Introduction

Nowadays, more and more consider-ation is being given to environmentalissues. Our efforts in this field havealready led to our being awarded theDanish Environmental Prize for devel-oping a plant for removing the poisonousnitric oxides from exhaust gases.

Formerly, noise was considered anecessary, but harmless, evil. Today,excessive noise is considered a formof pollution which, in the long run, maycause permanently reduced hearing.As a consequence, authorities now de-mand that noise levels are kept belowcertain specified limits.

One of the first countries to introducea standard for noise limits was theFederal Republic of Germany which,in 1968, issued a code regarding thenoise levels permitted on its ships.Today, there are numerous nationaland international codes which both re-commend, and demand, maximumpermissible noise levels in the variousparts of a ship.

The greater demand for noise limitationin the maritime area has, of course,aroused wide interest. Consequently,greater demands are now made on theengine designer to provide more de-tailed and precise information regard-ing the various types of noise emissionfrom the engine.

After a brief definition of what noiseactually is, this paper will attempt toclarify “noise” as applied to MANB&W’s two-stroke engines, and willthen go on to discuss the primary noisesources and types of engine-relatednoise emissions, noise level limitation,and the current situation in relation tonoise.

What is Noise?

A popular definition of noise is “an un-desirable sound”. To what extent asound can be characterised as noiseis, of course, a personal evaluation.However, if the sound level is so high

as to be damaging to health, it will nor-mally be considered by one and all asundesirable and, therefore, as noise.

Sound is the result of mechanical vibra-tions occurring in an elastic medium,e.g.air. When the air starts to pulsate, thevariations in air pressure will spreadfrom the source through the transfer ofenergy from molecule to molecule. Themore energy transferred, the higher thesound level.

Intensity of Sound

The physical intensity of sound, I− which expresses the volume of thesound − is defined as the energyemitted per second, per m2 of a sur-face which is at right angles to thedirection of propagation of the soundwave, as shown in Table 1 and Fig. 1.

Diesel Engines and the Environment - Noise

Intensity of sound

I = pu = ρcu2

and if we use k = √ρc, the corresponding mean effective soundpressure (p) and the pulsation velocity (u) may be stated as follows:

p = k x √ I and u = 1k x √ I

where,

I = Intensity of sound (W/m2)p = Mean effective sound pressure (N/m2)u = Mean effective pulsation velocity (m/s)c = Velocity of sound in medium (air) (m/s)ρ = Specific mass of medium (air) (kg/m3)k = √ρc ≅ √1.2 x 340 = 20

at normal ambient air temperature.

Reference for sound levels

Reference sound intensityIo = 10-12 W/m2

Given a sound intensity Io = 10-12 W/m2 and using the above formulas,we can state the corresponding reference mean effective sound pressure (po)and mean effective pulsation velocity (uo) as follows:

Reference sound pressure levelpo = 20 x √10−12 = 2 x 10-5 Pa (Pascal = N/m2)

Reference velocity leveluo = 1

20 x √10−12 = 5 x 10-8 m/s

Table 1: Sound wave formulas

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Sound Level MeasurementUnits

The International Standards Organisa-tion (ISO) has determined the followingreference values for acoustics:

Reference for sound intensity:Io = 10-12 W/m2

Reference for sound pressure :po = 2 x 10-5 Pa

The above-mentioned reference valuesrepresent sound intensity and soundpressure at the lowest levels perceptibleto the human ear.

As the ear is not particularly sensitiveand is just able to discern that a sound

has doubled in intensity, a linear divi-sion of the intensity would be imprac-tical. For this reason, decibel (dB) hasbeen introduced as a unit for measur-ing sound.

This unit is logarithmic and is definedas 10 times the logarithmic relationshipbetween the actual intensity of thesound and the reference value:

Sound intensity level (dB):LI = 10 x Log10 (I/Io);

re Io = 10-12 W/m2

As sound pressure squared corres-ponds to the intensity of the sound,the following corresponding values are

valid when we use sound pressure asa basis:

Sound pressure level (dB):Lp = 20 x Log10 (p/po);

re po = 2 x 10-5 Pa.

Normally, it is the sound pressure levelwhich is measured, and when nothingelse is given, it will be re 2 x 10-5 Pa.

