There are elegant, low-cost solutions to reduce fume or dust extract fan noise that is a common cause of complaints. They can also be used to reduce occupational noise hazards from these sources. Conventional extract fan silencing is not only subject to clogging, but can also increase backpressure, reducing system efficiency - not what you need from these extract systems.
A large weld booth fume extractor 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.