However, it is primarily and more directly related to molecular speed and thus temperature than it is to anything else. This next table is in the Metric System and indicates the speed of sound in kilometers per hour (km/h), meters per second (m/s), and knots (kts) for altitudes from 0 m to 30,000 m in 1,000 m increments. Since temperature and sound velocity normally decrease with increasing altitude, sound is refracted upward, away. According to the modern theory of sound, its speed in the Earths atmosphere depends only on temperature and does not depend on its density (height). For air, which is a mixture of molecules, you will need to use average values for the adiabatic constant and molecular mass. These pressure waves flow away from the airplane at the speed of sound, which at standard day temperature of 59 F, is 761 mph. One might also note that the temperature is lower as one ascends primarily because the pressure is lower as one ascends and then conclude that the speed difference is equally attributable to lower pressure (or lower density for that matter). The speed of sound varies with temperature. At higher temperatures the molecules have more energy and are moving faster than at lower temperatures, hence the speed of sound at higher temperatures is faster than at lower temperatures. ![]() Since molecular speed in a gas is a direct function of average molecular kinetic energy and that is a direct function of temperature, the speed of sound in a gas will also be a function of temperature.Īt the cruise altitude of the Concorde the atmospheric temperature is below that of the standard atmospheric temperature at sea level, so the speed of sound is also lower. The speed of sound in air at 0 oC (273.15 K) and absolute pressure 1 bar can be calculated as c (1.4 (286.9 J/K kg) (273.15 K))1/2 331.2 (m/s) where k 1.4 and R 286.9 (J/K kg) The speed of sound in air at 20 oC (293.15 K) and absolute pressure 1 bar can be calculated as c (1.4 (286.9 J/K kg) (293.15 K))1/2 343. Therefore, the sound wave can not possibly move through the gas at a speed greater than that of the individual molecules themselves, and in fact must move at a lower speed than that due to the random nature of molecular movement. ![]() A sound wave moving through a gas requires a small scale bulk movement of gas molecules back and forth as pressure at any locations builds or falls.
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