On the basis of the above, a sound in-tensity of 10-12 W/m2 corresponds toa sound level of 0 dB, and a sound in-tensity of 1 W/m2 corresponds to asound level of 120 dB. Incidentally,120 dB is the level at which the earbegins to feel pain − normal conversa-

Top offunnel

Sphericalpropagation

Bridge wing

R

Sound level at distance R2 compared to distance R1:

Area of a sphere A = 4πR2

Intensity I2 = I1 x (R1/R2)2

Sound level L2 = L1 - 20 x log10 (R2/R1)

In general, the sound level will be reduced by 6 dBfor each doubling of the distance from the noise source

Fig. 1: Spherically propagated sound waves − far-field lawDistance R from noise source (point source assumed)

dB130

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NR1020

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Centre frequencies of octave bands

31.5 63 125 250 500 1k 2k 4k 8k Hz

Fig. 2: ISO’s Noise Rating curvesOctave band pressure levels, re 2 x 10-5 Pa

2

tion is usually conducted at around55 dB.

At the so-called “far-field”, i.e. whereno sound is reflected and where soundwaves can be assumed to be propa-gated spherically, a doubling of the dis-tance will reduce the intensity of thesound to 1/4, corresponding to asound reduction of 6 dB, see Fig. 1.

The Influence ofSound Frequency

The sensitivity of the human ear isclosely related to frequency (Hz = vibra-tions per second). Sensitivity is low atlow frequencies, so it is often neces-sary to take measurements at differentfrequency ranges. Normally, thesemeasurements are made in the so-called octave bands. The octavebands are intervals between two fre-quencies where the upper frequency istwice as high as the lower.

Octave band frequencies, which arenamed according to their geometricalaverage frequencies, 31.5, 63, 125,250, etc. up to 16,000 Hz, are speci-fied by ISO. The audible frequencyrange for young people with undam-aged hearing is around 20-20,000 Hz.

As a result of the ear’s varying sensitiv-ity to combinations of different frequen-cies and sound levels, ISO has intro-duced special noise curves, and ISO’s“Noise Rating” curve sheet is veryoften used, see Fig. 2.

The groups of curves shown corres-pond, more or less, to the hearingcharacteristics of the ear with thesound level of the 1000 Hz octaveband used as a reference. As anexample, curve NR 60 shows that thesensitivity of the ear to 60 dB in the1000 Hz octave band roughly corres-ponds to its sensitivity to 75 dB inthe 125 Hz octave band.

If the sound pressure levels of thevarious octave bands for a given noisemeasurement are drawn-in on thecurve sheet, the octave band with the

highest NR-figure will give the resultingNR noise level for the measurementand, at the same time, show which fre-quency range(s) should be attenuated.

Another, simpler method of compen-sating for the ear’s subjective percep-tion is the use of sound level metersfitted with internationally standardisedfrequency weighting curves, i.e. elec-trical filtering curves, the so-called A,B, C, and now (for aeroplanes) also Dfilters. See Fig. 3.

In principle, to compensate for the fre-quency-dependent sensitivity of theear at various loudness levels, weight-ing curves A, B and C correct the ac-tual linear (un-weighted) noise levels inrelation to 1000 Hz corresponding to,respectively, the average ’Noise Rating’

curves NR 0-55 for A, NR 55-85 for B,and higher than NR 85 for C.

In particular, the A filter is often used togive the final results of a sound meas-urement as a single value. The measuredA-weighted value, designated dB(A), isalso regularly used, even in caseswhere the sound level is high and a Bor C-weighting curve would have beenmore appropriate.

A sound level obtained by linear meas-urement, i.e. without any correction forthe sensitivity of the ear, is designateddB(Lin).

0

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8k Hz

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4531.5 63

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-5

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500125

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Centre frequencies of octave bands250 4k

Fig. 3: Filtering (weighting) curves for sound level meters

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Primary Sources ofEngine Noise

On the basis of engine noise measure-ments and frequency analyses, it canbe ascertained that noise emissionsfrom the two-stroke engine primarilyoriginate from:

• The turbocharger, air and gaspulsations

• Exhaust valves• Fuel oil injection systems

and, to a certain extent,

• The chain drive.

The best way of reducing engine-re-lated noise is, naturally, to reduce thevibrational energy at the source or, ifthis is neither feasible nor adequate, toattenuate the noise as close to itssource as possible.

The different noise sources of the die-sel engine, of which the primary ones

are mentioned above, will, as a result,generate various types of noise emissionto the environment. The types of engine-related noise emission will be discussedin the next section.

