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1.1 This International Standard specifies requirements for instruments and test procedures for the measurement and reporting of sound pressure levels at crew and passenger locations in the interior of aircraft under steady flight conditions. The sound pressure levels may be used to determine various quantities for describing the acoustical environment in the interior of the aircraft. The procedures are intended to ensure uniformity in test results and to provide the basis for determination of measurement uncertainties. 1.2 This International Standard provides electroacoustical performance specifications for a complete measurement system from a microphone to the readout device. Various individual components of a measurement system may be selected so long as the total measurement system conforms to the specifications of this International Standard. 1.3 The preferred measurement procedure involves the recording of sound pressure signals with subsequent analysis into one-third-octave band sound pressure levels. Direct measurements may be made of one-third-octave-band sound pressure levels.