Acoustics and Sound  

Physics of Sound Waves > s.a. 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/)1/2, where c is the elastic constant; In a fluid, v = /, 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.
* 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 06.
@ Differential geometric viewpoint: Zloshchastiev APPB(99)gq/98 [superfluid He]; Bilic CQG(99)gq; Visser et al ht/01-in [BECs and Lorentz symmetry violation]; Fischer & Visser EPL(03)gq/02 [phonons]; > s.a. 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]; > s.a. spin models; wave phenomena.
@ Related topics: NS(90)jan20, p56; NS(91)jan19, p38-41; Stenflo PLA(96) [acoustic gravity waves]; news pw(05)nov, pw(07)jan [superluminal].

Other 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; Tested in water, air and solids (more complicated, since there are two types of sound waves there); 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 Casimir effect: Larraza & Denardo PLA(98); Larraza AJP(99)nov; Ford & Svaiter a0811-in [potentially observable aspects].
@ Related topics: news pw(08)jan [acoustic cloak]; > s.a. metamaterials; music [including hearing], physics teaching.

Ultrasound
* 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.
@ Medical applications: Crum & Hysynen PW(96); Vaezy et al pw(01)aug, ter Haar PT(01)dec [surgery]; Novario et al RNC(03) [medical diagnostic].

Infrasound
* 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, 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: Can travel around the world, using vsound(h) dependance and low dispersion to heat.
* 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].

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