Since 1 July 2014, IMO Resolution MSC.337(91) — the Code on Noise Levels on Board Ships — has been mandatory under SOLAS Regulation II-1/3-12 for applicable new ships of 1,600 gross tonnage and above. The marine generator set is one of the most acoustically significant continuously operating auxiliaries on board, and its noise and vibration signature determines whether the machinery space, the accommodation deck above it, and the bridge stay inside the applicable onboard noise limits — including 110 dB(A) in machinery spaces, 75 dB(A) in machinery control rooms, and 60 dB(A) or 55 dB(A) in cabins depending on ship size.
This guide covers what a shipowner, naval architect, or newbuild project manager needs to specify and verify: the regulatory backdrop, the two noise transmission paths (airborne and structureborne), the dB limits by ship space, vibration isolation mount selection principles, acoustic-enclosure design considerations, measurement under the MSC.337(91) noise-survey framework, the eight recurring installation observations from ASO Genset field-service projects, and an at-a-glance decision framework — with a downloadable Noise & Vibration Compliance Checklist for the commissioning engineer.
Table of Contents
- Why Marine Noise & Vibration Matter
- IMO Noise Code MSC.337(91) — Regulatory Overview
- Airborne vs Structureborne Noise Paths
- dB Limits by Ship Space
- Vibration Isolation Mount Selection
- Acoustic Enclosure & Insulation Design
- Active vs Passive Isolation
- Measurement & Compliance Verification
- 8 Common Installation Observations & Field Fixes
- FAQ
Quick Answer
Marine generator noise on applicable new SOLAS ships of 1,600 GT and above is controlled by IMO Resolution MSC.337(91) — the Code on Noise Levels on Board Ships. Key maximum A-weighted sound-pressure limits include 110 dB(A) in machinery spaces, 75 dB(A) in machinery control rooms, and 60 dB(A) or 55 dB(A) in cabins depending on ship size. Personnel entering spaces with nominal noise levels above 85 dB(A) should be required to wear suitable hearing protection.
Effective control requires both transmission paths to be addressed. Structureborne noise is reduced through correctly selected vibration isolators, controlled static deflection, suitable foundation design, and flexible service connections. Airborne noise is reduced through enclosure and room treatment, ventilation attenuators, and a spectrally matched exhaust-silencing system selected for the required insertion loss and allowable back pressure. Compliance is demonstrated through the noise survey required by MSC.337(91), using the applicable statutory and project measurement procedures.
Based on ASO Genset Field Experience
The engineering guidance in this guide draws on recurring observations from ASO Genset commissioning and after-sales projects across four marine installation contexts: offshore support vessels where statutory noise limits and contractual comfort targets are closely controlled; fishing-vessel projects where owners, yards, or class arrangements use MSC.337(91)-type noise-control principles as an engineering benchmark, even when the Code is not directly mandatory for the vessel type; yacht and workboat projects where accommodation-noise complaints drive post-delivery review; and steel-hulled inland/coastal vessels where structureborne transmission through welded foundations is a frequent contributor. Project examples are illustrative; applicable flag, class, contract, and manufacturer requirements take precedence.
Why Marine Generator Noise & Vibration Matter
The root reason MSC.337(91) exists is crew fatigue and safety. Chronic exposure to elevated sound pressure in accommodation and the wheelhouse degrades sleep quality, extends reaction times on watch, and is associated with an increase in operational incidents. The Code opens by stating three objectives: prevent hearing loss, facilitate clear speech and audible alarms, and provide a comfortable environment for rest and work.
Beyond crew welfare, there are three commercial reasons a shipowner should not defer the noise and vibration budget of the generator installation:
- Design review and onboard verification. Noise predictions are commonly required or expected during design review, and the applicable ship is verified through an onboard noise survey before entry into service.
- Charter contract compliance. Offshore vessel and yacht charter parties routinely specify accommodation-space noise limits below the applicable statutory minimum. Missing these targets can trigger acceptance disputes, remedial work, delayed delivery, or lost hire time.
- Long-term engine and hull integrity. Excessive structureborne vibration accelerates fatigue at engine foundations, stresses piping welds, and loosens electrical connections. Marine generator noise and vibration is therefore not just a comfort question — it is an equipment reliability question.
The generator set deserves particular attention as an acoustic and vibration source because it often operates for prolonged periods during voyage, manoeuvring, port stays, and night-time hotel-load conditions, sits directly on structural foundations that couple to occupied spaces, and generates both broadband combustion noise and narrow-band mechanical and electrical tonal components that human ears are highly sensitive to.
