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Speed of Light. The speed of light in vacuum, commonly denoted c, is a universal physical constant that is exactly equal to 299,792,458 metres per second (approximately 300,000 kilometres per second; 186,000 miles per second; 671 million miles per hour).
Wikipedia is a free content online encyclopedia written and maintained by a community of volunteers, known as Wikipedians, through open collaboration and the wiki software MediaWiki.Wikipedia is the largest and most-read reference work in history, and is consistently ranked among the ten most visited websites; as of April 2024, it was ranked fourth by Semrush, and seventh by Similarweb.
- Earth
- History
- Basic Concepts
- Equations
- Dependence on The Properties of The Medium
- Altitude Variation and Implications For Atmospheric Acoustics
- Details
- Effect of Frequency and Gas Composition
- Mach Number
- Experimental Methods
In Earth's atmosphere, the speed of sound varies greatly from about 295 m/s (1,060 km/h; 660 mph) at high altitudes to about 355 m/s (1,280 km/h; 790 mph) at high temperatures.
Sir Isaac Newton's 1687 Principia includes a computation of the speed of sound in air as 979 feet per second (298 m/s). This is too low by about 15%. The discrepancy is due primarily to neglecting the (then unknown) effect of rapidly fluctuating temperature in a sound wave (in modern terms, sound wave compression and expansion of air is an adiabati...
The transmission of sound can be illustrated by using a model consisting of an array of spherical objects interconnected by springs. In real material terms, the spheres represent the material's molecules and the springs represent the bondsbetween them. Sound passes through the system by compressing and expanding the springs, transmitting the acoust...
The speed of sound in mathematical notation is conventionally represented by c, from the Latin celeritasmeaning "swiftness". For fluids in general, the speed of sound cis given by the Newton–Laplace equation: 1. K s {\displaystyle K_{s}} is a coefficient of stiffness, the isentropic bulk modulus(or the modulus of bulk elasticity for gases); 2. ρ {\...
The speed of sound is variable and depends on the properties of the substance through which the wave is travelling. In solids, the speed of transverse (or shear) waves depends on the shear deformation under shear stress (called the shear modulus), and the density of the medium. Longitudinal (or compression) waves in solids depend on the same two fa...
In the Earth's atmosphere, the chief factor affecting the speed of sound is the temperature. For a given ideal gas with constant heat capacity and composition, the speed of sound is dependent solely upon temperature; see § Detailsbelow. In such an ideal case, the effects of decreased density and decreased pressure of altitude cancel each other out,...
Speed of sound in ideal gases and air
For an ideal gas, K (the bulk modulus in equations above, equivalent to C, the coefficient of stiffness in solids) is given by 1. γ is the adiabatic index also known as the isentropic expansion factor. It is the ratio of the specific heat of a gas at constant pressure to that of a gas at constant volume (C p / C v {\displaystyle C_{p}/C_{v}} ) and arises because a classical sound wave induces an adiabatic compression, in which the heat of the compression does not have enough time to escape th...
Effects due to wind shear
The speed of sound varies with temperature. Since temperature and sound velocity normally decrease with increasing altitude, sound is refracted upward, away from listeners on the ground, creating an acoustic shadow at some distance from the source. Wind shear of 4 m/(s · km) can produce refraction equal to a typical temperature lapse rate of 7.5 °C/km. Higher values of wind gradient will refract sound downward toward the surface in the downwind direction,eliminating the acoustic shadow on the...
Tables
In the standard atmosphere: 1. T0 is 273.15 K (= 0 °C = 32 °F), giving a theoretical value of 331.3 m/s (= 1086.9 ft/s = 1193 km/h = 741.1 mph = 644.0 kn). Values ranging from 331.3 to 331.6 m/smay be found in reference literature, however; 2. T20 is 293.15 K (= 20 °C = 68 °F), giving a value of 343.2 m/s (= 1126.0 ft/s = 1236 km/h = 767.8 mph = 667.2 kn); 3. T25 is 298.15 K (= 25 °C = 77 °F), giving a value of 346.1 m/s (= 1135.6 ft/s = 1246 km/h = 774.3 mph = 672.8 kn). In fact, assuming an...
General physical considerations
The medium in which a sound wave is travelling does not always respond adiabatically, and as a result, the speed of sound can vary with frequency. The limitations of the concept of speed of sound due to extreme attenuation are also of concern. The attenuation which exists at sea level for high frequencies applies to successively lower frequencies as atmospheric pressure decreases, or as the mean free path increases. For this reason, the concept of speed of sound (except for frequencies approa...
Practical application to air
By far, the most important factor influencing the speed of sound in air is temperature. The speed is proportional to the square root of the absolute temperature, giving an increase of about 0.6 m/sper degree Celsius. For this reason, the pitch of a musical wind instrument increases as its temperature increases. The speed of sound is raised by humidity. The difference between 0% and 100% humidity is about 1.5 m/sat standard pressure and temperature, but the size of the humidity effect increase...
Mach number, a useful quantity in aerodynamics, is the ratio of air speed to the local speed of sound. At altitude, for reasons explained, Mach number is a function of temperature.Aircraft flight instruments, however, operate using pressure differential to compute Mach number, not temperature. The assumption is that a particular pressure represents...
A range of different methods exist for the measurement of sound in air. The earliest reasonably accurate estimate of the speed of sound in air was made by William Derham and acknowledged by Isaac Newton. Derham had a telescope at the top of the tower of the Church of St Laurence in Upminster, England. On a calm day, a synchronized pocket watch woul...
Sigmund Freud (/ f r ɔɪ d / FROYD, German: [ˈziːkmʊnt ˈfrɔʏt]; born Sigismund Schlomo Freud; 6 May 1856 – 23 September 1939) was an Austrian neurologist and the founder of psychoanalysis, a clinical method for evaluating and treating pathologies seen as originating from conflicts in the psyche, through dialogue between patient and psychoanalyst, and the distinctive theory of mind and ...
Amyotrophic lateral sclerosis (ALS), also known as motor neurone disease (MND) or Lou Gehrig's disease in the United States, is a rare but terminal neurodegenerative disorder that results in the progressive loss of both upper and lower motor neurons that normally control voluntary muscle contraction.[3] ALS is the most common form of the motor ...
Venus is the second planet from the Sun. It is a terrestrial planet and is the closest in mass and size to its orbital neighbour Earth. Venus is notable for having the densest atmosphere of the terrestrial planets, composed mostly of carbon dioxide with a thick, global sulfuric acid cloud cover. At the surface it has a mean temperature of 737 ...
Joseph Vissarionovich Stalin (born Dzhugashvili; 18 December [O.S. 6 December] 1878 – 5 March 1953) was a Soviet politician and revolutionary who was the longest-serving leader of the Soviet Union from 1924 until his death in 1953. He held power as General Secretary of the Communist Party from 1922 to 1952, and Chairman of the Council of Ministers (head of government) from 1941 until his death.
