A sonic boom is the dramatic sound resulting from shockwaves created by an object traveling faster than the speed of sound. This phenomenon is the audible effect of a rapid pressure change that occurs as the object outruns the sound waves it generates. The energy released by this compressed wave is often compared to a clap of thunder, yet it is a continuous acoustic event following the supersonic source. Understanding the speed of a sonic boom requires examining the speed of the source, the physics of wave compression, and the atmospheric conditions that influence the speed of sound itself.
Defining the Speed Threshold (Mach 1)
The speed commonly associated with a sonic boom is the velocity an object must reach to exceed the speed of sound, which is designated as Mach 1. The Mach number is a ratio that compares the speed of an object to the local speed of sound in the surrounding medium. To produce a boom, an aircraft or other object must achieve a speed greater than Mach 1, meaning it is traveling in a supersonic regime.
The numerical value of Mach 1 is not a fixed constant but is dynamic, changing with the conditions of the atmosphere. At sea level and a standard temperature of 59 degrees Fahrenheit (15 degrees Celsius), the speed of sound is approximately 761 miles per hour (1,225 kilometers per hour). This is the speed threshold that must be crossed for a sonic boom to be generated near the ground.
When the object reaches Mach 1, it is traveling at the same rate as the disturbances it is creating, causing those waves to merge. Speeds below Mach 1 are called subsonic, while the transonic range (Mach 0.8 to Mach 1.2) is where the first shock waves begin to form. The speed beyond Mach 1 determines the angle of the resulting shockwave cone, with faster speeds creating a narrower cone.
Environmental Factors Influencing Speed
The speed of sound is highly dependent on the temperature of the air through which it travels. Sound waves are transmitted through collisions between air molecules; in warmer air, molecules move faster and transmit vibrations more quickly, increasing the speed of sound. Conversely, in colder air, molecular motion is slower, which decreases the speed of sound.
This temperature dependency means that the numerical speed required to reach Mach 1 changes significantly with altitude. As altitude increases, the air temperature generally decreases, causing the speed of sound to drop. For instance, at high altitudes, such as 36,000 to 60,000 feet, the speed of sound can be around 661 miles per hour, which is much slower than at sea level.
Humidity is another factor, though its effect is less pronounced than temperature. Humid air contains water molecules, which are lighter than the nitrogen and oxygen molecules that make up dry air. When water vapor replaces these heavier molecules, the average molecular weight of the air decreases, leading to a slight increase in the speed of sound. Pressure has a minimal effect on the speed of sound unless it is accompanied by a change in temperature.
The Physics of Shockwave Formation
The sonic boom itself is the result of pressure waves that have been compressed and merged because the source is moving faster than the waves can propagate away. When an aircraft travels at subsonic speeds, the pressure waves it creates radiate outward in all directions, including forward, notifying an observer of its approach before it arrives. As the object accelerates toward Mach 1, the waves in front of it begin to pile up, since the object is catching up to its own disturbances.
Upon exceeding Mach 1, the object leaves the sound waves behind, and the accumulated pressure waves merge into a single, intense shockwave. This shockwave forms a three-dimensional cone that trails the supersonic object, similar to the wake behind a boat moving faster than the water waves it creates. The observer hears the boom only when the edge of this conical shockwave passes over their location on the ground.
The pressure signature of a typical sonic boom is characterized by an abrupt rise in pressure, followed by a linear decrease to a negative pressure, and then a sudden return to ambient atmospheric pressure. This profile is often called an “N-wave” because of its shape on a pressure-time graph. These two sudden pressure changes are perceived as two distinct booms, which is why a supersonic aircraft often produces a “double boom.”
Measuring a Sonic Boom’s Intensity
The intensity of a sonic boom reaching the ground is primarily measured in terms of peak overpressure, the maximum increase in pressure above the normal atmospheric pressure. This overpressure is usually quantified in pounds per square foot (psf) or in pounds per square inch (psi). For large supersonic aircraft, the peak overpressure typically ranges from 1 to 2 psf, which is a very small change in pressure.
Despite the small pressure change, the sound is perceived as loud because the change occurs so rapidly. Loudness is also measured in decibels (dB), often using specialized metrics like Perceived Loudness in Decibels (PLdB) to represent human annoyance. A typical sonic boom from a large aircraft can register between 110 and 140 dB, comparable to a thunderclap or explosion.
The intensity experienced by an observer is significantly affected by the distance between the aircraft and the ground. Higher-altitude flights generally result in a lower-intensity boom because the shockwave dissipates more energy as it travels a greater distance through the atmosphere. Atmospheric turbulence can also distort the shockwave, causing the measured overpressure values to vary considerably along the ground track.