When an aircraft travels faster than the speed of sound, it creates a loud noise known as a sonic boom. How does this happen exactly, and what important facts surround this phenomenon?
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Sonic boom explained
There’s a scientific reason why a fast-travelling aircraft can create a loud, thunder-clap sound, known as a sonic boom. It’s all to do with shock and sound waves.
A flying plane creates sound waves. If the plane is travelling slower than the speed of sound, these waves form in front of the plane (similar to the waves a boat makes on water).
However, if the plane goes faster than the speed of sound (approximately 700 mph), it breaks the sound barrier. The sound waves that normally gather at the front of the plane can’t get out of the way of each other, and so they accumulate and become compressed, forming a single shock wave.
This shock wave extends to the rear of the plane in a widening cone shape. As the plane continues to fly, the edge of this cone transmits a loud bang directly below its flight path towards the ground, known as the boom carpet. The sound is so noisy because the waves contain a lot of sound energy.
Often, you can hear the sound of a sonic boom without being able to pinpoint the aircraft in the sky. Other times, you might be able to glimpse the trail at the rear of the aircraft.
Although supersonic air travel might seem like a recent invention, scientists have understood about sonic booms for a long time. In particular, Austrian physicist Ernst Mach described shock acoustic waves in the 1870s, when he developed a method to measure airspeed according to the speed of sound. For example, a plane flying at the speed of sound is described as going Mach 1, while a plane travelling at twice the speed of sound is Mach 2.
Up until World War II, shock acoustic waves were mainly obtained from artillery, going by the name of ballistic waves. However, sonic booms from aircraft first came to light in 1947, when Chuck Yeager piloted an X-1 aircraft to Mach 1.07, at 42,000 feet.
Other sonic boom sounds
As history shows, early research into sonic booms looked at how missiles behaved when fired at speed. Indeed, the loud sound that a bullet makes in flight acts as a mini sonic boom. Yet, it’s not just aircraft and bullets that cause sonic booms. Studies have shown that some animals also break the sound barrier when catching prey.
Although a sonic boom can sometimes be harmful to people and animals, it won’t deafen the pilot. In fact, the pilot won’t hear it but rather be able to view the pressure waves surrounding the aircraft – much like seeing the waves left in the wake of a moving boat.
Factors influencing sonic booms
Sonic booms can be influenced by a wide range of factors, including the size, weight and shape of the aircraft, as well as how high the plane is flying, its flight path, the type of manoeuvres it makes, and the weather, wind, temperature or atmospheric conditions.
Big and heavy aircraft displace a lot of air, so they tend to produce louder and stronger sonic booms compared to small or light aircraft. Very long aircraft may even produce double sonic booms. If a plane breaks the speed of sound at a low altitude, the shock wave may be strong enough to shatter glass in buildings.
You can’t ignore the sound of a sonic boom when a supersonic jet flies by, but you can control acoustic sound in your own environment, by choosing a wide range of high-quality noise control products and acoustic solutions from Sound Planning.
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