How Do Sonic Booms Occur in Physics?
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.
FAQs on What Is a Sonic Boom?
1. What is a sonic boom in Physics?
A sonic boom is the sound associated with the shock waves created whenever an object travels through the air faster than the speed of sound. It is an impulsive noise, often compared to a loud thunderclap or explosion, which is generated by the sudden change in air pressure caused by a supersonic object.
2. How exactly does an object moving faster than sound create a sonic boom?
As an object moves through the air, it creates pressure waves. At subsonic speeds, these waves travel ahead of the object. However, when the object exceeds the speed of sound (Mach 1), it outruns its own pressure waves. These waves cannot dissipate and instead accumulate and merge into a massive shock wave. This wave contains a sharp rise and fall in pressure, which is heard on the ground as the characteristic 'boom' when the shock wave passes.
3. What is the difference between subsonic, supersonic, and hypersonic speeds?
These terms describe an object's speed relative to the speed of sound in a medium:
Subsonic: Speed that is less than the speed of sound (Mach < 1).
Supersonic: Speed that is greater than the speed of sound (Mach > 1). This is the condition required to create a sonic boom.
Hypersonic: Speed that is significantly greater than the speed of sound, typically defined as Mach 5 or higher.
4. Does the size and shape of an aircraft affect the sonic boom it produces?
Yes, the characteristics of the aircraft play a crucial role. A larger and heavier aircraft displaces more air, which generally results in a stronger shock wave and a louder sonic boom. Conversely, the shape is also important; a longer, more slender aircraft with a sharp nose can create a weaker, more distributed pressure wave. Modern research focuses on shaping aircraft to turn the sharp 'boom' into a gentler 'thump'.
5. What are the primary effects of a sonic boom?
The most obvious effect of a sonic boom is a loud, startling noise. The associated overpressure wave can also cause physical effects, such as:
Vibrating and rattling buildings and windows.
In very rare and extreme cases of low-altitude supersonic flight, it can cause minor structural damage like shattered glass.
It can be alarming to people and animals on the ground, which is why supersonic flight is generally prohibited over land.
6. Why don't pilots inside a supersonic aircraft hear the sonic boom?
Pilots do not hear the sonic boom because they are travelling with the aircraft, ahead of the shock wave it creates. The sound energy and the shock wave itself form a cone-shaped wake that trails behind and away from the aircraft. People on the ground hear the boom only when this cone passes over them, but the source of the sound always remains behind the pilot.
7. Is a sonic boom a single event that happens only when an aircraft breaks the sound barrier?
This is a common misconception. A sonic boom is not a one-time event. An aircraft generates shock waves and a sonic boom continuously for the entire duration that it travels at supersonic speeds. An observer on the ground hears it as a momentary event only because the cone-shaped shock wave sweeps over their location at a specific instant.
8. Can phenomena other than aircraft create sonic booms?
Yes, any object that travels faster than sound can create a sonic boom. Examples include:
The tip of a bullwhip, which is often cited as the first man-made object to break the sound barrier.
Spacecraft re-entering Earth's atmosphere.
Meteors entering the atmosphere at high speeds.
High-velocity rifle bullets.