IMO Noise Code MSC.337(91) — Regulatory Overview
IMO Resolution MSC.337(91) was adopted on 30 November 2012 and made mandatory under SOLAS Regulation II-1/3-12 for applicable new ships of 1,600 gross tonnage and above meeting one of these construction dates:
- Building contract signed on or after 1 July 2014, or
- In the absence of a building contract, keel laid (or similar stage of construction) on or after 1 January 2015, or
- Delivery on or after 1 July 2018
For ships within its mandatory scope, MSC.337(91) supersedes the earlier voluntary guidance in IMO Resolution A.468(XII). A.468(XII) remains a recognised reference for vessels outside the mandatory scope and for legacy fleet-noise practice.
What the Code Governs
- Maximum permitted A-weighted sound pressure levels in defined spaces on board
- Noise measurement methodology (survey during sea trial)
- Personnel exposure limits and hearing-protection zones
- Noise-control measures in the design and construction phase
Two Tiers by Ship Size
The Code applies two different tables of noise limits depending on gross tonnage:
- Ships 1,600 GT ≤ GT < 10,000 GT: baseline limits — for example 60 dB(A) in cabins
- Ships ≥ 10,000 GT: tightened limits, typically 5 dB(A) lower in accommodation spaces — for example 55 dB(A) in cabins
Excluded Vessel Categories
Certain vessel types fall outside the mandatory scope of MSC.337(91):
- Dynamically supported craft and high-speed craft
- Fishing vessels
- Pipe-laying barges, crane barges, pile-driving vessels, and dredgers
- Mobile offshore drilling units (MODUs)
- Pleasure yachts not engaged in trade
- Ships of war and troopships
- Ships not propelled by mechanical means
For excluded categories, the Code is often used as a voluntary engineering benchmark, a class-society recommendation, or an owner/contractual noise-target reference — not as a statutory requirement.
Statutory Roles — Administration and Class Society
Flag Administrations are responsible for statutory implementation of MSC.337(91). Classification societies (ABS, DNV, Lloyd's Register, Bureau Veritas, CCS — see our ABS vs DNV vs CCS comparison) may review noise predictions, survey procedures, and compliance documentation when acting as recognized organizations on behalf of the flag Administration, or when applying their own class and comfort-notation requirements. Some societies, including DNV through its COMF-V notation, also offer voluntary comfort standards that may impose materially lower limits, depending on the notation, comfort rating, vessel type, and space category.
What Is NOT Covered
MSC.337(91) is a noise code — it does not itself establish general shipboard vibration acceptance criteria. Habitability vibration in accommodation and occupied working spaces is commonly assessed under the applicable class or contractual requirements. ISO 20283-5:2016 serves as the current ISO standard for vibration measurement, evaluation, and reporting with regard to habitability on passenger and merchant ships, particularly ships intended for voyages of 24 hours or more; it is a habitability-vibration standard, not a component of the statutory MSC.337(91) noise framework. ISO 6954:2000 remains visible in some legacy specifications but has been withdrawn and replaced. Machinery vibration, equipment integrity, and occupational hand-arm vibration may require separate standards and acceptance criteria.
Airborne vs Structureborne Noise Paths
Every generator noise problem on board a ship involves a combination of two physically distinct transmission paths. Understanding which path dominates in a given case is the first move in any remedial exercise — the wrong diagnosis wastes budget on the wrong mitigation.
| Path | Mechanism | Where It Dominates | Primary Mitigation |
|---|---|---|---|
| Airborne | Sound waves travel through engine-room air, penetrate bulkheads/decks, and radiate into adjacent spaces | In the machinery space, in spaces sharing a bulkhead with an unenclosed genset, and along the exhaust discharge path | Acoustic enclosure, mass-loaded bulkhead insulation, spectrally matched exhaust silencing |
| Structureborne | Mechanical vibration is transmitted from engine feet into the hull, propagates through steel structure, and re-radiates as noise from remote panels | In accommodation and bridge spaces distant from the machinery space, especially cabins directly above the engine room | Vibration isolation mounts under the genset, flexible piping/exhaust connections, floating floors in accommodation |
Why the Distinction Matters
Owners occasionally commission a costly acoustic enclosure to solve a cabin-noise complaint, only to find the enclosure barely moves the dB reading at the affected cabin — because the complaint was structureborne. Conversely, adding softer isolation mounts to a machinery-space noise complaint often has limited effect on the machinery-space reading, because the airborne path was dominant.
Diagnosing Path Dominance
Meaningful diagnosis is quantitative. Standard techniques include narrow-band vibration spectra (accelerometer at engine feet, foundation, and receiver locations), source-order analysis to separate shaft-rotation and firing-order contributions, transfer-path analysis to attribute receiver noise to specific routes, operational deflection shapes, and coherence measurements. The path whose contribution dominates the spectrum at the complaint receiver is the path whose budget should be treated first.
