Circular silencer designer
Enter the duct diameter, liner thickness, silencer length and airflow for a round (tube) duct silencer — with an optional central pod for larger sizes — and get the airway velocity, pressure loss and an ASHRAE-based insertion-loss estimate.
How to use this calculator
- Enter the airflow and duct diameter.
- Add a central pod diameter if the design uses one.
- Enter the liner thickness and silencer length.
- Pick the facing material and pod nose/tail shape.
- Optional: enter the unattenuated octave-band spectrum to see the attenuated result.
- Read the airway velocity, pressure loss and insertion loss.
What this calculates
A round (tube) duct silencer is normally sized so its airway matches the connecting duct's own diameter at both ends — the acoustic lining sits outside that airway, between it and the (larger) casing, so a plain lined silencer causes no velocity step and no contraction/expansion loss, only friction along its lined wall. Larger sizes often add a central pod (bullet) inside the airway for better acoustic performance at a given pressure-loss budget — that pod reduces the free flow area to an annulus, which does introduce a real contraction/expansion loss, the same physical effect this site's splitter silencer designer already models for splitters occupying duct cross-section. Setting pod diameter to 0 here collapses the model back to the plain, pod-less case automatically (area ratio = 1, so the contraction/expansion terms vanish on their own) rather than needing a separate formula branch.
Aairway = π/4 × (D² − dpod²) free flow area
Dh = D − dpod annulus hydraulic diameter (= D when dpod=0)
Kc = 0.5 × (1 − Aairway/Aduct) contraction (duct → airway)
Ke = (1 − Aairway/Aduct)² expansion (airway → duct)
Δp = KcρVairway²/2 + f(L/Dh)ρVairway²/2 + KeρVairway²/2
Unlike the splitter silencer designer, this calculator has no VDI 2081 (or other verified-source) correlation to reproduce for round/tube silencers — so this general engineering model (Darcy-Weisbach friction + Borda-Carnot contraction/ expansion, both already used and checked against reference points elsewhere on this site) is the only pressure-loss method here, not a cross-check alongside a standard's own figure. It's ordinary, independently-verifiable fluid mechanics, not a named silencer standard's tabulated coefficients — treat the result accordingly: good for comparing design options and rough sizing, not a substitute for a manufacturer's tested performance data.
The "Technical drawing" panel in the results draws the geometry to scale — a dimensioned cross-section (looking along the flow: casing, airway and pod, if any, as concentric circles) and a longitudinal section (a cylinder's side profile is a plain rectangle, so the casing and airway boundaries are drawn as parallel lines along the length, with the pod, if fitted, as a simple tapered shape) — a schematic for checking the arrangement makes sense, not a fabrication/shop drawing. The "3D model" panel renders the same geometry with the casing semi-transparent — drag to rotate, scroll to zoom. It's a visualization aid, not a CAD model: no gauge-accurate profiles, welds, gaskets or real bolt threads.
Why no self-generated flow-noise figure
The splitter silencer designer reports a self-generated flow noise figure from VDI 2081-1 Equation (49) — checked there against that standard's own published worked example. That formula and its verification are specific to splitter silencers; nothing here has been checked against an equivalent worked example for round/tube geometry, and this site's policy is not to ship unverified precision. Rather than guess at a plausible-looking number, this figure is simply not calculated here — the outlet noise estimate below reflects attenuation only, not any added self-noise from airflow past the lining or pod.
Insertion loss — ASHRAE lined-duct estimate (unverified)
Since there's no verified insertion-loss formula for this geometry either, the octave-band table below borrows the same real published source the splitter tool uses: the 2019 ASHRAE Handbook — HVAC Applications, Chapter 49, Tables 17 & 18 — measured insertion loss for a straight rectangular sheet-metal duct lined with 25 mm or 50 mm fiberglass, matched here by P/A (perimeter/area) ratio, the same parameter ASHRAE's own Table 16 organizes duct attenuation by. P/A = 4/Dh for any duct cross-section — a general identity, not shape-specific — so the round airway's own hydraulic diameter (or the pod annulus's, if fitted) plugs directly into the same lookup the splitter tool already uses for its rectangular gaps. This remains a reasonable acoustic-engineering analogy, not a citable method for round silencers specifically, and has no worked example here to verify it against; whenever your airway's P/A ratio falls outside the table's own tested range (common for round geometry, same as for splitter gaps), the result is flagged with the nearest reference size used.
Treat this table as a rough order-of-magnitude guide for early sizing decisions, not a design value — and definitely not a substitute for a manufacturer's own tested octave-band insertion loss for the actual silencer construction you specify.
Outlet noise estimate
If you enter a fan's octave-band inlet sound power spectrum (optional, under "I know my fan's noise in octave bands" in the inputs), each band is reduced by the ASHRAE insertion-loss estimate above, then the resulting spectrum is A-weighted and logarithmically summed to one overall level — with no separate self-noise term added (see above). Use it to compare design options, not as a substitute for a manufacturer's tested acoustic submittal.