ISO 5136 specifies a method for testing ducted fans and other air-moving devices to determine the sound power radiated into an anechoically terminated duct on the inlet and/or outlet side of the equipment.

The method is applicable to fans which emit steady, broad-band, narrow-band and discrete-frequency sound and to air temperatures between - 50 °C and + 70 °C. The test duct diameter range is from 0,15 m to 2 m. Test methods for small (d < 0,15 m) and large (d > 2 m) test ducts are described in the informative annexes H and I, respectively.

The maximum mean flow velocity at the microphone head for which the method is suitable, depends on the type of microphone shield used, and is as follows:

foam ball 15 m/s; nose cone 20 m/s; sampling tube 40 m/s.

Above these values the suppression of turbulent pressure fluctuations by the microphone shield may be insufficient.

The method described in ISO 5136 is applicable to a sound source in which a fan is connected to ducts on at least one side. It is also applicable to other fan/attenuator combinations or equipment incorporating fans which can be considered as "black boxes".

Examples of fans and other equipment covered by ISO 5136 are

ducted centrifugal fans, ducted axial flow fans, ducted mixed-flow fans, ducted air-handling units, ducted dust-collection units, ducted air-conditioning units, and ducted furnaces.

ISO 5136 is also applicable to other aerodynamic sources such as boxes, dampers and throttle devices provided that a quiet air flow delivered by an auxiliary fan is available, and the signal-to-noise ratio of sound pressures to turbulent pressure fluctuations in the test duct is at least 6 dB.

It is not applicable to non-ducted fans or equipment.