The Exhaust — A Third Sub-Path
Exhaust discharge is technically airborne but is often treated as a separate sub-path because it can dominate open-deck and accommodation noise even when the engine room itself is well insulated. A silencer not matched to the actual exhaust spectrum will radiate low-frequency exhaust pulses that mass-loaded bulkhead insulation cannot block. Marine generator exhaust systems typically use one or more silencer elements selected to meet the required spectral insertion-loss and back-pressure limits.
dB Limits by Ship Space
The core specification the marine generator installation must be designed against is the table of maximum permitted A-weighted sound pressure levels by space type, published in MSC.337(91) Chapter 4. The values below reflect the Code's typical limits; project targets may be tightened by a class-society comfort notation or a charter contract.
| Ship Space | 1,600 ≤ GT < 10,000 | GT ≥ 10,000 | Notes |
|---|---|---|---|
| Machinery spaces | 110 dB(A) | 110 dB(A) | Hearing protection required for entry; the 110 dB(A) limit assumes compliant hearing protectors are worn |
| Machinery control room | 75 dB(A) | 75 dB(A) | Speech-intelligibility priority |
| Workshop | 85 dB(A) | 85 dB(A) | Workshops forming part of machinery spaces are covered by the applicable machinery-space limit |
| Non-specified work spaces / other work areas | 85 dB(A) | 85 dB(A) | Default working-space category |
| Navigation bridge and chartroom | 65 dB(A) | 65 dB(A) | Alarm audibility priority |
| Cabins & hospitals | 60 dB(A) | 55 dB(A) | Rest-quality priority; ≥10,000 GT tightened by 5 dB |
| Messrooms & recreation rooms | 65 dB(A) | 60 dB(A) | ≥10,000 GT tightened by 5 dB |
| Offices | 65 dB(A) | 60 dB(A) | ≥10,000 GT tightened by 5 dB |
| Galley (no food-processing equipment) | 75 dB(A) | 75 dB(A) | Working-space category |
| Open recreation deck | 75 dB(A) | 75 dB(A) | External space |
Personnel Exposure — Three Distinct Concepts
Regardless of the space-by-space table above, MSC.337(91) uses three separate exposure concepts that should not be conflated:
- 24-hour equivalent exposure level Lex(24) = 80 dB(A): the Code uses 80 dB(A) as the 24-hour equivalent exposure criterion, evaluated across work, rest, and recreation periods.
- Hearing protection: personnel entering spaces with nominal noise levels above 85 dB(A) should be required to wear suitable hearing protection, and exposure-controlled zones must be signposted. Machinery spaces at 110 dB(A) are always hearing-protection zones.
- C-weighted peak limit 135 dB(C): unprotected personnel exposure above this instantaneous peak level is not permitted.
What This Means for Generator Installation
The tightest constraint on the marine generator installation is typically the cabin limit (55–60 dB(A)), because cabins sit near or above the machinery space and receive both airborne and structureborne contributions from the genset. Achieving 55 dB(A) in a cabin above a running large auxiliary requires an integrated noise budget across enclosure design (H2-6), isolation mount selection (H2-5), and hull-panel damping, informed by a full source-to-receiver path budget rather than a simple subtraction from the machinery-space limit.
Vibration Isolation Mount Selection
Correctly designed vibration isolation is one of the most cost-effective structureborne-noise reduction measures available on a marine generator installation. A correctly designed mount set can materially reduce transmitted vibration into the hull when selected and installed for the actual excitation spectrum and load path. A poorly specified mount set, by contrast, can amplify vibration at resonant frequencies and make the problem measurably worse.
Three Mount Families
| Mount Type | Material | Typical Natural Frequency | Typical Use |
|---|---|---|---|
| Elastomeric (rubber-in-shear) | Manufacturer-approved elastomer compound selected for the service environment | 8–15 Hz | Auxiliary marine gensets, up to typical mid-power ranges |
| Coil spring | Steel spring in housing | 3–6 Hz | Larger main and auxiliary sets; high isolation efficiency; requires snubbers for shock/roll |
| Composite (spring + elastomer) | Steel spring with elastomer pad in series or parallel | 4–8 Hz | Common on comfort-notation newbuilds |
Sizing Principle — Frequency Ratio
Vibration isolation begins to attenuate transmission when the frequency ratio — engine excitation frequency divided by mount natural frequency — exceeds approximately √2 ≈ 1.4. Below that ratio the mount amplifies vibration. Practical marine installations aim for a ratio of 3 or higher as a design target; actual isolation efficiency also depends on damping, mounting configuration, and the specific excitation spectrum, so the mount vendor's tested transmissibility data — not a single percentage rule — should govern acceptance.
