Speed is commonly expressed in miles per hour (MPH), but some velocities extend far beyond everyday human experience. These extreme speeds are categorized based on their relationship to the speed of sound, revealing unique physical phenomena.
Understanding Supersonic Speed
Supersonic speed refers to any velocity that exceeds the speed of sound, which is designated as Mach 1. The Mach number is a ratio comparing an object’s speed to the speed of sound in its surrounding medium. For an object moving through dry air at sea level with a temperature of 68°F (20°C), Mach 1 is approximately 767 miles per hour (MPH).
The speed of sound is not a fixed value; it changes based on atmospheric conditions. Temperature is the most significant factor influencing the speed of sound, with higher temperatures leading to faster sound propagation. Altitude also plays a role because temperature typically decreases with increasing altitude, which in turn reduces the speed of sound. Humidity also slightly increases the speed of sound by reducing the overall density of the air.
The Sound Barrier and Sonic Booms
When an object approaches and then exceeds the speed of sound, it encounters the “sound barrier.” As the object compresses the air in front of it, the pressure waves it continuously generates cannot propagate ahead. Instead, these waves combine and accumulate, forming intense shock waves.
These shock waves spread outward from the object in a cone-shaped pattern, similar to a boat’s wake. When this compressed air reaches an observer, it is perceived as a sudden, loud noise known as a sonic boom. A sonic boom sounds much like an explosion or a clap of thunder.
Sonic booms are a continuous effect, not single events occurring only when an object crosses Mach 1. As long as the object travels at supersonic speeds, it generates these shock waves. The intensity and characteristics of a sonic boom are influenced by the object’s speed, size, shape, and altitude. Larger or faster objects generally create stronger sonic booms.
Speed Categories: Subsonic, Supersonic, and Hypersonic
Speed is broadly categorized relative to the speed of sound, defining distinct flight regimes. Speeds below Mach 1 are classified as subsonic. Most commercial airplanes, like airliners, operate in this regime, typically cruising at speeds around Mach 0.78 to Mach 0.82. In subsonic flight, air flows smoothly around an aircraft, and pressure disturbances can travel ahead of the object.
Supersonic speeds encompass velocities from Mach 1 up to Mach 5. This range includes objects like military fighter jets and some specialized experimental aircraft.
Beyond Mach 5, speeds are referred to as hypersonic. At these extreme velocities, which can reach over 3,800 miles per hour, the behavior of air molecules changes significantly, leading to intense heating and complex aerodynamic challenges. Examples of objects that achieve hypersonic speeds include certain experimental aircraft, missiles, and spacecraft during re-entry into Earth’s atmosphere.