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The primary measurement procedure recommended in this ARP includes the recording of sound pressure signals in the interior of an airplane during steady state cruise conditions with analysis after the flight into octave band (or one-third octave band) sound pressure levels. Alternatively, spectral analyses of the sound pressure signals may be performed directly on board an airplane. Though less flexible than the primary measurement procedure, the alternative procedure offers the advantage of immediate availability of frequency band or frequency weighted sound pressure levels that are consistent with those measured in accordance with the procedures in ISO 5129. In view of the range of possible measurement this ARP provides electroacoustical performance requirements for a complete 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 complies with the requirements. Recommendations of this ARP are not intended to satisfy the stringent requirements for airplane design or development. Also, they may not be directly applicable to measurements of sound pressure levels in the interior of helicopters or general aviation airplanes where special constraints on measurement locations and test procedures may exist. Specific recommendations for measurement of interior sound pressure levels in helicopters are given.