Excitation Frequency Is Not Just Shaft Speed
The dominant excitation seen by the isolation mounts is a combination of shaft rotational frequency and its orders, engine firing orders, alternator electromagnetic forces, ancillary machinery on the skid, and structural resonances. In a four-stroke engine, each cylinder fires once every two crankshaft revolutions; for an evenly firing engine, the principal aggregate firing frequency is therefore related to the number of cylinders and is commonly expressed as one or more engine orders. Isolation-mount sizing should be checked against the complete measured or predicted excitation spectrum rather than a single rotational-frequency value.
Weight Distribution Matters
Each mount in the set must carry the correct share of the genset's static weight so that all mounts operate at the intended deflection. A genset skid rarely has its centre of gravity at the geometric centre — module layout, tank position, and alternator asymmetry all shift the CG. The mount vendor should be supplied with the per-mount static load derived from the actual centre-of-gravity data, not just the total genset weight, or the softer mounts will bottom out while the stiffer ones stay under-loaded.
Material Selection for the Service Environment
Marine engine-room environments combine fuel, lube oil, salt spray, ozone, and elevated temperature. The elastomer compound must be selected — and approved by the mount manufacturer — for the specific combination expected at the installation. No single compound is universal; some elastomers well-suited to oil resistance perform poorly in ozone or vice versa. Housings should be corrosion-protected to match the environment. Automotive mounts not designed, tested, and approved for the marine service environment should not be used on a marine genset skid.
Flexible Connections Are Part of the System
A vibration mount set is only as effective as the weakest connection to the isolated mass. Rigidly bolting a fuel line, coolant hose, exhaust connection, or electrical conduit between the genset and the ship structure short-circuits the mounts. Every service connection to a vibration-isolated genset should be a flexible connection — braided-metal fuel hose, elastomeric coolant hose, bellows-type exhaust connection, and electrical connections with adequate flexibility, bend radius, slack, and strain relief.
Maintenance Expectations
Vibration isolators are not maintenance-free. Periodic inspection is required for elastomer ageing, contamination, corrosion, loss of design deflection, snubber engagement, and physical damage. Replacement intervals depend on service environment and mount type, not on running hours alone.
Acoustic Enclosure & Insulation Design
Where vibration isolation attacks the structureborne path, the acoustic enclosure attacks the airborne path. A well-designed marine genset enclosure can deliver substantial airborne insertion loss between the genset and the surrounding machinery-space air; the actual site reduction achieved depends on enclosure design, ventilation openings, silencer selection, penetrations, and the measurement condition. Manufacturer-stated insertion loss should be treated as a laboratory reference, not a guaranteed site outcome.
Enclosure Layers
A marine acoustic enclosure is a layered assembly, not a single wall:
- Outer skin: marine-grade steel or aluminium panel; provides structural strength and fire performance to the applicable class requirement
- Damping layer: viscoelastic bitumen or polymer sheet bonded to the outer skin; suppresses panel-radiated resonance
- Absorptive core: mineral wool, glass fibre, or melamine foam; absorbs mid- and high-frequency airborne noise inside the enclosure
- Facing: perforated stainless steel sheet or protective cloth to retain the absorptive core and resist oil/fuel contamination
- Ventilation openings: lined ducted paths for combustion air intake and cooling air exhaust — the acoustic weak point requiring most attention
The Ventilation Trade-Off
A sealed box is the ideal noise barrier — but a diesel genset requires combustion air and cooling air per the engine manufacturer's airflow and heat-rejection data. Untreated intake and discharge openings can substantially reduce enclosure insertion loss, especially where they provide a direct line-of-sight transmission path. The standard mitigation is a lined splitter attenuator — a duct with parallel absorptive baffles that lets air through while absorbing sound. Splitter geometry must be sized by an acoustic-insertion-loss and pressure-drop calculation for the specific spectrum and airflow, not by a rule-of-thumb ratio.
Exhaust Silencing
Even a well-sealed acoustic enclosure will radiate audible noise if the exhaust discharges unsilenced. Marine genset exhaust systems typically use one or more silencer elements selected to meet the required spectral insertion-loss and back-pressure limits. Solutions vary by application:
- Reactive (chambered) silencer: targets low-frequency exhaust pulses
- Absorptive (packed) silencer: targets mid- and high-frequency broadband exhaust noise
- Water-injected exhaust: used on some accommodation-critical vessels; cools the exhaust gas by seawater injection and can further reduce discharge sound level, subject to product- and project-specific test conditions
Whether one silencer element is sufficient or multiple stages are required depends on the exhaust spectrum, the acceptable back-pressure envelope, and the receiver location — not on a universal rule.
