Monday, April 2, 2012

 Flying 3 times the speed of sound. SR 71 Black bird.
For some, sonic boom is like hearing a gun fire, but for me it is much more than just the sound of a gun shot. True, the intensity of sonic boom depends on the size of the objects that creates it and the distance at which the hearer hears it. Well, I lived very close to a military base where fighter airplanes flew very low and produced this annoying sound frequently. Later in life when I became a member of Concorde supersonic operations in Bahrain the sound what I experienced used to be window shattering enormous booms, two thunders one after the other, because the size of aircraft was much larger than the fighters. Sonic boom could be a fun at an air show, but if you live under a supersonic flight path on a regular base, the novelty would die out soon. Concorde was allowed to make supersonic flight only transatlantic Miami to London. It had tough time in making London to Singapore via Bahrain, because they had to chose only deserted land as their flight path to prevent causing sonic boom over residential areas. If Concorde travelling at the speed of only 2 Mach (2 times the speed of sound) and producing such noise pollution what about the intensity of sonic boom created by SR 71 black bird aircraft travelling at 3 Mach speed (3 times the speed of sound)

Anglo French Supersonic Transport. Flies twice the speed sound
What is sonic boom 
Sonic boom is an ear splitting noise created by shock waves that is generated when an object is crossing the speed of sound or the sound barrier. The boom is made by the sudden release of pressure that is built up by the shock wave. Even though the change in pressure very small, as the change takes place so rapidly you hear the boom. We do not hear all the sonic booms, only the powerful ones becomes audible by the times it reaches the ground. Shock waves are the primary reason for sonic boom, but there are much more to understand about sonic boom than just shock wave. Supersonic speed of an object, Medium through which the sound can travel which is air, and of course the shock waves these are the basics of a sonic boom. 

All aircraft produce two booms one from the nose and one from the tail. As aircraft length is short, the distance between two releasing points of shock waves are smaller so the booms are generally heard as one. As shock waves spread across the landscape like a cone becoming larger and larger from the release point towards the rear, sonic booms are continuously created along the flight path. If you continued to move with the plane at the same supersonic speed you would hear continuous booms. Otherwise, you hear the booms only when the plane is above you. 

Cone created by shock wave
Supersonic speed
Sound travels through air around 750 miles per hour depending on the atmospheric conditions. This speed of sound is said to be the sound barrier. If an airplane fly above the speed of sound it is known to be the supersonic flight. 

What is Mach number
Mach number is the speed of aircraft in relation to the speed of sound. For example, 1 Mach means the speed of aircraft is equal to the speed of sound. When both of these speeds are equal we say the aircraft is flying at 1 Mach speed. If it flies twice the speed of sound we say aircraft speed is 2 Mach.

What was the need of defining the speed of aircraft in Mach numbers is because at Mach 1 or above speed shock wave develop on the surface of aircraft. Shock waves creates increase in drag, loss of lift and loss of aircraft control. below Mach 1 speed shock waves disappears.

What is shock wave ?
Sonic boom as a result of  shock wave. 
When an airplane travels through the air it produces sound waves. If the plane travels less than the speed of sound, the sound waves can propagate ahead of the plane. As the airplane breaks the sound barrier it pushes on the sound waves in front of the aircraft. Sound waves stick to its speed limit because they cannot move faster than its barrier speed even though they are being pushed by the aircraft which is moving faster than the sound waves them selves. So, instead of moving away the sound waves get piled up one against the other, as they are continuously being created by the fast moving aircraft. These piled up waves are called the shock waves. Most powerful shock waves are at the tip and at the tail of the air plane.  

Air as medium
Imagine an airplane flying through the air. As the plane travels it pushes the air molecules out of its way continuously creating waves of compressed air. These are pressure waves moves away from airplane in all directions like ripples moving away when you drop a pebble in the pond. Unlike the ripples, these pressure waves moves away at the speed of sound. 

Can we increase or decrease the intensity of sonic boom ?
Yes we can, if you increase size of an aircraft the intensity of a sonic boom will also be increased because of the large displacement of air. So also, lower the altitude of airplane to the ground more intense will be the sonic boom. This is because the shock waves will have less distance to travel before hitting your ear. There fore to decrease the intensity of sonic boom decrease the size of the plane and fly it higher altitude. 

Space shuttle reaching out for vacuum flight.
Will a space shuttle create sonic boom?
No, Space shuttle travels many times faster than speed of sound but travels in vacuum surrounding. For sound to travel there should be some medium such as air to carry the vibrations. Because there is nothing in the space across which sound can be conducted, no shock waves are made and no sonic boom is created. In fact it is completely silent out in space.

Is thunder a sonic boom ?
Yes, it is in a way, lightening heats up the air to a very high temperature. As the air is heated up it expands rapidly and cools down quickly. This causes shock waves in the air. Sound of thunder is caused by this shock wave. But a true sonic boom is the result of an object travelling through air faster than the speed of sound. 

Lightening and thunder happen at the same time but we see the lightening first and hear the sound later. This is because speed of light is much greater than the speed of sound.

Have you ever broken the sound barrier ?
If the answer is no, it is easier than you think to break the sound barrier. All that you need a bull  whip and some amount of practice under guidance. Watch this link you will know what is a sonic boom and how can you create it at home. By practice, the tip of bull whip can be made to travel at supersonic speed and the "swish" sound you hear will be the sonic boom created by you. 

Warning: Do not practice the whip until you are quite familiar with it.