Key takeaways
- Mass is only half the job. The mass law gives you about 6 dB per doubling of surface mass — but a 5% opening caps the whole enclosure at roughly 19 dB.
- Use the ISO 15667 reference build: 1.5 mm outer steel (10–15 kg/m²), 50 mm mineral wool at 48–96 kg/m³, perforated liner at about 30% open area, 6 mm safety glass for windows.
- Absorption stops the build-up, not the escape. Lining at least 50% of the internal surface prevents reverberant gain inside the box, which is worth 5–10 dB on its own.
- Ventilation is the weakest link. Specify silencers by octave-band dynamic insertion loss, typically 10–30 dB, not by a single headline number.
- Isolate the machine from the enclosure and the enclosure from the floor. One rigid connection turns the panel into a loudspeaker.
- Target the A-weighted band around 500 Hz — that is what governs the perceived level for most industrial sources.
Acoustic enclosure design is the discipline where a 3 mm gap costs more decibels than 50 mm of extra insulation. A European rental-fleet operator asked us for a soundproof generator enclosure targeting 75 dB(A) at 1 m from a 105 dB(A) set. Our first build measured 84. The panels were correct — 1.5 mm outer skin, 50 mm rock wool, perforated liner, exactly the reference build. The problem was a 3 mm continuous gap under the access door and an unsilenced combustion-air intake the size of a brick. We sealed the door with a compression gasket and a double-lip threshold, fitted an acoustic louver with a 900 mm silenced duct, and re-measured: 76 dB(A). Nine decibels, from two fixes, none of which involved more steel or more wool.
This guide is what we now apply before we cut metal for any noise-control project: the three mechanisms that actually reduce sound, the mass law with real numbers for steel thicknesses, the reference panel build-up, how much absorption to specify, why leaks dominate, how to ventilate without throwing the performance away, and a 7-step build specification you can put straight into an RFQ.
Three Mechanisms, and What Each One Is Worth
An acoustic enclosure reduces noise three ways. They are not alternatives — a real enclosure uses all three, and if one is missing the other two underperform.
- Sound transmission loss (STL) — the panel acts as a barrier. Sound striking the outer skin is partly reflected and partly converted to heat through internal damping; only a fraction emerges on the far side. STL is governed by surface mass, stiffness, damping and frequency.
- Sound absorption — porous infill behind a perforated liner soaks up energy by viscous friction as air oscillates in the material's pores. This does not block sound; it stops it bouncing around inside and building up a reverberant field that pushes more energy through the walls. Worth roughly 5–10 dB by itself.
- Vibration isolation — resilient mounts and flexible connections stop structure-borne energy bypassing the panels entirely. Skip this and the enclosure frame becomes a radiating surface of its own.
A useful mental model: the barrier sets the ceiling, the absorption lets you approach it, and the isolation stops you quietly losing 10 dB out of the back door.
The Mass Law, With Real Numbers
For a single homogeneous panel in the mass-controlled region, transmission loss increases by about 6 dB for every doubling of surface mass, and by the same 6 dB for every doubling of frequency. That second half matters: every panel is much worse at low frequency, which is why generator and compressor enclosures are hard.
| Steel thickness | Surface mass (kg/m²) | Typical mid-band TL (dB, ~500 Hz) | Comment |
|---|---|---|---|
| 1.0 mm | 7.9 | ≈ 25 | Light hoods and small covers only |
| 1.5 mm | 11.8 | ≈ 28 | The ISO 15667 reference outer skin; the industry default |
| 2.0 mm | 15.7 | ≈ 31 | Outdoor enclosures needing impact resistance as well |
| 3.0 mm | 23.6 | ≈ 34 | Diminishing returns start here; weight climbs fast |
| 5.0 mm | 39.3 | ≈ 39 | Usually better solved with a cavity than with plate |
Read the last column carefully. Going from 1.5 mm to 3.0 mm costs you 12 kg/m² of steel to buy 6 dB. Going from 3.0 mm to 5.0 mm costs you another 16 kg/m² for 5 dB. Above about 20–25 kg/m², an absorptive cavity with a resilient layer is almost always the cheaper decibel — and it does not need a bigger crane.