Class Society and Fire Rules Constrain the Enclosure
An acoustic enclosure cannot be designed for noise performance alone. Its materials, ventilation arrangement, penetrations, detection coverage, and maintenance access must comply with the applicable flag and class fire-safety requirements and must not compromise any A-class divisions surrounding the machinery space. Whether an A-60, A-0, or other boundary rating applies depends on the space arrangement and the governing fire-control plan. For custom offshore and hazardous-area installations, see our companion guide on custom offshore generator cabin design.
Active vs Passive Isolation
The vibration isolators and acoustic enclosures discussed above are passive — they attenuate noise and vibration through material properties that do not adapt to operating condition. Active vibration-control systems have also been investigated and applied in specialist marine installations, particularly where low-frequency structureborne vibration remains difficult to control using passive measures alone.
An active system pairs the passive mount with accelerometers on both the isolated mass and the base structure, actuators (typically piezoelectric or electromagnetic) placed in parallel with the passive mount, and a digital controller that computes counter-forces to cancel residual vibration in real time. When correctly tuned, active isolation can provide additional low-frequency reduction in the frequency band where passive mounts are least effective — achievable improvement depends on the excitation spectrum, the passive baseline, sensor placement, and control-loop bandwidth.
When Active Isolation Is Worth Considering
- Small marine vessels (yachts, patrol craft) with thin hulls where structureborne noise reaches accommodation easily
- Comfort-notation newbuilds where accommodation-noise targets are tighter than the statutory minimum
- Retrofits where hull and foundation cannot be changed and further passive treatment has run out of options
When Passive Alone Is Typically Sufficient
- Larger merchant and offshore vessels where hull mass and passive isolation deliver the required reduction economically
- Fishing vessels and workboats where the operating environment is unfriendly to precision sensors and actuators
- Projects without the operations budget for the periodic calibration active systems require
Passive isolation remains the standard approach across the great majority of marine generator installations. Active isolation is a specialist tool where passive options have been thoroughly explored and further reduction is still required.
Measurement & Compliance Verification
Design review and onboard verification are separate stages. During design, the yard, owner, flag Administration, and classification society may review noise predictions and planned mitigation measures. Before the ship enters service, an onboard noise survey is carried out under the applicable statutory and project procedures, and a Noise Survey Report is prepared in accordance with MSC.337(91).
Applicable Measurement Standards
- ISO 2923 — Acoustics — Measurement of noise on board vessels. A widely used shipboard-noise measurement standard that may be specified for MSC.337(91) surveys where required by the project, flag Administration, or classification society.
- ISO 20283-5:2016 — the current ISO standard for vibration measurement, evaluation, and reporting with regard to habitability on passenger and merchant ships. Machinery vibration and occupational hand-arm vibration may require separate standards.
Sea Trial Noise Survey — What Actually Happens
During sea trial, a competent noise-measurement specialist accepted by the Administration, or acting under the applicable flag/class survey arrangement, takes calibrated Class 1 sound-level-meter readings at the defined measurement positions across the vessel — bridge, cabins on different decks, mess room, machinery control room, engine room walkway, workshop, galley, and open recreation deck. Measurements are taken under the Code-specified normal service condition — generally at not less than 80% of maximum continuous rating — with normal auxiliary machinery operating including the generator set(s). A specific project may specify a higher operating point (for example 85% MCR) as a project condition; that is not a universal statutory rule.
The measurement report compares each point to the applicable limit for that space category, and any exceedance triggers rework — most commonly additional accommodation-side insulation, silencer upgrades, or (in the worst case) genset foundation modifications.
Vibration Survey
A habitability-vibration survey typically uses tri-axial vibration measurements at representative occupied-space locations selected under the applicable class, contractual, or ISO procedure. Acceptance or guideline values vary by space use, vessel type, frequency range, and project comfort target. Yacht owner suites and other premium spaces may carry stricter contractual targets, but these should not be presented as universal ISO requirements. Noise and habitability-vibration surveys are often coordinated because the combined data can help identify possible structureborne contributions.
Predelivery Testing vs Post-Delivery Follow-Up
The sea trial survey is the formal compliance milestone. Consider an owner-requested follow-up survey after the vessel has accumulated representative operating experience, particularly where contractual comfort targets or unresolved complaints remain — this can capture behaviour that sea-trial conditions did not exercise, such as night-time cabin measurements under generator-only load or resonances excited only in specific rpm bands. For the sea-trial procedure we use during marine generator commissioning, see our marine generator commissioning & sea trial checklist.
