Acoustics—Determination of sound power radiated into a duct by fans and other air-moving devices—In-duct method
1 Scope
1.1 General
This Standard 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.
Note: For the sake of brevity. Wherever the term “fan” occurs in the text, it means “fan or other air-moving device”.
The method is applicable to fans which emit steady, broad-band, narrow-band and discrete-frequency noise and to air temperatures between −50°C and +70°C. The test duct diameter range is from 0.15m to 2m. Test methods for determining the sound power of fans in diameter (d<0.15m) and diameter (d>2m) test ducts are described in the 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:
—Foam ball: 15m/s;
—nose cone: 20m/s;
—sampling tube: 40m/s.
Above these values the suppression of turbulent pressure fluctuations by the microphone shield (see 3.9) may be insufficient.
It is expected that sound power tests will be conducted in conjunction with airflow parameters measurement in accordance with the methods specified in GB/T 1236. Therefore, the test duct will normally incorporate a “star” type flow straightener on the outlet side of the fan which will minimize swirl (see 5.2.9 and 7.3). Where it is permissible to delete the straightener as, for example, with large fans to installation category C according to IGB/T 1236, the method is limited to a swirl angle of 15°. (Example of the method for determining the angle of swirl is given in Annex J.)
Note: The installation categories defined in GB/T 1236 imply that the fan is either ducted on the outlet side only (category B), on the inlet side only (category C) or on both sides (category D).
1.2 Types of applicable sound source
The method described in this Standard 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”.
Fans and other equipment covered by this Standard are:
——ducted centrifugal fans;
——ducted axial flow fans;
——ducted mixed-flow fans;
——ducted air-handling units;
——ducted dust-collection units;
——ducted air-conditioning units;
——ducted air heater.
This Standard is also applicable to other aerodynamic sound sources such as static pressure box, air valve 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 (see 7.2.1).
An alternative method (reverberation room method) to determine the power level of the regenerated noise of air flow of such aerodynamic sound sources, which does not require the measurement of sound pressure in a flow environment, is described in GB/T 25516. The method was originally devised for the determination of the regenerated noise of air flow level of ducted attenuators. The sound power is determined in a reverberation room connected to the test duct via a transition element.
In the case of the ducted fans which are closely installed to the attenuators, the signal-to-noise ratio of sound pressures to turbulent pressure fluctuations may be insufficient when using the in-duct method. Therefore, the method described in GB/T 25516 is recommended for such fan/attenuator combinations.
This Standard is not applicable to non-ducted fans or other equipment.
