The sound barrier represents a fundamental limit to how fast an object can travel through the air. For decades, this speed threshold was considered an obstacle that aircraft designers and pilots struggled to overcome. Accelerating a machine past its own emitted noise drove significant advances in aerodynamics and propulsion. Understanding this barrier requires exploring the nature of sound and the physics that govern its speed through the atmosphere.
Defining the Speed of Sound and Mach 1
The speed required to break the sound barrier is defined by the Mach number. Mach 1 is the precise speed of sound in the surrounding medium, meaning an object traveling at Mach 1 moves exactly as fast as the pressure waves it creates. The scale is straightforward: Mach 0.8 is 80% of the speed of sound, while Mach 2 is twice that speed.
Under standard atmospheric conditions at sea level, the speed of sound is approximately 761 miles per hour (1,225 kilometers per hour), or about 343 meters per second. This standard is based on an air temperature of 59 degrees Fahrenheit (15 degrees Celsius). Objects moving below Mach 1 are in subsonic flight, where their pressure waves move ahead of them.
As an object’s speed nears Mach 1, it enters the transonic range, typically defined as Mach 0.8 to Mach 1.2. This range is where the most dramatic aerodynamic effects, including the formation of shock waves, begin to occur. Once an object fully exceeds Mach 1, it enters supersonic flight, continuously leaving its sound behind.
The Physics of the Pressure Barrier
Sound travels through the air as a compression wave, a mechanical disturbance that causes air molecules to vibrate and transmit energy. When an object moves through the atmosphere, it continuously generates these pressure waves. In subsonic flight, the pressure waves travel faster than the object, propagating outward and ahead of the object’s approach.
As the object accelerates toward Mach 1, it catches up to the pressure waves it creates. The waves pile up immediately ahead of the object, resulting in a massive, high-pressure zone. This intensely compressed air creates immense resistance, historically called the sound barrier. The force required to push through this dense pressure front causes a sharp increase in aerodynamic drag. Once the object has sufficient thrust to overcome this resistance, it continues into supersonic flight.
Environmental Factors That Change the Required Speed
The specific speed required to achieve Mach 1 is not constant because the speed of sound is highly dependent on the properties of the air. For any given gas, the speed of sound is governed almost entirely by temperature: as temperature increases, air molecules move faster, allowing them to transmit the pressure wave energy more quickly. Conversely, when the temperature drops, the speed of sound decreases significantly.
This temperature dependence is why the speed required to reach Mach 1 changes with altitude. Since temperature generally decreases as altitude increases, the sound barrier is easier to break at high altitudes. For example, at 30,000 feet, the speed of sound may be closer to 678 mph (1,091 km/h). Temperature remains the primary factor determining the local speed of sound and the necessary velocity for Mach 1.
The Phenomenon of the Sonic Boom
Once an object exceeds Mach 1, it continuously generates a shock wave rather than ordinary sound waves. This shock wave is a single, strong disturbance formed from the accumulated compression waves that could not propagate ahead of the object. The shock wave expands backward and outward from the object in a conical shape, often referred to as a Mach cone.
The familiar sonic boom is the audible result of this pressure cone sweeping across a listener on the ground. When the shock wave passes, it causes a sudden, rapid change in air pressure, which the human ear perceives as a loud, explosive noise. Contrary to common belief, the sonic boom is not a single event that occurs only at the moment the barrier is broken.
Instead, the boom is generated continuously for the entire duration of supersonic flight, following the aircraft along its flight path. A visual phenomenon, the vapor cone, is often associated with the moment of breaking the barrier. This temporary, cone-shaped cloud of condensed water vapor surrounds the object, caused by the sudden drop in air pressure and temperature near the shock wave.