Fume or dust extract fan noise is a very common cause of noise complaints and can also be an occupational noise hazard. Conventional extract fan silencing is not only subject to clogging, but can also increase backpressure, reducing system efficiency - not what you need from these systems.
A large weld booth fume extract fan system generated high-level low-frequency tones at 35Hz and harmonics that caused extensive complaints in nearby offices. Diagnostic measurements proved that the major problem was an in-duct standing wave resonance in the manifold that was amplified by panel resonances in the filter enclosure. The elegant solution was to insert an air-guide plate into the ductwork to reduce turbulence, fitting an acoustic absorbent plug into an end cap to create an anechoic termination and applying stiffening ribs to detune the noise radiating panels.
These measures were fitted within a day at very low cost, not only eliminating the low-frequency tones (30dB reduction), but also dramatically improving the fume extract fan efficiency and hence reducing running costs.