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Sound power or acoustic power is the rate at which sound energy is emitted, reflected, transmitted or received, per unit time. The SI unit of sound power is the watt (W).〔 It is the power of the sound force on a surface of the medium of propagation of the sound wave. For a sound source, unlike sound pressure, sound power is neither room-dependent nor distance-dependent. Sound pressure is a measurement at a point in space near the source, while the sound power of a source is the total power emitted by that source in all directions. Sound power passing through an area is sometimes called sound flux or acoustic flux through that area. ==Mathematical definition== Sound power, denoted ''P'', is defined by〔Landau & Lifshitz, "Fluid Mechanics", Course of Theoretical Physics, Vol. 6〕 : where *f is the sound force of unit vector u; *v is the particle velocity of projection ''v'' along u; *''A'' is the area; *''p'' is the sound pressure. In a medium, the sound power is given by : where *''A'' is the area of the surface; *''ρ'' is the mass density; *''c'' is the sound velocity; *''θ'' is the angle between the direction of propagation of the sound and the normal to the surface. For example a sound at SPL = 85 dB or ''p'' = 0.356 Pa in air (''ρ'' = 1.2 kg·m−3 and ''c'' = 343 m·s−1) through a surface of area ''A'' = 1 m2 normal to the direction of propagation (''θ'' = 0 °) has a sound energy flux ''P'' = 0.3 mW. This is the parameter one would be interested in when converting noise back into usable energy, along with any losses in the capturing device. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Sound power」の詳細全文を読む スポンサード リンク
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