8 Common Installation Observations & Field Fixes
Across ASO Genset marine field-service projects, eight recurring installation observations account for many of the accommodation-noise and vibration issues encountered on marine generator installations. Each is preventable at specification stage.
| # | Observation | Symptom | Field Fix |
|---|---|---|---|
| 1 | Mount natural frequency too close to excitation frequency | Accommodation vibration comparable to or worse than a rigidly-bolted installation | Recalculate mount natural frequency against the actual excitation spectrum; specify softer coil-spring or composite mounts with a design frequency ratio of 3 or higher |
| 2 | Rigid service connection short-circuits the isolation mount | Isolation ineffective despite correctly sized mounts | Replace rigid fuel/coolant/exhaust/electrical connections with flexible equivalents having adequate bend radius, slack, and strain relief |
| 3 | Automotive-grade or unsuitable elastomer compound on marine skid | Premature deterioration from fuel, oil, ozone, temperature, or seawater exposure | Replace with manufacturer-approved marine-grade mounts selected for the specific service-environment envelope, in corrosion-protected housings appropriate to the location |
| 4 | Undersized ventilation attenuator | Enclosure insertion loss falls short of the required spectral performance at site | Recompute splitter geometry with an insertion-loss and pressure-drop calculation for the actual airflow, spectrum, and receiver location; add second stage if constrained |
| 5 | Exhaust silencer not matched to the actual exhaust spectrum | Deck-space or accommodation exhaust noise dominates other paths | Reselect silencer element(s) to meet the required spectral insertion-loss and back-pressure limits; add complementary stage(s) as required by the spectrum |
| 6 | Weight distribution across mounts not verified before install | Some mounts fully compressed while others under-loaded; isolation degrades unevenly | Provide vendor with per-mount static-load data derived from actual centre-of-gravity; re-shim after dry-fit; re-verify deflection under running load |
| 7 | Accommodation deck plating not damped | Cabin noise above the applicable limit despite a compliant machinery-space reading | Bond viscoelastic damping tiles to accommodation deck plating below cabin floors; add floating floor if the required reduction is large |
| 8 | Sea trial noise survey run at low load or in port | Sea-trial report passes; owner feedback appears post-delivery at operating load | Insist on the Code-specified normal-service condition (generally ≥80% MCR in normal seaway); consider an owner-requested follow-up survey after representative operating experience where contractual comfort targets or unresolved complaints remain |
For the broader marine generator engineering fundamentals that these fixes build on — sizing, cooling, wet-stacking, IMO Tier III emissions, class-society certification — see our marine generator overview.
Field Note — Mount Changes Post-Delivery
Changing vibration isolation mounts — or their type, arrangement, or mounting configuration — on a delivered vessel may require review or approval depending on the applicable class rules and the scope of change. Coordinate any post-delivery isolation change with the vessel's class surveyor early in the planning stage so paperwork does not delay the next class renewal.
FAQ
What is the maximum permitted noise level for a marine generator room?
Under IMO Resolution MSC.337(91) — the Code on Noise Levels on Board Ships, mandatory under SOLAS Reg II-1/3-12 for applicable new ships ≥1,600 GT — a machinery space may not exceed 110 dB(A). The machinery control room is capped at 75 dB(A). Personnel entering spaces with nominal noise levels above 85 dB(A) should be required to wear suitable hearing protection; the 110 dB(A) machinery-space limit assumes compliant hearing protectors are worn.
What is the noise limit for an accommodation cabin on a SOLAS vessel?
60 dB(A) for ships between 1,600 GT and 10,000 GT; 55 dB(A) for ships ≥10,000 GT — the larger-vessel tier is tightened by 5 dB relative to the smaller-vessel baseline. Class-society comfort notations (for example DNV COMF-V) may impose materially lower limits, depending on the notation, comfort rating, vessel type, and space category. Charter contracts for offshore vessels and yachts often specify below-statutory accommodation limits as well.
Does SOLAS MSC.337(91) apply to my vessel?
MSC.337(91) applies to applicable new SOLAS ships of 1,600 gross tonnage and above that meet the relevant building-contract, keel-laying, or delivery dates (building contract on or after 1 July 2014; or absent a contract, keel laid on or after 1 January 2015; or delivery on or after 1 July 2018). Excluded vessel categories include dynamically supported craft and high-speed craft, fishing vessels, pipe-laying barges, crane barges, pile-driving vessels, dredgers, mobile offshore drilling units, pleasure yachts not engaged in trade, ships of war and troopships, and ships not propelled by mechanical means. Existing ships and major conversions should be assessed against the applicable flag, class, and project requirements rather than assumed to fall automatically under the same rule set.
