Knowledge of sound speed

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　The speed of sound, also known as “sonic velocity,” refers to the speed at which sound waves propagate through a medium. Its size varies depending on the properties and state of the medium. Generally speaking, the value of the speed of sound is higher in solids than in liquids, and higher in liquids than in gases. The speed of sound in air, under standard atmospheric conditions, is approximately 340 meters per second, or 1224 kilometers per hour. The speed of sound also changes with atmospheric temperature; in the troposphere, as altitude increases, the temperature drops and the speed of sound decreases. In the lower stratosphere, the temperature remains constant with height, and the speed of sound also remains constant at 295.2 meters per second. The laws of air flow and the aerodynamic properties of aircraft differ significantly when the flight speed is below or above the speed of sound; therefore, when studying the movement of aircraft in the atmosphere, the speed of sound is a very important reference value. 　　Taking a tuning fork as an example, when we strike it, the fork vibrates. As it vibrates, it pushes the air around it back and forth, causing the pressure of the air to vary from high to low. This leads to variations in the density of air molecules, which then spread outward through collisions between these molecules; thus, the sound waves generated by the tuning fork are transmitted outward. When sound waves propagate, the direction of vibration of the air molecules is the same as the direction in which the wave moves; such waves are called \"longitudinal waves\". 　　Substances such as air that can transmit sound waves are what we call “media”. Sound waves require a medium in order to be transmitted; in a vacuum, where there is no medium for sound waves to travel through, no sound can be heard. In addition to air, liquids (such as water) and solids (such as wood, glass, steel) also serve as media for sound. Moreover, because the molecules in liquids and solids are arranged more closely together, sound travels faster through them than through air. The speed of sound in water is about five times that in air, and in steel it is nearly twenty times faster than in air. 　　In daily life, sound is mostly transmitted through air. The speed of sound in air was measured for the first time in history in the year 1708. At that time, an Englishman named Deham stood on the roof of a church, watching the cannons that were being fired 19 kilometers away. He measured the time elapsed between the flash from the cannons and the sound of the explosions, and by taking the average of multiple measurements, he obtained a value for the speed of sound that is quite close to the current figure: 343 meters per second at 20°C. Factors affecting the speed of sound The sound waves emitted from a sound source propagate outward at a certain speed, which means that the energy of those sound waves also propagates outward at that same speed. As is known, sound waves can travel through all materials (except a vacuum). Its propagation speed is determined by certain physical properties of the sound-transmitting medium, primarily its mechanical properties. For example, the speed of sound is related to the density and elastic properties of the medium, and therefore it also changes with state parameters such as the temperature and pressure of the medium. The speed of sound in gases is about several hundred meters per second, and it increases as the temperature rises. At 0°C, the speed of sound in air is 331.4 meters per second, while at 15°C it is 340 meters per second; for every 1°C increase in temperature, the speed of sound increases by approximately 0.6 meters per second. Generally, the speed of sound is highest in solid media, lower in liquid media, and lowest in gas media. Furthermore, the speed of sound in a non-uniform medium varies from place to place. The sound speed in an anisotropic medium varies with the direction of propagation. 　　In some cases, the speed of sound is also related to the amplitude, frequency, and mode of vibration of the sound wave itself (such as the speed of sound for longitudinal waves and transverse waves). If the size of the propagation medium is not large enough, its boundaries also have an effect on the speed of sound. Therefore, in order to accurately represent the acoustic properties of a sound-transmitting medium using the value of the speed of sound, free from the influence of its geometric shape, it is generally necessary to specify the speed of sound wave propagation under the condition that the size of the medium is large enough (theoretically infinite). Sometimes, for practical convenience, the speed of sound under certain special conditions is also listed, such as the speed of sound in solid rods. Sound speed in some media: Vacuum – 0 m/s (i.e., sound cannot travel there). Air (15°C) – 340 m/s; Air (25°C) – 346 m/s. Cork – 500 m/s. Kerosene (25°C) – 1324 m/s. Distilled water (25°C) – 1497 m/s. Seawater (25°C) – 1531 m/s. Copper (rod) – 3750 m/s. Marble – 3810 m/s. Aluminum (rod) – 5000 m/s. Iron (rod) – 5200 m/s