Sonic Boom: What cause that earth shattering sound?

I once lived near an aviation range. Living there had its own perks. I used to see lot of planes in the sky daily, when I was kid. On one such time, I was looking out of window while studying through the sunny day. All was quiet, and I vaguely remember, what I saw was a fighter plane. It must be going at blazing fast pace, because it appeared to be faster than normal planes. I was admiring the machine and it goes towards the horizon as fast as it approached. I was about to look away, but suddenly an earth shattering boom like an explosion or God of Thunder's voice, filled the surroundings. The shockwave was so strong that even the glass panes of the windows and water Glass on the table threatened to break apart. It was quite an experience for me and took some time to understand what I experienced was Sonic Boom.

What is sonic boom?

Sonic boom is a sound shockwave, which is created by fast moving object through a medium, when an object surpass the speed of sound. In my case the medium was Air and Object was Jet Plane.

The thunderous sound heard during sonic boom is a buildup energy. An object said to be moving faster than speed of sound which is Mach 1, when its speed gets beyond approximately 1,235 km/h (767 mph) at sea level and 20 °C (68 °F).

Why do we hear sonic boom?

Think Air like invisible water all around us. Now, a fast-moving object in Air will create an invisible "Cone" like wave in 3-dimensional space or pressure waves, just like pebble skidding through water but only in 3D. If aircraft move slowly, these pressure waves will get dispersed, before aircraft moves to the new location. However, moving at supersonic speed, these pressure waves cannot get out of the way of an aircraft and are forced together to get compressed. These compressed shockwaves get pushed aside by moving aircraft faster than sound, this energy gets released due to the sudden change in air pressure. When this shockwave reaches ground, we hear a sonic boom.

It is interesting to know that person travelling at the speed of sound do not hear any sonic boom, he only see the buildup fog due to compression of air by pressure waves.

There are several factors which can affect the intensity of shockwave like larger the aircraft, more air it will displace and hence stronger the shockwave. Thus, the composition of the object has direct effect on shockwave for example - weight, dimensions, Flight properties, height and atmospheric conditions.

There are some interesting terms associated with Sonic Boom like

Double Boom

There are two shockwaves created by an airplane one is at the front and other at the tail. The aircraft moving at much faster pace generates more two-separate distinct sonic boom like NASA space shuttles.

Boom Carpet

The intersection of cone of influence, which is created by shockwave and the earth’s plane is called Boom Carpet. It is the region where people will feel the effect of shockwave directly.

Technically, Maximum intensity of sonic boom is beneath the aircraft, and it decreases with lateral distance from the aircraft. For example, an aircraft flying at supersonic speed on an altitude of 50,000 feet can produce a sonic boom cone of about 50 miles wide.

The units in which Sonic booms are measured is, psf or pound per square foot, above normal atmospheric pressure, which accounts to be 2116 psf/14.7 psi also known as overpressure.

Overpressure below 1psf does no structural damage and a minor annoyance. 2 - 5psf can do minor damage. More than that can be damaging. For example, Concorde generates about 1.94 psf at the speed of Mach 2 at altitude of 52,000 feet. While Space Shuttle's shockwave is 1.25 psf at speed of Mach 1.5 at 60,000 feet during landing approach. If you observe, the space shuttle generates less psf due to higher altitude, despite being of greater size and weight because higher altitude means less air molecules to create shockwave.

Supersonic planes industry is started gaining popularity again, after Concorde. Richard Branson of Virgin Group announced purchase order of 10 supersonic airplane, with full size prototype to be developed by 2020 and passenger flights will come in market by 2023.

Sonic Boom is fascinating natural phenomenon whether it is created by a bull's whip, gun, plane or thunder, it will always fascinate us ...

FootNote

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Credits

Articles

https://en.wikipedia.org/wiki/Sonic_boom

https://www.nasa.gov/centers/armstrong/news/FactSheets/FS-016-DFRC.html

https://science.howstuffworks.com/question73.htm

https://www.wired.com/2017/03/supersonic-planes-mounting-comeback-without-earth-shaking-boom/

https://wonderopolis.org/wonder/what-is-a-sonic-boom

https://en.wikibooks.org/wiki/Acoustics/Sonic_Boom

Images

Wikimedia, Flickr, Pixabay

Videos

https://www.youtube.com/watch?v=GstwLblPpIM