page_title: Sound
section_title: Speed of sound
hierarchical_section_title: Sound / Physics of sound / Speed of sound
caption_attribution_description: Off the coast of Pusan, South Korea: An F/A-18 Hornet assigned to Strike Fighter Squadron One Five One (VFA-151) breaks the sound barrier in the skies over the Pacific Ocean. VFA-151 is deployed aboard USS Constellation (CVN 64). This is an edited version (reduced grain).
context_page_description: In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid.
  In human physiology and psychology, sound is the reception of such waves and their perception by the brain. Only acoustic waves that have frequencies lying between about 20 Hz and 20 kHz, the audio frequency range, elicit an auditory percept in humans. In air at atmospheric pressure, these represent sound waves with wavelengths of 17 meters to 1.7 centimetres. Sound waves above 20 kHz are known as ultrasound and are not audible to humans. Sound waves below 20 Hz are known as infrasound. Different animal species have varying hearing ranges.
context_section_description: The speed of sound depends on the medium the waves pass through, and is a fundamental property of the material. The first significant effort towards measurement of the speed of sound was made by Isaac Newton. He believed the speed of sound in a particular substance was equal to the square root of the pressure acting on it divided by its density:
  This was later proven wrong when found to incorrectly derive the speed. The French mathematician Laplace corrected the formula by deducing that the phenomenon of sound travelling is not isothermal, as believed by Newton, but adiabatic.  He added another factor to the equation—gamma—and multiplied
  
  by
  ,
  thus coming up with the equation
  .
  Since
  ,
  the final equation came up to be
  ,
  which is also known as the Newton–Laplace equation. In this equation, K is the elastic bulk modulus, c is the velocity of sound, and  is the density.  Thus, the speed of sound is proportional to the square root of the ratio of the bulk modulus of the medium to its density.
  Those physical properties and the speed of sound change with ambient conditions.  For example, the speed of sound in gases depends on temperature.  In 20 °C (68 °F) air at sea level, the speed of sound is approximately 343 m/s (1,230 km/h; 767 mph) using the formula v [m/s] = 331 + 0.6 T [°C].  In fresh water, also at 20 °C, the speed of sound is approximately 1,482 m/s (5,335 km/h; 3,315 mph). In steel, the speed of sound is about 5,960 m/s (21,460 km/h; 13,330 mph).  The speed of sound is also slightly sensitive, being subject to a second-order anharmonic effect, to the sound amplitude, which means there are non-linear propagation effects, such as the production of harmonics and mixed tones not present in the original sound (see parametric array).
  If relativistic effects are important, the speed of sound is calculated from the relativistic Euler equations.
caption: U.S. Navy F/A-18 approaching the speed of sound. The white halo is formed by condensed water droplets thought to result from a drop in air pressure around the aircraft (see Prandtl–Glauert singularity).[12]
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