Earlier investigations by Stahl and Arguello (Stahl, B,, and Arguello, G.,
"Schallerzeugung und Schalldampfung in einer Rohrstromung stromab einer uns
tetigen Querschnittserweiterung," Acustica, Vol. 65, No. 2, 1988, pp. 75-84
), who studied the sound field in a circular duct generated by a jet how in
dicated that flow separation occurring in a diffuser attached to the jet no
zzle leads to increased sound levels in the low-frequency regime. This obse
rvation formed the basis for the present study. Experiments were made with
five centrifugal fan impellers. Cross-correlation studies involving the uns
teady wall pressures in the impeller and the casing led to the following in
sights: the low-frequency noise of centrifugal fans is generated by classic
al flow separation regions located on the shroud and the blade suction side
s of the impeller. The design of the shroud is of particular importance: a
large radius of curvature in combination with a smaller impeller exit width
improves the flow along the shroud. As a result, how separation is reduced
, the flow leaving the impeller is more uniform and less turbulent, and the
low-frequency noise is reduced. The flow in the fan casing also contribute
s to the low-frequency noise: here the how along those parts of the volute
that are close to the impeller periphery, i.e., the region close to the cut
off is Important.