How do I reduce marine generator vibration transmitted to accommodation?
Attack the structureborne path directly: install correctly sized elastomeric, coil-spring, or composite vibration-isolation mounts with a design frequency ratio of 3 or higher against the actual excitation spectrum. Where the lowest relevant excitation frequency is the 25 Hz shaft-rotation frequency of a 1,500 rpm set, a frequency ratio of approximately 3 would correspond to a mount natural frequency of about 8 Hz or lower; final selection must be based on the complete excitation spectrum, multi-degree-of-freedom mount behaviour, per-mount loading, and the supplier's transmissibility data. Replace every rigid service connection (fuel, coolant, exhaust, electrical) with a flexible equivalent to prevent short-circuiting of the mounts; and add viscoelastic damping tiles to accommodation deck plating above the machinery space. Per-mount static load must be derived from actual centre-of-gravity data — nominal genset weight alone is not sufficient.
What is the difference between airborne and structureborne noise?
Airborne noise travels as sound waves through air, penetrating bulkheads and decks into adjacent spaces — dominant in the machinery space itself and in spaces sharing an unenclosed bulkhead with the genset. Structureborne noise travels as mechanical vibration through the ship's steel structure and re-radiates as noise from remote hull panels — dominant in cabins and bridge spaces distant from the machinery. Airborne is treated with acoustic enclosures and mass-loaded insulation; structureborne is treated with vibration isolation mounts and flexible service connections. Most real complaints involve both paths.
Which vibration isolator type is best for a marine generator set?
For auxiliary marine gensets across typical mid-power ranges, elastomeric mounts (natural frequency 8–15 Hz) using a manufacturer-approved compound selected for the actual service environment — fuel, oil, ozone, temperature, seawater — are the workhorse. For larger sets and for comfort-notation newbuilds, coil-spring or spring-elastomer composite mounts (natural frequency 3–8 Hz) deliver higher isolation efficiency but require snubbers for shock and roll. No single elastomer compound is universal — the correct choice depends on the specific service environment. Automotive mounts not designed, tested, and approved for the marine service environment should not be used on a marine genset skid.
How is marine generator noise measured for compliance?
Shipboard airborne-noise compliance is demonstrated through the survey and reporting framework required by MSC.337(91), using calibrated instrumentation and the measurement conditions specified by the Code and the project. ISO 2923 may be used where required by the flag Administration, classification society, or contract. Habitability vibration is assessed under the applicable class or contractual requirements, with ISO 20283-5:2016 as the current ISO standard for habitability vibration on passenger and merchant ships intended for voyages of 24 hours or more.
Is active vibration isolation worth it for a marine generator?
Only in specific cases. Active systems can add low-frequency isolation on top of a well-designed passive mount, but require accelerometers, actuators, a digital controller, and ongoing calibration. They can pay off on small marine vessels (yachts, patrol craft) where thin hulls make structureborne noise hard to control passively, on comfort-notation newbuilds with sub-statutory accommodation targets, and on retrofits where passive options have been exhausted. For most larger merchant, offshore, and workboat installations, correctly sized passive isolation delivers the required reduction more economically.
What are the SOLAS noise limits for each shipboard space category?
The IMO Code on Noise Levels On Board Ships (Resolution MSC.337(91), mandatory under SOLAS regulation II-1/3-12) sets maximum permissible sound pressure levels in dB(A) for shipboard spaces on vessels of 1,600 gross tonnage and upward keel-laid on or after 1 July 2014. Limits vary by space category and by ship size. Selected limits for ships ≥ 10,000 GT: machinery spaces (continuously manned) 90 dB(A) with hearing protection required at 85 dB(A) and above; machinery spaces (non-continuously manned) 110 dB(A); machinery control rooms 75 dB(A); workshops 85 dB(A); non-specified work spaces 90 dB(A); navigating bridge and chartrooms 65 dB(A); listening posts including bridge wings 70 dB(A); radio rooms (equipment operating) 60 dB(A); cabins and hospitals 60 dB(A); messrooms 65 dB(A); recreation rooms 65 dB(A); open recreation areas 75 dB(A); offices 65 dB(A); galleys (no processing equipment) 75 dB(A); serving pantries 75 dB(A). Ships below 10,000 GT typically have slightly higher permissible limits (+5 dB in most categories). Passenger vessels have additional detailed limits for public spaces, cabins, and open decks. Habitability vibration limits are set separately under ISO 20283-5:2016 (superseding the older ISO 6954:2000) and referenced by comfort class notations; SOLAS itself does not set a numeric vibration limit — it is addressed through class rules and the survey framework. For the complete authoritative list, refer directly to Table 1 of the Code, and verify limits against the current MSC.337(91) text and any flag administration or classification society interpretive guidance.
