Key takeaways
- A 50 mm batt absorbs 0.15 at 125 Hz and 0.90 at 500 Hz, so lining buys almost nothing in the band where a canopy is already short of loss.
- Perforated sheet needs at least 30% open area; 2 mm holes on a 5 mm pitch give 12.6% and reflect most of the sound before it reaches the wool.
- Leaving 10% of the internal area unlined costs 1.9 dB against a fully lined canopy; 25% bare costs 3.7 dB, which is more than doubling the skin buys.
- Fix batts with impaling pins or washers at 300 mm centres and never compress them below 80% of nominal thickness, or the compressed patches stop working in the 250–500 Hz band.
- Keep lining out of the cooling air path: a batt across the radiator face raises air-on temperature 5–8 °C and derates the set for a gain of a few dB.
Genset acoustic lining is the cheapest 5 dB in a canopy and the easiest to lose. A 50 mm mineral wool batt behind perforated sheet costs a few hundred rupees per square metre and cuts the reverberant field inside the enclosure, which lowers the sound pressure driving the panels. Fit it badly — compressed, saturated, or covering only 80% of the surface — and the canopy measures like an unlined box.
The reason is that absorption is a per-square-metre property and transmission is a per-path property. A lined panel works because the wool removes energy each time sound passes through it. A bare patch does not, and sound finds the bare patch. Ten percent of the internal area left unlined costs 1.9 dB against a fully lined enclosure. Twenty-five percent costs 3.7 dB, which is more than doubling the skin gauge would buy in the same band.
This guide covers genset acoustic lining as a specification: thickness and density against frequency, the perforated facing that lets sound reach the wool, coverage at apertures and baffles, fixing methods that survive vibration, and the contamination that quietly kills performance. The numbers are for a 40–125 kVA canopy, and the same logic holds at larger sizes.
Genset acoustic lining: which frequency does it actually fix?
None of the low ones. Lining works from about 250 Hz upward, and a canopy is usually short of loss at 125 Hz.
Absorption is a function of thickness relative to wavelength. At 125 Hz the wavelength is 2.74 m, so a 50 mm batt is λ/55 thick and the sound barely notices it. At 500 Hz the wavelength is 0.69 m and the same batt is λ/14, deep enough to absorb most of the incident energy.
Typical figures for mineral wool at 32–48 kg/m³ make the point.
| Octave band | 50 mm batt | 100 mm batt | What the lining does |
|---|---|---|---|
| 125 Hz | 0.15 | 0.45 | Almost nothing at 50 mm; thickness is the only lever |
| 250 Hz | 0.60 | 0.90 | First useful band; 100 mm is clearly better |
| 500 Hz | 0.90 | 0.98 | 50 mm is enough; extra thickness buys little |
| 1 kHz | 0.95 | 0.99 | Saturated; density and facing matter more than depth |
| 2 kHz | 0.95 | 0.99 | Saturated |
The table settles the thickness argument. If the canopy is short at 125 Hz, 50 mm of wool will not fix it, and 100 mm helps but does not solve it — mass, sealing and the low-frequency spectrum of the engine decide that band. If the canopy is short at 500 Hz and above, 50 mm is enough and the money should go on coverage and sealing instead.
What the absorption coefficient is worth in decibels
Absorption reduces the reverberant field, and the reduction is set by the room constant R = S × α / (1 − α). Take a canopy with 16 m² of internal surface. Unlined at α = 0.1, R = 16 × 0.1 / 0.9 = 1.78 m². Fully lined at α = 0.9, R = 16 × 0.9 / 0.1 = 144 m².
The theoretical change in reverberant level is 10 log₁₀(144 / 1.78) = 19 dB. Nobody measures 19 dB, because absorption only controls the reverberant field and sound also leaves through the panels and the leaks. Once those paths dominate, more absorption stops helping. The realistic gain from lining a canopy is 3–8 dB in the 500 Hz–2 kHz bands, which is still the cheapest few dB in the enclosure.
That gap between theory and measurement is why lining is often over-specified after a failed test. Adding 50 mm more wool cannot beat a leak path or a panel that is already transmitting. Fix the path first, then the absorption.
