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Decibel scale <br /> A linear numbering scale used to define a logarithmic amplitude <br /> scale, thereby compressing a wide range of amplitude value to a <br /> small set of numbers. <br /> Diffraction <br /> The scattering of radiation at an object smaller then one <br /> wavelength and the subsequent interference of the scattered <br /> wavefronts. <br /> Sound Pressure Level <br /> The fundamental measure of sound pressure. Defined as: <br /> Lp = 20 log p/po dB <br /> where p is the RMS value (unless otherwise stated) of sound <br /> pressure in pascals, and po is 20uPa for measurements in air. <br /> Hertz <br /> The unit of frequency measurement , representing cycles per second. <br /> Infrasound <br /> Sound at frequencies below the audible range, i. e. below about <br /> 16Hz . <br /> Loudness <br /> Subjective inpression of the intensity of a sound. <br /> Far field <br /> Distribution of acoustic energy at a very much greater distance <br /> from a source than the linear dimensions of the source itself; the <br /> region of acoustic radiation used to the source and in which the <br /> sound waves can be considered planar. See also: diffraction. <br /> Free field <br /> An environment in which there are no reflective surfaces within the <br /> frequency region of interest. <br /> Near field <br /> That part of a sound field, usually within about two wavelengths <br /> from a noise source, where there is no simple relationship between <br /> sound level and distance. <br /> Noise reduction coefficient, NRC <br /> The arithmetic average of the sound absorption coefficients of a <br /> material at 250, 500, 1000 and 2000 Hz. <br />