How much does it cost to reduce marine generator noise?
Marine generator noise reduction cost depends on the required dB reduction and the approach chosen. Order-of-magnitude estimates for pre-project scoping: Passive vibration isolation upgrade (elastomeric or spring mounts) — typically USD 2,000–8,000 per genset for retrofit of a 100–500 kVA marine generator, reducing structureborne noise transmission by 10–15 dB when properly sized; included in newbuild spec at minimal marginal cost. Acoustic enclosure around the generator — USD 15,000–60,000 for a bolt-on marine enclosure on a 100–500 kVA generator, delivering 15–25 dB(A) airborne reduction at 1 m from the enclosure surface. Machinery space acoustic treatment (wall absorbers, floating deck layers, deckhead absorbers) — USD 30,000–200,000 for a machinery space, providing 5–15 dB airborne reduction in adjacent spaces. Double-elastic mount system (isolator on both engine base and foundation) — USD 8,000–20,000 per genset for an additional 5–10 dB structureborne reduction beyond single-elastic. Exhaust silencer upgrade (residential to critical grade) — USD 3,000–15,000 per generator. Active vibration isolation — USD 30,000–80,000 per genset installed; only justified for extreme comfort targets on smaller vessels. Full comfort-notation newbuild package (DNV COMF-V(1) or equivalent) — typically adds 3–5% to total vessel construction cost. Retrofit costs are generally 40–100% higher than the same measure specified at newbuild stage. Owner priorities should be: (1) meet mandatory SOLAS MSC.337(91) limits, (2) achieve any contractual charter or flag-state comfort requirements, (3) invest in additional comfort measures only where the vessel's operational profile (yacht, cruise, offshore accommodation) justifies the return.
What is a comfort class notation and do I need one for my vessel?
A comfort class notation is a voluntary classification society notation that certifies the vessel meets noise and vibration limits stricter than the mandatory SOLAS MSC.337(91) values. Comfort notations demonstrate premium habitability for passenger vessels, yachts, offshore accommodation platforms, and select naval or research vessels. Major class society comfort notations include: DNV COMF-V (vibration) and COMF-N (noise) with numerical class levels 1 (highest) through 3; Lloyd's Register PCAC (Passenger and Crew Accommodation Comfort) with levels 1 to 3; ABS COMF (Comfort) and HAB (Habitability) with numerical grades; RINA COMF with grades 1 to 3; BV COMF and CCS COMFORT notations follow similar structures. Comfort notation values are typically 5–15 dB(A) lower than the corresponding SOLAS MSC.337(91) limits, depending on notation grade and space category — for example, a cabin under DNV COMF-N(1) may require 50 dB(A) versus the SOLAS 60 dB(A) statutory limit. When you need it: cruise vessels and passenger ferries almost always require comfort notation by charter and flag-state agreements; motor yachts and superyachts (24 m and above) increasingly specify it for owner and charter market expectations; offshore accommodation vessels (flotels, walk-to-work) often require it under oil-and-gas operator standards. When you probably don't need it: commercial cargo vessels (bulk, container, tanker) — SOLAS statutory compliance is sufficient; workboats and tugs — comfort notation rare and expensive to achieve given machinery-dense compact layouts. Cost implication: adding a comfort notation at newbuild specification typically adds 3–8% to construction cost. Retrofit to comfort standard is usually uneconomic and requires major dockyard work touching most vessel systems — always specify the comfort notation grade at contract signing rather than plan for retrofit.
Free Download: Marine Generator Noise & Vibration Compliance Checklist
A printable engineering checklist covering dB limits by ship space, the eight common installation observations and field fixes, vibration-mount selection principles, and the MSC.337(91) noise-survey framework and applicable shipboard habitability-vibration standards. Based on ASO Genset marine commissioning and after-sales projects across ABS, DNV, LR, BV, and CCS class arrangements.
Download PDF ChecklistNeed MSC.337(91) Compliance for Your Marine Generator?
ASO Genset engineers marine generator packages with matched vibration isolation, acoustic enclosures, ventilation attenuation, and spectrally selected exhaust-silencing systems sized to your accommodation-noise budget and applicable statutory, class, and contractual comfort targets. Send us your vessel GT, deck plan, operating profile, and any comfort-notation targets for a technical review.
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- ABS vs DNV vs CCS Marine Generator Classification — How each class society handles noise-plan approval and comfort notations.
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- Marine Generator IMO Tier III Compliance — Companion emissions-compliance guide for shipowners planning newbuild specification.