Thickness, density and open area: the lining table
The middle column is the specification for a 40–125 kVA canopy; the right column is what the fabrication route holds.
| Parameter | Typical value | Ganesh limit or note |
|---|---|---|
| Lining thickness | 50 mm standard, 100 mm for low-band shortfall | 100 mm costs 4.8 kg/m² and 100 mm of internal clearance per face |
| Density | 32–48 kg/m³ | Above 64 kg/m³ the batt turns reflective at mid frequencies |
| Facing | None, or glass cloth on the airstream side | Foil facing reflects and defeats the lining |
| Perforated sheet | 0.8–1.0 mm, ≥ 30% open area | Laser cut and formed; 3 mm hole at 5 mm pitch gives 28% |
| Perforation pitch | 5 mm square, 3 mm hole | Checked against the sheet certificate, not the sample |
| Fixing | Impaling pins or washers at 300 mm centres | Batt compressed to no less than 80% of nominal |
| Coverage | 100% of internal panel area, including baffles | Unlined apertures cost 1.9 dB at 10% of area |
| Batt weight, 50 mm at 48 kg/m³ | 2.4 kg/m² | 16 m² of lining is about 38 kg on the canopy |
| Clearance to moving parts | 25 mm minimum | Lining must not touch the fan guard or the alternator |
| Air path | No lining across the radiator face | Air-on rise of 5–8 °C if a batt blocks the core |
| Service apertures | Lined door cards, removable | Access panels must come out without destroying the batt |
Five failure modes and what each costs
Perforation open area below 30%
The facing is an acoustic window. Below about 20% open area it behaves as a reflector at 500 Hz and above, and the wool behind it is invisible to the sound. The failure is common because a 2 mm hole on a 5 mm pitch looks like a fine mesh and is cheap to punch.
Specify open area as a number, not a pattern. 30% minimum, 40% preferred. At 40% the sheet is still strong enough to protect the batt and take a powder coat.
Batt compressed at the fixings
A washer or impaling pin pulls the batt down locally. Compress 50 mm wool to 25 mm under a 30 mm washer and that patch absorbs like a 25 mm batt — roughly half the 250 Hz performance over the compressed area. Spread over every fixing at 300 mm centres, that is a meaningful share of the panel.
Use impaling pins with a large head, or washers at 40 mm diameter, and tighten until the batt is held rather than crushed. If the batt is 50 mm, the fixing should leave at least 40 mm under the washer.
Coverage gaps at baffles and apertures
The internal baffles that turn the air path are also acoustic surfaces. If they are left bare, the sound that would have been absorbed at the turn passes through. The same applies to cable entry boxes, the exhaust collar and the frame around the radiator opening.
Calculate the cost before arguing about it. With a lined panel at 25 dB and a bare patch at 17 dB, 10% bare area gives a combined 23.1 dB — a 1.9 dB loss. At 25% bare, the combined figure is 21.3 dB and the loss is 3.7 dB.
Saturation with oil, water or dust
Mineral wool is open-cell. Diesel mist, lube oil and monsoon humidity fill the pores, and a saturated batt loses most of its absorption while gaining weight. The weight is what fails the fixings, and the sag is what leaves the top of the panel bare.
On a canopy that sees oil mist, use a glass-cloth facing on the airstream side, or a foil-faced batt with the foil on the warm side — not on the room side, where it would reflect. Replace battings at service intervals rather than waiting for a complaint.
Lining across the cooling path
A batt fitted across the radiator face, or a loose flap that drops into the fan guard, blocks airflow. The air-on temperature rises 5–8 °C, the set derates, and the acoustic gain is a few dB at best. The lining belongs on the panels and the baffles, not in the duct.
Keep 25 mm clear of moving parts and line the baffle faces, not the gap between them.
Fixing the lining so it survives service
A batt is a heavy, porous, friable slab held to a vertical panel by friction and a few fixings. On a canopy that vibrates at the engine firing frequency, friction is not a fixing method.
Three details matter. First, support the batt mechanically with impaling pins welded or riveted to the panel, at 300 mm centres in both directions. Adhesive alone fails at the first hot cycle, and the failure is invisible behind the perforated sheet until the batt drops onto the fan guard.
Second, cap the pins with a 40 mm washer so the load is spread over a patch rather than a point. A 30 mm washer on 50 mm wool compresses a cone of material around it; a 40 mm washer with a domed head compresses less and holds better.
Third, clamp the edges. A batt cut 5 mm oversize stands in the bay by friction and compresses behind the return; one cut to nominal size or under falls out when the panel vibrates. Cut oversize, fit dry, and let the return hold the edge.
Where the lining will be replaced, specify a removable perforated card held by quarter-turn fasteners rather than a welded-in sheet. The cost is a few fasteners; the alternative is cutting the sheet off on site.