Watch the low end as well. Below the panel's coincidence and resonance region the mass law stops holding, and a stiff, undamped 1.5 mm panel can perform worse at 125 Hz than the table suggests. That is where constrained-layer damping — a viscoelastic layer bonded to the skin — earns its money, and it is a standard option on our noise-control builds.
The Reference Panel Build-Up
ISO 15667 gives a construction that is a safe starting point for most machinery enclosures, and it is the one we quote unless the customer has a measured spectrum that says otherwise:
- Outer shell — 1.5 mm steel sheet; if another material is used, choose the thickness to reach a minimum surface mass of 10–15 kg/m².
- Absorbent lining — 50 mm mineral wool (or glass wool) at 48–96 kg/m³ density. Thicker for low-frequency sources.
- Perforated cover — around 30% open area, 1.0–1.6 mm, protecting the infill while remaining acoustically transparent. Below about 25% open area the facing starts reflecting rather than transmitting.
- Windows — 6 mm safety glass; laminated or double-glazed where higher reduction is needed.
The standard also notes what experienced builders learn the hard way: the A-weighted sound emission around 500 Hz determines the perceived insulation, so that is the band to optimise when you cannot optimise all of them.
| Mineral wool thickness | 250 Hz | 500 Hz | 1 kHz | 2 kHz | Typical NRC |
|---|---|---|---|---|---|
| 25 mm | 0.25 | 0.60 | 0.85 | 0.90 | 0.70 |
| 50 mm | 0.55 | 0.85 | 0.95 | 0.95 | 0.90 |
| 100 mm | 0.85 | 0.99 | 0.99 | 0.99 | 0.95 |
Look at the 250 Hz column. This is the whole argument for 50 mm as a default rather than 25 mm: the low-frequency absorption more than doubles, and low frequency is exactly where the mass law is weakest. Specify linings by banded absorption measured to ASTM C423 or ISO 354, not by a single NRC figure on a datasheet.
Leaks: Where Acoustic Enclosures Actually Fail
Sound behaves like water — it finds the hole, and once it has one the panel performance becomes irrelevant. Measured on unlined enclosures:
| Open area as % of total surface | Maximum achievable reduction | Practical example |
|---|---|---|
| 10% | ≈ 9 dB | An unsilenced louver plus a door gap |
| 5% | ≈ 19 dB | A poorly sealed access door alone |
| ≈ 0% (fully sealed) | Limited by panel TL, typically 25–35 dB | Compression gasket all round, every penetration booted |
Translated into a build instruction: seal every joint with a compression sealing strip, use double-lip or labyrinth thresholds on doors, and treat every penetration as a design item. Cable pass-throughs get acoustic boots or a lined tunnel; pipework gets a flexible rubber sleeve; shafts get a lined slot or a brush seal. The enclosure is only as quiet as its noisiest hole.
One that catches people out: the enclosure must not touch the machine. A single rigid bracket, a bolted conduit, or a cable tray screwed to both the frame and the enclosure panel creates a flanking path that can cost 10 dB regardless of how good the wall is. Where contact is unavoidable, keep the number of mounting points to a minimum and put resilient elements at each one.
Ventilation Without Losing the Decibels
Almost every machine in an acoustic enclosure needs air: for cooling, for combustion, or to dilute an explosive atmosphere. Openings are the classic weak link, so airflow paths must be designed as acoustic ducts rather than as holes.
- Acoustic louvres and duct silencers — sized to deliver the same transmission loss as the panels, so the ventilation path does not become the weakest acoustic element.
- Specify by dynamic insertion loss (DIL) across octave bands, not by a single headline figure. Commercial silencers typically deliver 10–30 dB or more in the mid and high bands, and much less at 125 Hz.
- Forced ventilation — where natural flow through silenced louvres is not enough, the fan and ducting must themselves be treated, or you have simply moved the noise source.
- Check the thermal side — a quiet enclosure that cooks the machine is a failed enclosure. Pair this work with our thermal management guide and our perforated vent pattern guide.
