Acoustics — Determination of sound power radiated into a duct by fans and other air-moving devices — In-duct method

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