A sonic boom is the thunder-like noise heard on the ground when an object travels through the air faster than the speed of sound. This explosive sound is not a continuous phenomenon but rather a momentary effect that occurs as the resulting pressure disturbance passes over an observer. The boom is fundamentally a consequence of rapidly moving objects disturbing the air and creating shock waves. The intense, singular “boom” is only heard as the accumulated energy cone sweeps across a location.
Understanding Mach 1 and Normal Sound Propagation
Sound travels through the atmosphere as a wave of pressure, essentially a vibration propagating through the air. For an object moving at subsonic speeds, the pressure waves it creates travel ahead of the object in all directions. This allows the air molecules to move out of the way smoothly, and the sound is heard before the object arrives. The speed of sound is not constant; it varies significantly based on air temperature, with warmer air allowing sound to propagate faster.
The critical reference point is Mach 1, which represents the local speed of sound in the surrounding medium. At sea level and standard temperatures, Mach 1 is approximately 767 miles per hour (1,235 kilometers per hour). The Mach number is a ratio comparing the object’s speed to the speed of sound in that specific environment. Since temperature generally decreases with altitude, Mach 1 also decreases at higher elevations.
The Barrier: How Sound Waves Stack Up
As an object accelerates toward Mach 1, it begins to catch up to the pressure waves it is constantly generating. These waves start to compress or stack up in front of the moving object, similar to the bow wave created by a boat moving through water. The accumulating pressure waves form an intense, single shock wave known historically as the “sound barrier.”
This compression causes a sudden increase in aerodynamic drag, known as wave drag, which historically made it difficult for aircraft to accelerate beyond this speed. The “barrier” is not a physical wall but a region of highly compressed air molecules. Once the object exceeds Mach 1, it travels faster than the pressure disturbances it creates, leaving the accumulated energy behind.
Forming the Shockwave and the Signature “N-Wave”
When an object maintains a speed greater than Mach 1, it continually generates shock waves that trail behind it in a conical shape, often called a Mach cone. The shock wave acts like a wake, spreading outward and rearward from the aircraft. The explosive sound is heard only when the edge of this cone passes directly over an observer on the ground. This creates a “boom carpet” that follows the aircraft along its supersonic flight path.
The characteristic pressure signature of a sonic boom is known as the N-wave. This name comes from the shape of the pressure graph, which features two distinct shocks. The first shock is a rapid rise in air pressure above the normal atmospheric pressure, followed by a linear decrease to a negative pressure (below ambient). The second shock is a swift return to the normal atmospheric pressure, and these two sharp pressure changes are heard as the distinct, double-tap sound of the boom.
Why Booms Vary in Intensity
The intensity of a sonic boom experienced on the ground is determined by a combination of factors, not solely the aircraft’s speed. The altitude of the object is a significant factor because the shockwave dissipates as it travels. A higher altitude means the shockwave travels a greater distance, allowing its energy to spread out and resulting in less intense overpressure on the ground.
Atmospheric conditions also play a role, as temperature and wind gradients can cause the sound waves to refract or bend. For instance, a decrease in air temperature with altitude can bend the shock waves upward, potentially preventing the boom from reaching the ground. The size, weight, and shape of the aircraft influence the initial strength of the shockwave. Aircraft with more slender designs displace less air, which helps mitigate the strength of the resultant shock waves.