This International Standard specifies requirements for the electroacoustical performance of systems of instruments used to measure sound for the purposes of aeroplane noise certification, and recommends methods by which tests may be made periodically to verify that the performance continues to comply with the requirements given within stated tolerances. In general, a sound measurement system for this purpose comprises a combination of instruments extending from a microphone through data recording and processing devices to a suitable output. Particular measurement systems, differing in their composition, perform the necessary functions in different ways and operate on either analogue or digital principles. The purpose of this International Standard is to ensure that different measurement systems have the same electroacoustical characteristics within the stated tolerances under specified reference environmental conditions. This standard does not provide recommendations for installation of microphones or microphone windscreens, nor requirements for measurement and analysis procedures used in aeroplane noise certification, but gives only the performance specifications for the measurement systems used to provide one-third-octave-band sound pressure levels averaged over a period of time. Certain of the requirements apply to the complete measurement system, including any means of recording a time waveform of the sound pressure signal to be measured prior to analysis. Other requirements apply specifically to the microphone which generates an electrical signal in response to the sound pressure received. Still further requirements apply only to the instruments used to operate on that signal in order to provide an output in the form of one-third-octave-band sound pressure levels. The requirements of this International Standard apply to the instruments used to measure the sound produced by jet- or propeller-driven transport-category aeroplanes of maximum certificated takeoff mass over 9 000 kg, or their propulsion systems when installed on a suitable outdoor engine test stand.