Reading time 7 minutes, 1,400 words
To attract readers with a dramatic headline, I stand guilty of employing a marketing gimmick. The Concorde fleet was grounded for many reasons including its high operating cost, and an unfortunate accident. Let me redeem myself by providing a brief explanation. Concorde aircrafs could fly at roughly twice the speed of sound. Any speed higher than the speed of sound is called ‘supersonic’ speed. Concords were supersonic jets that covered London to New York in about 3.5 hours as against a minimum of 7 hours taken by other commercial jet aircrafts. This high speed was its key attraction. While flying above the speed of sound, aircrafts make large thud sounds like that of an explosion, and for this reason, USA banned the flights from flying above the speed of sound over land. How much did this ban contribute to the failure of Concorde, I am not certain, but surely it makes a catchy headline. This loud explosive sound is called ‘sonic boom’, a phenomena that we shall explore here.
Understanding Sound:
Let us study the nature of sound by taking the example of a bass drum. When we strike the drum with a mallet, causing the membrane to vibrate, the previously still air around the drum is set into motion. The vibrating membrane pushes the layer of air in its contact, initiating a chain reaction. Each layer of air particles collides with the next layer and then recoils, creating a sequential disturbance in the adjacent layers of air. This disturbance travels through the air, creating moments of compression, where air particles are pressed together, and rarefaction, where they are pulled apart. Although the disturbance at a specific location is momentary, it moves forward, constituting a wave. It is only the wave that moves forward not the particles. This process of the wave moving forward, known as wave propagation, is depicted in the figures below:
When this disturbance falls on our ear, the eardrum vibrates. Through a physiological process, that we need not describe here, our ear converts this vibration into an electrical signal in our auditory nerve, which the brain interprets as a sensation that we call sound.
Properties of Sound:
This feature of sound propagation is the same in all mediums, solid, liquid and air. Propagation of such pressure waves through any medium will be sound. The speed at which the waves move forward depends on the type of the medium. In air, sound wave travels at a speed of about 340 meter/ second. This speed depends on temperature, pressure and quality of air.
Moving source of sound:
So far we have discussed only a single drum strike, now imagine a car with a hooter. Say it is, 1000 meters away from us and it hoots three times. The first hoot at time =0, the second hoot at time = 1 second and third hoot at time= 2 second. Taking the speed of sound to be 340 meter/second, let us consider four scenarios:
A. Car is stationary:
We hear the first hoot at time = 2.94 second, which is the time taken, by sound, to travel 1,000 meter. Then at time 3.94, we hear second hoot (i.e. time for the sound to travel + time at which the hoot occurs), and lastly at time 4.94 we hear the third one.
B. Car is moving towards us at 100 meter/second:
We hear the first hoot at time = 2.94 second. By the time of the second hoot, the car has moved 100 meters and the sound has to travel only 900 meters so we hear it at time 3.65 second. The third hoot has to travel only 800 meters so we hear it at time 4.35 second.
C. Car is moving towards us at 340 meter/second:
We hear the first hoot at time = 2.94 second. By the time of the second hoot, the car has moved 340 meters and the sound has to travel only 660 meters so we hear it at time 2.94 second. The third hoot has to travel only 320 meters so we hear it at time 2.94 second. We observe that when speed of the car is same as speed of sound, we shall hear all the three hoots together as one loud hoot.
D. Car is moving towards us at 400 meter/second:
We hear the first hoot at time 2.94 second. By the time of the second hoot, the car has moved 400 meters and the sound has to travel only 600 meters so we hear it at time 2.76 second. The third hoot has to travel only 200 meters so we hear it at time 2.59 second. We can see that when speed of the car is higher than the speed of sound, we shall hear the last hoot first followed by the second hoot and we hear the first hoot last.
The above scenarios are summarised in the table below:
It may be noted that in these scenarios the time interval between hoots is 1 second. In scenario ‘C’ where speed of the car is 340 meter/second, we hear all the hoots together, since the car and the sound travels at the same speed. For any speed above the speed of sound, at a certain time interval all the hoots will be heard together. In the case of the car speeding at 400 meter/second, this interval works out to be at 1.18 second. Accordingly, we can conclude that even in scenario ‘D’, there will be a sonic boom, It will be addition of sounds produced in intervals of 1.18 seconds.
Whenever the source of sound travels at a supersonic speed, we hear a loud burst, caused because the sound produced at different instances adds up and falls on our ear as one loud blast. This blast is called the sonic boom. It is also known as breaking the sound barrier.
In Supersonic Jets:
When a regular aircraft is flying above us, we hear its humming sound much before the aircraft crosses us, that is because sound is traveling faster than the aircraft and hence the sound reaches us before the aircraft.
Whereas, in the case of a supersonic jet, the aircraft will reach us before its sound reaches us, the first indication we get of the presence of a supersonic aircraft is the sonic boom. By the time we hear this sonic boom the aircraft would have already crossed us. This sonic boom is the hum of the aircraft at various instances adding up and falling on our ear simultaneously as one loud bang. This results in a deafening thunder, followed by the hum of the aircraft that has already passed.
When a sound travels towards us, we perceive it at a higher pitch and when it goes away from us we hear a lower pitch, this is called the ‘Doppler Effect’. It is experienced when we hear an approaching train. We are ignoring Doppler Effect of sound from supersonic jets in the present discussion.
Vapour Cone:
Figure 5, above, is often confused as the picture of an aircraft breaking the sound barrier, which is misleading. This cone of cloud is unrelated to sonic booms and can be witnessed even in cases when the aircraft is traveling at speeds lower than that of sound. As the aircraft moves at high speed it pushes air to its front, consequently there will be less air behind the aircraft thus making it an area of low pressure. Low pressure results in low temperature according to laws of physics. If there is sufficient moisture in the air, then this moisture condenses into a cloud. We see this cloud behind such aircrafts. It should not be confused as breaking the sound barrier.
An often quoted example of sonic boom is the cracking of whip. To lash a whip, first the whip is rotated in a circle and then you swing it forward, while the tip of the whip is moving forward, abruptly its handle is pulled back. In the process, the tip of the whip travels at a speed faster than sound, and produces a sonic boom that is called the "Cracking of the Whip". This was perhaps often experienced in the past, but now we dont see whips in daily life anymore.
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