Two-Stroke Engine NoiseEmissions

On the basis of theoretical calculationsand actual measurements, we employcomputer models − please refer to ourpaper: “MAN B&W Computerised En-gine Application System” − to provideour customers with data regarding thesound levels of the following engine-related noise emissions, which aretypical of our two-stroke engines:

1. Exhaust gas noise (gas pulsations)

2. Airborne noise (engine room noise)

3. Structure-borne noise excitation (vibration in engine feet)

1. Exhaust gas noise

Our constant-pressure turbochargedtwo-stroke diesel engines are, unlikethe former impulse turbocharged en-gines, equipped with a large exhaustgas receiver located between the gasoutlets of the cylinders and the turbo-charger(s).

Thanks to its ideal location, i.e. closeto the noise source, this gas receiveralso functions as an exhaust gassilencer, in particular attenuating thelow-frequency gas pulsations.

Fig. 6a curve 1 shows a 6L80MC en-gine, running at nominal MCR, wherethe calculated octave band analysis ofthe exhaust gas noise from an exhaustgas system without boiler and withoutsilencer has been drawn in.

The noise level calculation is based ona distance of 15 metres from the top ofthe funnel to the bridge wing. Thecurve sheet shows that the noise levelin the octave band frequencies be-tween 125 and 1,000 Hz is decisive for

Fig. 4a: Absorption silencer

AttenuationdB40

30

20

10

031.5 63 125 250 500 1k 2k 4k 8k Hz

Centre frequencies of octave bands

Fig. 4b: Typical noise attenuation for a 25 dB(A) absorption silencer

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the total noise level of NR 81, and thatthe A-weighted sound level correspondsto 86 dB(A). The dB(A) figure is calcu-lated by accumulating the intensities ofthe octave band sound levels, includ-ing the A-weighted attenuation, asshown in Fig. 3.

Fig. 6b shows the similarly calculatednoise levels for a nominally rated6S26MC engine where the distancefrom the funnel top to the bridge wingis 7 metres.

To keep noise below a maximum per-missible level of, for example, 65 dB(A)on the bridge wing, a relatively volu-minous 25 dB(A) exhaust gas silencerof the absorption type will normally beadequate, as this attenuates the domi-nant frequency ranges in question.

As the exhaust gas arrangement itself(for example the exhaust gas boiler)can generate noise, we recommendthat the exhaust gas silencer is in-serted as close to the funnel top aspossible.

The most frequently used absorptionsilencer is a flow silencer, i.e. a pipewith sound-absorbing wall material

(mineral or glass wool). Fig. 4a showssuch a flow silencer which, apart fromhaving good attenuating qualities in thehigh-frequency ranges can, by virtue ofits size, also be used to attenuatesome of the lower frequency ranges.

The typical noise attenuation achievedwith such a silencer type is shown inFig. 4b as a function of the octaveband frequencies.

2. Airborne noise

Engine room noise is primarily gene-rated by emissions from the individualengine components and their surfaces,which cause the air to pulsate.

The average engine noise levelsmeasured, for example according to’CIMAC’s Recommendations for Meas-urements of the Overall Noise forReciprocating Engines’, or other similarstandards, are used to express thetypical airborne sound pressure levelof the engine.

The calculated average sound levelcorresponds to the average value ofsound intensity measured at different

points around the engine. Measuringpoints are - depending on the enginesize - located at two or three heightsaround the engine, and at a distanceof approximately one metre from theengine surface. Along each side of theengine, the number of measuringpoints at each level must equal half thenumber of cylinders. Fig. 5 showswhere these measuring points couldbe located.

In general, depending, of course, onthe type of engine, the average air-borne noise level of a nominally ratedengine will be around 104 dB(A),whereas the maximum level measuredaround the engine, and normally near aturbocharger, will be about 108 dB(A).

Fig. 6a curve 2 shows the average air-borne noise level calculated for a nomi-nally rated 6L80MC engine with anoise level of approximately NR 101and 105 dB(A) for an engine with high-efficiency turbochargers, (curve 2A)and approximately NR 98 and 103dB(A) for an engine with conventionalturbochargers (curve 2B). The dif-ference in noise levels originates fromthe difference in noise emission fromthe turbochargers themselves. Ingeneral, the higher the turbochargerefficiency, the higher the noise emissionfrom the turbocharger and the engine.

