What it is and when to use it. When a central air-handling unit or ventilation fan runs, its noise travels down the duct and is re-radiated at every diffuser — the "whoosh" heard in offices and hotel rooms is duct-borne fan noise, not the machine itself. The standard cure is a duct silencer inserted right after the flexible connector at the unit discharge. HANWEI silencers are splitter-type: galvanized casing, parallel absorptive splitters dividing the airway into narrow passages — air flows through, sound is consumed. Dynamic insertion loss is 15–25 dB(A) at rated flow, with pressure drop held at ≤50 Pa, the number that matters most on HVAC projects because every pascal comes off the fan's external margin.
The triangle: attenuation, pressure drop, velocity. These three fight each other, and good selection balances all of them. Attenuation is set by the required room level per GB 50118 (30 dB night bedrooms, 40 dB offices) minus the unit's sound power and duct natural decay. Pressure drop is capped at 50 Pa in our designs; when the fan margin is tight we enlarge the cross-section rather than squeeze the airway. Velocity is the third gate: above 10 m/s between splitters, airflow itself generates regenerative noise and the silencer becomes a new source — we keep 6–8 m/s, adding a transition section where the duct is too small. Low-frequency-heavy units (large AHUs, slow centrifugal fans) get a combined reactive-resistive body whose expansion chamber pulls the 100–250 Hz boom down. Acceptance follows GB/T 4760 insertion-loss measurement, with the calculation sheet delivered alongside.
Moisture is the HVAC-specific killer. Cooling supply air condenses on duct surfaces, and unwrapped glass wool cakes within two seasons — absorption halved, wool sagging. Our splitters wrap the wool in water-repellent glass fabric and the facing carries an anti-condensation coating; on a two-unit comparison we visited years apart, the unwrapped competitor silencer could literally squeeze water from its wool. Field lessons also cover placement — always after the flexible connector (it cures structure-borne transmission) and never at a sharp bend — and sizing honesty: static lab values overstate real performance, so our datasheets quote dynamic (with-flow) figures, and pressure drop is written into the contract, not left off the quotation.