What Ganesh actually runs
Perforated panels are laser cut on the 12 kW fibre laser with the 40 ft × 16 ft bed, which takes a full canopy side in one blank and keeps the perforation away from the fold line. Panels are formed on the 200 T, 5-axis brake with the 4,450 mm bed, so a 2,400 mm perforated return is one hit rather than a joined pair. Batts are cut to the panel bays, fitted on impaling pins at 300 mm centres and closed behind the perforated sheet before the canopy is assembled.
Coating runs through seven-tank pretreatment and the 18 ft × 14 ft powder booth where the drawing calls for a coated perforated panel, with the perforation checked for blow-through and the DFT measured on the flat, not on an edge. Canopies for Mahindra Powerol programmes have gone through this route, and the genset parts page lists the inputs that let the quotation be built in one pass.
We do not measure absorption coefficients. The α values in the table are published material data, and the canopy's dB(A) result is tested on the assembled set by the OEM or a third-party laboratory. If the result misses, the first check is coverage and perforation, not thickness.
What to put on the drawing
- dB(A) target at 1 m and the octave-band spectrum if the acoustic consultant has one.
- Lining thickness, density and facing, by panel area.
- Perforated sheet thickness, hole size, pitch and minimum open area.
- Fixing type and centres, and the maximum permitted compression.
- Coverage map, including baffles, door cards and the panels around apertures.
- Airstream faces and the contamination the lining must resist.
- Clearance to the fan guard, alternator and any moving part.
- Whether the lining is replaceable in service, and how the panel comes off.
The coverage map is the item that most drawings leave out, and it is the one the shop needs. "Line the canopy" is not a specification; a marked elevation with each bay, each baffle face and each door card called out is. It also gives the inspector something to count against.
When lining is the wrong spend
If the canopy is short at 125 Hz, lining is not the answer; the panel mass, the door seal and the exhaust penetration are. If the target needs more than about 10 dB below what a lined single-skin canopy achieves, the route is a double-skin panel with a decoupled inner leaf, not a thicker batt.
And if the set is going into a plant room with room treatment, the lining budget may belong to the room rather than the canopy. The genset canopy hub covers the related guides, the acoustic enclosure design guide sets the band targets the lining has to serve, and the louvre panel guide covers the opening that usually leaks more than the lining absorbs. Send the canopy drawing and the acoustic target through request a quote and we will check thickness, perforation and coverage before pricing.
The check that takes ten minutes
Before any canopy is signed off, measure the perforated sheet open area on a sample, count the batts against the coverage drawing, and press a thumb into each one to confirm it springs back. Those three checks catch the failures that a dimensional inspection misses, and they cost less than one acoustic test.
A lining that is specified but not installed is worse than no lining at all, because the drawing says the canopy is treated and the diagnosis starts in the wrong place.
Frequently asked questions
What thickness of acoustic lining does a genset canopy need?
50 mm of 32–48 kg/m³ mineral wool is the working specification for a 40–125 kVA canopy. Go to 100 mm only if the 125–250 Hz bands are short; 100 mm raises absorption at 125 Hz from about 0.15 to 0.45 but costs twice the weight and 100 mm of internal clearance.
Does higher density always mean better acoustic lining?
No. Above about 64 kg/m³ the batt becomes reflective at mid frequencies and the gain flattens. 32–48 kg/m³ is the useful range for a canopy, and thickness matters more than density below 250 Hz.
How much open area should the perforated sheet have?
At least 30%, and 40% is better. A 2 mm hole on a 5 mm square pitch gives (0.785 × 2²) / 25 = 12.6%, which reflects most of the sound before it reaches the lining. A 3 mm hole on a 5 mm pitch gives (0.785 × 9) / 25 = 28%.
Why does the canopy get louder after a few years?
Because the lining has sagged, compressed or absorbed oil and water. A saturated batt loses most of its absorption, and a sagged one leaves the top of the panel bare. Check the fixings and the batt weight before changing the panel gauge.
Can lining be fitted after the canopy is built?
On a bolted canopy, yes, through the access doors, if the batts are cut to the panel bays and the fixings are reachable. On a welded canopy with the lining behind perforated sheet, no — the sheet has to come off, which means the canopy comes apart.
Sources and further reading
- ISO 11654 — Acoustics: sound absorbers for use in buildings, rating of sound absorption · reviewed 3 Nov 2026
- ISO 717-1 — Acoustics: rating of sound insulation in buildings and of building elements · reviewed 3 Nov 2026
- IS 513 — Cold reduced low carbon steel sheets and strips · reviewed 3 Nov 2026
Send the canopy acoustic target
Share the enclosure drawing, the dB(A) target, the genset model and the quantity. We check lining thickness, perforation and coverage before quoting.
Request a quote