The “sound barrier” refers to challenges and phenomena emerging when an object approaches and surpasses the speed of sound waves through a medium. This metaphorical barrier highlights changes in physics and aerodynamics at extreme velocities. Understanding this transition involves exploring the nature of sound and its interactions with a moving object. The journey from subsonic to supersonic speeds reveals an interplay of pressure, waves, and energy.
Understanding the Sound Barrier
The “sound barrier” is not a physical wall but a term describing the aerodynamic effects on objects moving at or above the speed of sound. Sound travels as pressure waves propagating through a medium like air. The speed of sound is not constant; it changes based on the medium’s properties, such as temperature and density. Sound travels faster in warmer, denser air.
In dry air at 20 degrees Celsius (68 degrees Fahrenheit) at sea level, the speed of sound is approximately 343 meters per second (about 767 miles per hour). Scientists use the Mach number to describe an object’s speed relative to the speed of sound. Mach 1 represents the speed of sound; an object traveling at Mach 1 moves at the speed of sound. Speeds below Mach 1 are subsonic, while speeds above Mach 1 are supersonic.
The Physics of Breaking the Sound Barrier
As an object moves through the air, it creates pressure waves that propagate away from it at the speed of sound. When the object travels at subsonic speeds, these pressure waves move ahead of the object, preparing the air molecules for its approach. Air can then smoothly flow around the object. As the object’s speed increases and approaches Mach 1, it begins to catch up with its own pressure waves.
At Mach 1, the object moves as fast as the pressure waves it generates. This causes the waves to pile up directly in front of the object, forming a region of highly compressed air. This accumulation of pressure waves creates wave drag, making further acceleration challenging. Once the object exceeds Mach 1, it outruns these accumulated pressure waves.
As the object continues to accelerate past Mach 1, it leaves its pressure waves behind. Instead of smooth air flow, the object continuously generates new pressure waves that cannot outrun it. These waves coalesce into conical shock waves that trail behind the object. These shock waves are characterized by rapid changes in air pressure, temperature, and density. The energy within these shock waves causes the audible phenomenon associated with breaking the sound barrier.
The Sonic Boom Explained
The audible effect of an object breaking the sound barrier is the sonic boom. This loud sound is not a single explosion but the continuous effect of shock waves generated by a supersonic object as they expand and reach an observer on the ground. As the conical shock wave trails behind the object, it sweeps across the landscape below, creating a momentary but significant pressure change. This rapid change in air pressure is perceived as a sudden, loud sound resembling thunder or an explosion.
The characteristics of a sonic boom depend on several factors, including the object’s size, shape, altitude, and atmospheric conditions. Larger objects and lower altitudes produce more intense booms because the shock waves have less distance to dissipate their energy. Objects traveling supersonically generate these shock waves, meaning a sonic boom is heard only when the conical shock wave passes over an observer. For example, an aircraft flying at supersonic speeds generates a continuous “boom carpet” on the ground below, rather than a single point of sound.
Common Objects That Break the Sound Barrier
Numerous objects, both natural and man-made, are capable of breaking the sound barrier, each achieving supersonic speeds through different mechanisms. Supersonic aircraft, such as military fighter jets and the now-retired Concorde passenger airliner, are designed to operate at speeds above Mach 1. These aircraft use engines and aerodynamic designs that manage pressure changes and drag associated with supersonic flight. Their pointed noses and swept wings help to minimize the strength of the shock waves they generate.
Another common example is the tip of a whip, which can easily exceed the speed of sound. The whip’s design, tapering from a thick handle to a thin tip, allows the energy from a flick of the wrist to be concentrated at the end. As the loop of the whip travels down its length, its speed increases, causing the tip to accelerate past Mach 1 and create a miniature sonic boom, the characteristic “crack.”
Bullets fired from firearms also routinely travel at supersonic speeds. The explosive force within the cartridge propels the bullet out of the barrel at supersonic velocities, creating a small sonic boom that contributes to the distinct sound of a gunshot. Meteors entering Earth’s atmosphere at high speeds also generate sonic booms, often heard as rumbling or thunder-like sounds as they descend.