The concept of the “sound barrier” refers to an invisible threshold that objects encounter when accelerating to speeds approaching that of sound. This aerodynamic challenge historically represented a significant obstacle in aviation. Understanding this phenomenon involves recognizing how an object’s movement interacts with the very medium it travels through.
Understanding the Speed of Sound
Sound is a form of energy that travels as a wave of pressure, propagating through a medium by causing its particles to vibrate. These vibrations create alternating regions of compression and rarefaction, moving outward from the sound source. The “speed of sound” quantifies how fast these vibrations travel. This speed is not fixed; it varies depending on the properties of the medium and the environmental conditions it encounters.
The term Mach 1 is used to represent the speed of sound in a specific environment. For instance, in dry air at sea level and a temperature of 20°C (68°F), the speed of sound is approximately 343 meters per second (1,235 kilometers per hour or 767 miles per hour). This measurement serves as a reference point for evaluating an object’s velocity relative to sound waves. This speed changes, meaning Mach 1 is not a constant value.
The Phenomenon of the Sound Barrier
As an object, such as an aircraft, approaches the speed of sound, it begins to “catch up” to the pressure waves it generates. These pressure waves, normally spreading ahead, compress and pile up in front of it. This accumulation leads to increased aerodynamic drag and other effects, historically challenging supersonic speeds. The air ahead of the object receives no warning of its approach, causing it to “plow” through the air.
When the object surpasses Mach 1, it leaves these compressed waves behind, forming a distinct shockwave. This shockwave is a sudden, sharp change in air pressure, density, and temperature, propagating outward in a cone shape behind the object. When this shockwave reaches an observer, they hear a loud, impulsive noise known as a “sonic boom.” This sound is similar to thunder or an explosion and is caused by the sudden onset and release of pressure.
Visual effects can accompany the breaking of the sound barrier, especially in humid conditions. A condensation cloud, often cone-shaped or disk-shaped, can briefly form around the aircraft. This occurs because the rapid pressure drop within the shockwave causes the air to cool suddenly, leading to the condensation of water vapor into visible droplets. This visual evidence highlights the physical changes occurring as an object transitions from subsonic to supersonic flight.
Factors Affecting Sound Speed
Sound speed is influenced by several physical properties of the medium. Temperature is a primary factor, especially in gases like air. Sound travels faster in warmer air because molecules possess more kinetic energy at higher temperatures, causing them to vibrate and transmit sound waves more quickly. Conversely, in colder air, molecules move more slowly, reducing the speed of sound.
Altitude impacts sound speed because temperature generally decreases with increasing altitude. This means Mach 1 is typically lower at higher altitudes. For example, while Mach 1 is approximately 1,235 km/h (767 mph) at sea level and 20°C, it can drop to around 1,062 km/h (660 mph) at an altitude of 35,000 feet. The reduction in temperature, rather than pressure directly, is the main reason for this change.
The medium also plays a role in determining sound speed. Sound waves propagate most slowly in gases, faster in liquids, and fastest in solids. This difference arises from the molecular structure and elasticity of materials; particles are more closely packed in solids, allowing vibrations to transfer more efficiently than in liquids or gases. For instance, sound travels at about 343 m/s in air, but it can reach approximately 1,481 m/s in water and around 5,120 m/s in iron.