Typical Source and Target Levels
| Source | Typical unmitigated level | Common target | Note |
|---|---|---|---|
| Diesel generator set, 200–500 kVA | 100–110 dB(A) at 1 m | 70–75 dB(A) at 1 m | Low-frequency dominant; needs mass plus damping |
| Screw compressor | 85–95 dB(A) at 1 m | 75–80 dB(A) at 1 m | Strong tonal content; check the 500 Hz–1 kHz bands |
| Vacuum pump / blower | 85–95 dB(A) at 1 m | 75 dB(A) at 1 m | Vibration isolation is critical |
| HVAC chiller / rooftop unit | 80–90 dB(A) at 1 m | 60–65 dB(A) at the property line | Often a barrier screen, not a full enclosure |
| Industrial shredder / granulator | 95–105 dB(A) at 1 m | 80–85 dB(A) at 1 m | Impact noise; needs a heavy, damped shell |
Seven Steps to an Acoustic Enclosure Specification
- Measure the source in octave bands. A single dB(A) number is not a design input. If that is all you have, treat the 500 Hz band as a conservative proxy.
- Calculate the required insertion loss at the receiver — source level at the receiver without mitigation minus the target level. Use ISO 9613-2 for the propagation part.
- Choose the enclosure type: full acoustic shed for continuous heavy plant, partial enclosure or shroud for intermittent or localised sources.
- Size the panel from the mass-law table plus the required insertion loss, and pick the infill thickness from the absorption table.
- Design the ventilation as a duct with silencers specified by octave-band DIL, and verify the thermal balance.
- Specify the sealing and the isolation — compression strips on every joint, double-lip thresholds, acoustic boots on every penetration, resilient mounts under the machine and, where needed, under the enclosure.
- Define verification: sound power to ISO 3744 or sound pressure at a stated distance and operating condition, measured before and after, with the measurement positions written into the contract.
What We Build, and What We Need From You
Send us the octave-band spectrum (or the machine model and duty cycle), the target level and the measurement distance, the airflow requirement, and whether the enclosure is indoor or outdoor. We will return a one-page Acoustic Build Pack within 24 hours — panel construction, infill specification, silencer schedule, sealing detail, isolation layout and a fixed-price line item. Use the contact form or email gongtianbao@xhlmarketing.com. For enclosures that also have to survive transport and site handling, see our vibration and shock testing guide. We build the enclosure to the acoustic specification; the final in-situ measurement is a site responsibility, and we will tell you honestly what the build can be expected to deliver.
Frequently Asked Questions
How much noise reduction can a sheet metal acoustic enclosure achieve?
A single-wall steel panel gives roughly 25–35 dB of transmission loss in the mid-band depending on thickness. A properly built double-wall enclosure with 50 mm mineral wool, a perforated liner and sealed joints typically achieves 25–35 dB of insertion loss, and up to 40–45 dB for a high-performance build with an added mass barrier. The practical ceiling is almost always set by leaks and unsilenced ventilation, not by the panel.
Why is my enclosure underperforming even with thick insulation?
Because sound finds the hole. In an unlined enclosure a 10% opening limits the achievable reduction to about 9 dB, and a 5% opening to about 19 dB, regardless of panel quality. Look first at the door gap, the air intake, cable pass-throughs with no acoustic boot, and any rigid connection that lets structure-borne vibration bypass the panel.
What panel build-up should I specify?
Start from the ISO 15667 reference: 1.5 mm outer steel (or anything giving 10–15 kg/m² surface mass), 50 mm mineral wool at 48–96 kg/m³, a perforated cover at about 30% open area, and 6 mm safety glass for windows.
How do I ventilate without losing the noise reduction?
Treat every airflow path as an acoustic duct. Fit acoustic louvres or duct silencers matched to the panel transmission loss, and specify them by octave-band dynamic insertion loss rather than a single number — typically 10–30 dB in the mid and high bands. Treat the fan and ducting too, or they become new sources.
Does thicker steel always mean quieter?
Up to a point. Doubling surface mass buys about 6 dB, so 1.5 mm to 3.0 mm is worth roughly 6 dB; doubling again buys another 6 dB for a lot of weight and cost. Past about 20–25 kg/m² an absorptive cavity with a resilient layer is usually the cheaper decibel, because real-world performance is limited by flanking paths rather than by the panel.