Abstract
A window‐type ventilation fan of 150 mm in diameter with a flow rate of 170 cfm is studied. The fan has four rotor blades and the downstream motor is held by two metal struts. Acoustic measurement in an anechoic chamber reveals an asymmetric pattern of sound intensity at the blade passing frequency with a major beam in the front and a minor beam at the back of the fan. The beams are basically aligned with the rotational axis, implying a thrust‐noise mechanism, but there is irregularity in the distribution. Analysis shows that such irregularity may be caused by the interference between the leading order thrust noise and the higher order noise mechanisms. Modifications of the front and back geometries reveal that the dominant tone noise mechanism lies in the vortex shedding from the four rims of the short circular exit passage which is housed in a square outer frame. Correction of the exit duct geometry reduces the tone noise power by about 5 dB, and the pattern of the residual sound intensity is changed drastically. Comparison is also made between the directivity of the ventilation fan with that of a computer cooling fan with similar features.
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