reflect the waves and produce a doubled-up booming effect. Grassy and shrubby cover on the ground can lessen the impact of the sonic waves by absorbing them. 

In the present scenario, there are no standards prescribed legally for sonic booms. But work is underway by the legislators and industry experts in framing guidelines to lessen the impact of sonic booms. 

Conclusion

To summarise, sonic booms are produced when objects travel beyond the speed of sound, due to the concentration of pressure waves to form shock waves, which possess a huge amount of energy. The sonic boom from aircraft can cause ground-level damage, i.e can break glasses of buildings and houses or shatter windows. The pilots themselves cannot hear the sonic booms but can manage to see a wave emanating from behind the aircraft they're carrying. 

Experts and scientists are trying hard to reduce the effects of the sonic boom on the ground level. Although this a wonderful physics phenomenon to read about, your experience with sonic booms may not be just as pleasing. So, Can sonic booms kill you? Well, technically yes. High-intensity ultrasonic waves can kill you, but such an incidence is very very rare, especially if you are in your home. High-intensity ultrasonic waves from overhead aircraft carriers are more likely to shatter your windows and cause damage to your property. This is the reason they are not routed through populated areas. Experts are framing some rules regarding supersonic jets. Scales are to be formed regarding measuring and comparing the intensity of sonic booms generated by aircraft for the safety of people on the ground.