While speed is often measured in miles or kilometers per hour, a different system is needed when objects approach or exceed the speed of sound. This system uses Mach numbers, which express an object’s speed as a ratio relative to the speed of sound in its surrounding environment. This ratio provides a consistent way to describe high-speed travel, as the speed of sound itself is not a fixed value.
The Speed of Sound
The speed of sound represents the rate at which sound waves propagate through a medium. This speed is not constant; it changes based on environmental factors, with temperature being the primary influence.
As air temperature decreases, the speed of sound also decreases because the air molecules move more slowly, transmitting vibrations less efficiently. Altitude also affects the speed of sound due to temperature changes in the atmosphere. At sea level, under standard atmospheric conditions (around 20 degrees Celsius or 68 degrees Fahrenheit), the speed of sound in dry air is approximately 343 meters per second. This translates to about 1,235 kilometers per hour or 767 miles per hour.
Decoding the Mach Number
The Mach number, named after Austrian physicist Ernst Mach, quantifies an object’s speed by comparing it to the local speed of sound. It is a dimensionless ratio, meaning it has no units. When an object travels at the speed of sound, its speed is designated as Mach 1.
“Mach 2” signifies that an object is moving at twice the local speed of sound. For instance, if the speed of sound at a given altitude and temperature is 767 miles per hour, then Mach 2 would be approximately 1,534 miles per hour. In metric terms, if the speed of sound is 1,235 kilometers per hour, Mach 2 equates to roughly 2,470 kilometers per hour. Flight speeds are generally categorized: subsonic (below Mach 1), transonic (around Mach 1), supersonic (Mach 1 to Mach 5), and hypersonic (above Mach 5).
The Impact of Supersonic Travel
When an object travels at or above the speed of sound, it creates distinct physical phenomena. As the object pushes through the air, it compresses the air molecules in front of it, forming pressure waves. At supersonic speeds, these pressure waves cannot propagate away from the object fast enough and coalesce into shockwaves.
These shockwaves extend outward from the object in a cone shape. When they reach the ground, they are perceived as a loud, explosive sound known as a sonic boom.
Where Mach 2 Matters
Speeds around Mach 2 have found significant application, predominantly in military aviation and, historically, in commercial air travel. Modern fighter jets, such as the F-15 Eagle, F-22 Raptor, and the MiG-25, are designed to operate at or above Mach 2 for tactical advantages like rapid interception and quick ingress or egress from combat zones. Reconnaissance aircraft, like the SR-71 Blackbird, also utilized such speeds for intelligence gathering, allowing them to outrun threats.
In the commercial sector, the Anglo-French Concorde was a notable example, routinely cruising at Mach 2.04, reducing transatlantic flight times from hours to mere minutes. While supersonic commercial travel offered significant time savings, it faced challenges including high fuel consumption, substantial operational costs, and noise restrictions due to sonic booms over populated areas. These factors ultimately led to the retirement of commercial supersonic passenger services, though research continues into quieter, more efficient designs.