Acoustics and Sound  

Physics of Sound Waves > s.a. fluids; Phonon; wave equations.
* Properties: Sound waves are longitudinal in fluids, but can be both longitudinal and transverse (in general with different propagation speeds) in solids.
* Propagation speed: In an elastic solid it is v = (c/ρ)1/2, where c is the elastic constant; In a fluid, v = λ/τ, the ratio of the mean free length to mean free time; In a medium with negative specific heat, it is imaginary; In air, v is proportional to T 1/2; The maximum in solids and liquids on earth seems to be 36 km/s, from a combination of the fundamental constants c, α and mp / me; The greatest measured speed is in diamond.
* Most intense sound: Macroscopically, up to 500 psi; Sonoluminescence has more.
@ General references: Lindsay TPT(63); Campbell & Greated 87; Rossing AJP(87)jul; Bregman 90; Raichel 00; Borovitskaya 15 [I]; Goldsmith 16 [I].
@ Differential geometric viewpoint: Zloshchastiev APPB(99)gq/98 [superfluid He]; Bilić CQG(99)gq; Visser et al ht/01-conf [BECs and Lorentz symmetry violation]; Fischer & Visser EPL(03)gq/02 [phonons]; Visser & Molina-París NJP(10)-a1001 [barotropic irrotational fluid flow]; Gibbons & Warnick CP(11)-a1102 [geometry of sound rays in a wind]; Bensoam a1304; > s.a. emergent gravity [acoustic spacetimes]; lorentzian geometry; wormholes.
@ Acoustic torsion: Garcia de Andrade gq/03 [and superfluids], PRD(04), gq/04 [turbulence], PLA(05)gq [vorticity], PLA(05) [and breaking of acoustic Lorentz invariance].
@ Propagation in various media: Fletcher AJP(74)jun [gas, adiabatic assumption]; Martynov TMP(06) [liquids and gases]; Bacon TPT(12) [speed of sound in air vs temperature]; Deckert et al a1406 [quantum gas of interacting bosonic atoms]; Peano et al PRX(15) [unidirectional propagation]; news sn(20)oct [maximum speed in solids and liquids on earth]; > s.a. spin models; wave phenomena.
@ Related topics: NS(90)jan20, p56; NS(91)jan19, 38-41; Stenflo PLA(96) [acoustic gravity waves]; news pw(05)nov, pw(07)jan [superluminal]; Aref et al ch(16)-a1506 [quantum acoustics with surface acoustic waves]; Becker et al PRX(18) [removing reflections from the boundaries].

Applications and Effects > s.a. black-hole analogs [acoustic]; electromagnetism [electroacoustics]; sonoluminescence.
* Photoacoustics: The process of producing sound with light (the opposite of sonoluminescence), discovered by Alexander G Bell in the XIX century; Applied to detect tiny gas leaks by heating the gas with a laser [@ news pn(00)jun].
* Thermoacoustics: 1997, Engines so far are not very efficient, but they are ecological refrigerators or prime movers with no moving parts.
* In geophysics and astronomy: Sound waves probe the interior of the Earth, Moon, Sun, and other objects.
* In cosmology: Relativistic sound waves propagating in the early universe left an imprint that is still discernible in the cosmic microwave background and in the large-scale distribution of galaxies.
* Acoustic Time Reversal Mirrors: Devices that record a sound wave from a source and generate a new one that behaves as if the original traveled backwards in time; They have been tested in water, air and solids (more complicated, since there are two types of sound waves there), and they can be applied to locating defects in solids (airplanes, kidney stones).
@ General references: Munk ThSc(93)sep [ocean warming]; Kuperman & Lynch PT(04)oct [in shallow water]; news pn(06)jul [sand dunes].
@ Thermoacoustics: Swift PT(95)jul, Garrett AJP(04)jan [engines]; news pn(07)jun [turning heat into electricity].
@ Time reversal: news pn(95)nov; Fink CP(96), PT(97)mar; Fink SA(99)nov.
@ In cosmology: Eisenstein & Bennett PT(08)apr; Corasaniti & Melchiorri PRD(08).
@ Acoustic cloaking devices: news pw(08)jan; Chan Phy(11)jan [for ultrasonic water waves]; Popa et al PRL(11) + news bbc(11)jun; news pw(12)jan; news sci(13)mar [3D]; Zhu et al PRX(14) [acoustic PT symmetry and one-way cloaking].
@ Related topics: Snieder & Wapenaar PT(10)oct [imaging with ambient noise]; Altfeder et al PRL(10) + news ns(10)oct + focus(10)oct ["phonon tunneling" or "propagation across a vacuum"]; Lepri & Casati PRL(11) + news msnbc(11)may [one-way acoustic mirrors]; Torrent & Sánchez-Dehesa PRL(12) + news pw(12)may [acoustic analog of graphene]; news sn(13)jul, sfg(14)jan [levitation]; news pw(14)jan ["one-way circulator"]; Denardo et al AJP(14)feb [acoustic radiation force]; news pm(14)may [acoustic tractor beam]; Marzo et al PRL(18) [acoustic trap]; Soper PRR(20)-a1908 [sound waves move matter]; McKenna PT(20)jan [sounds of nature]; > s.a. casimir effect [acoustic analog]; Lasers; metamaterials; music [including hearing, psychoacoustics, architectural acoustics]; physics teaching; technology.

* Idea: Sound whose frequency is either higher than those audible by humans, or high enough that molecules of the medium experience almost no collisions over one period of the wave.
* Applications: In medicine, destroying cancer cells; Stopping internal bleeding (ASA meeting, 06.1998); Ultrasound imaging (1999, without physical contact through impedance matching); RUS, Resonant Ultrasound Spectroscopy, developed in 1988 by A Migliori for elasticity measurements; Ultrasound amplification by self-excited resonance (uaser), an acoustic analog of lasers.
@ General references: Maynard PT(96)jan [RUS]; Povey CP(98) [and food]; Maris SA(98)jan [picosecond pulses]; Cheeke 12.
@ Medical applications: Crum & Hynynen PW(96)aug; Vaezy et al pw(01)aug, ter Haar PT(01)dec [surgery]; Novario et al RNC(03) [medical diagnostic]; Wang et al PRL(13) [measuring local blood flow speed in living tissue].
@ Other applications: Potter et al PRL(14) [acoustic imaging technique to reveal cracks in structures]; news pw(15)feb [twisted light]; news pt(15)dec, pt(16)feb [super-resolution imaging].

* Sources: Some large mammals, such as elephants, rhinoceros and whales use f just below 20 Hz for communication; Volcanoes, meteorites, ocean swell (f ~ 0.2–0.3 Hz), tornadoes and hurricanes (from a location not in the eye), explosions; A background of about 70 db with f ~ 0.1–1 Hz is normal; The Earth's solid interior produces a constant hum of a few mHz.
* Propagation: Infrasound can travel around the world, using the vsound(h) dependance and low dispersion to heat; Diatomic molecules in the air do absorb energy from acoustic waves in the sonic range, but not from infrasound.
* Effects: Frequencies above 1 Hz can be sometimes felt with our bodies; Infrasound at about 17 Hz has been shown to induce a sense of uneasiness, sadness and anxiety in an audience.
* Applications: Monitor location and nature of avalanches, tornadoes and atmospheric physics, meteorite impacts, volcanoes, nuclear weapons tests.
@ References: Bedard & Georges PT(00)mar [atmospheric]; Hedlin & Romanowicz pw(06)aug [review and global network]; Feder PT(18)aug [monitoring].

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