Atmospheric General Circulation Models
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In a gas,
ACOUSTIC RADIATION Alain Weill Bur. Jussieu, LATMOS, Laboratoire Atmosphere Milieux Observations Spatiales, Paris, France
Definition Acoustic. One branch of physics which studies sound. The word acoustic comes from the Greek word akoustikos. “which is related to hearing.” Sound. It comes from the Latin word sonum: “which is related to the hearing sensation created by perturbation of the material medium (elastic, fluid, solid).” In physics, it is a vibration, generally in a gas, created by expansion and compression of gas molecules. Sound waves propagate in the fluid medium and do not propagate in the vacuum. Sounds can be produced in the atmosphere and oceans by living animals or by structures through interaction with the wind, as, for example, trees murmuring, mountains roaring, river sounds, and waves breaking and can be created by various instruments such as music instruments, microphones, speakers, and transducers and also by instruments developed for remote sensing such as SONAR (Sound Navigation and Ranging), ADCP (Acoustic Doppler Current Profiler), and SODAR (Sound Detection and Ranging or what are called echo sounders for atmosphere and ocean). A sound propagating in a medium is characterized by its speed c: c2 ¼ @P=@r
(1)
where P is the pressure and r the density, and @ is derived.
c ¼ ðgP=rÞ1=2
(2)
where g is the heat capacity ratio. Notice that sound speed in the air for standard conditions of temperature and pressure near the surface is close to 340 m/s, while at the ocean surface it is close to 1,500 m/s, which is faster. This will have an incidence on different ways for acoustic signal processing to be done in the ocean and atmosphere. Sound or rather a sound wave is a mechanical pressure oscillation, which is generally longitudinally propagating. Period T. It is the signal duration corresponding to the time when the sound wave is reproduced identically. Frequency. f ¼ 1/T (T in s and f in hertz). Frequency audio spectrum (distribution of acoustic energy as function of frequencies can be divided in four zones related to human hearing power: 0–20 Hz infrasound (not audible), 20–300 Hz is low-pitched, 300–6,000 Hz is medium range, 6,000–20,000 Hz is high-pitched, more than 20,000 Hz are ultrasonic sounds (not audible). Sound amplitude. It corresponds to acoustic pressure fluctuation of the medium Dp (amount of energy in the sound wave) measured at one point of a surface S. It is the ratio of pressure P by the surface element S. I ¼ P=S in W/m2 For a spherical acoustic source, the intensity at distance r is Ir ¼ P ðsound power of the sourceÞ 4p r2 Radiation. It is the way acoustic wave energy radiates and concerns acoustic rays from the acoustic sources through
E.G. Njoku (ed.), Encyclopedia of Remote Sensing, DOI 10.1007/978-0-387-36699-9, © Springer Science+Business Media New York 2014
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ACOUSTIC RADIATION
the concerned medium. For example, from a microphone radiating along different directions, we are interested in the radiation diagram corresponding to the knowledge of rays (
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