A sound wave is a pressure disturbance that propagates through the air. This disturbance can be caused by a vibration (for example the vibrations of the vocal cords) or even turbulence in an air flow, such as a whistle.
Take the example of a loudspeaker: it transforms an electrical signal into vibrations of its membrane. When the membrane goes ahead, it pushes the air in front of it allowing an overpressure. But when she backs up, she creates a low pressure zone.
These disturbances propagate close to close, creating what is called a sound wave. Sound waves can travel through different media. Their speed (or celerity) depends on the properties of the medium. In air, the speed of sound depends mainly on temperature.
At a temperature of 20°C, the speed of sound is 342 m/s. At 0°C, the speed of sound is 331 m/s. As a first approximation, the speed of sound in air c is proportional to the square root of the temperature T in degrees Kelvin (°K = °C + 273.15).
The speed of sound propagation depends on the medium. For example, in water at 15°C sound propagates at about 1500 m/s. In steel, the speed of propagation is about 5000 m/s.
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This article takes place in our Acoustic 101 serie!