Fig. 6b shows the corresponding aver-age airborne noise level calculations fora 6S26MC engine. Because of thereverberations of sound in the engineroom, the sound pressure based noiselevels measured in the vessel may be1-5 dB higher than the calculatedsound intensity based noise levels.

Measurements show that the turbo-charger noise has a dominant in-fluence on the total average airbornenoise level, an influence which hasbecome greater and greater becauseof the increasingly efficient and highpowered engines demanded by theshipyards and shipowners.

The maximum noise level measurednear a turbocharger will normally beabout 3-5 dB(A) higher than the aver-

Fig. 5: Example of location of measuring points on a diesel enginein accordance with ’Cimac Recommendations of Measurement’

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age noise level of the engine, using thehigh figure for high efficiency turbo-chargers. Often it is the maximumnoise level measured at an engine thathas to meet the specified noise limit re-quirements.

Especially in large diesel engines, itmay sometimes − to meet the noiselimit requirements − be necessary to in-troduce additional noise reductionmeasures, see Table 2.

These measures may reduce the maxi-mum noise levels by 3-5 dB(A) andsometimes more, depending on theirextent. For further information regard-

ing noise reduction measures, see ourpaper “Noise and Vibration Optimised11-cylinder Diesel Engine for Propul-sion of 4,800 TEU Container Vessel”.

It would be extremely difficult to meetstricter requirements with regards tomaximum engine room noise level of,for example, 105 dB(A) instead of 110dB(A), especially in view of the in-fluence of sound reverberations andthe noise emitted by other machinery.The possibility of reducing the noisefrom an existing engine is greatlylimited because, as previously men-tioned, the noise stems from many dif-ferent sources, and because the noisetransmission paths − through which vi-brational energy is transferred from onearea to another through the engine −are numerous.

However, in principle, the transmissionof airborne noise from the engine roomto other locations, e.g. accommodationquarters, normally has no influence onthe actual noise level in these locations.

3. Structure-borne noise excitation

Vibrational energy in the engine is pro-pagated, via the engine structure, tothe engine bedplate flanges, i.e. the“feet” of the engine. From here, theenergy is transferred to the ship’s tanktop, and then outwards to the ship’s

100

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Maximum2A

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1. Exhaust gas - distance 15 metres (re 2 x 10-5 Pa)2. Airborne - average and maximum (re 2 x 10-5 Pa)3. Structure-borne - engine feet, vertical (re 5 x 10-8 m/sec)

Centre frequencies of octave bands

40

NR50

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100

Fig. 6a: ISO’s NR curves and noise levels for a 6L80MC engine withA) high efficiency T/C and/or B) conventional T/C. MCR: 20,580 kW at 93 r/min

1. Internal absorption material in scavenge air pipe

2. Ring diffusor absorption plate in the top of the scavenge air cooler

3. External insulation of the scavenge air cooler

4. Additional absorption material at the engine and/or at the engine roomwalls (yard’s responsibility)

5. Additional turbocharger intake silencer attenuation(turbocharger maker’s responsibility)

6. Additional attenuation material at the turbocharger’s inspection cover

Table 2: Additional noise reduction measures on diesel engines

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structure which starts to vibrate andthus emits noise.

Among the sources which can gener-ate vibrational energy are the pulsescaused by the combustion process ofthe engine and the reciprocating move-ment of the pistons.

The vibrational energy transferred be-tween the contact surfaces of the en-gine bedplate and the ship is largelyamplitude-dependent, so the velocitycan normally be employed as a unit ofmeasurement. Like the sound pressurelevel, the velocity is best expressed indB, see also Table 1:

Velocity level (dB):Lv = 20 x Log10 (v/vo);

re vo = 5 x 10-8 m/sec.

The reference velocity used correspondsto the previously used intensity andsound pressure reference values andhas therefore been selected by MANB&W Diesel. According to thelatest ISO Standard, the referencevalue 10-9 m/sec is now used in thisnorm.

Figs. 6a and 6b (curve 3) also showthe structure-borne noise excitationlevels from a nominally rated 6L80MC

and a 6S26MC engine, given as a verti-cal vibration velocity level in the enginefeet.

Incidentally, the vibration velocity levelin a two-stroke engine is, on average,approximately 15-20 dB lower than ina four-stroke engine which, therefore,may sometimes have special vibrationisolators (resilient mountings) built-inbetween the engine feet and the tanktop of the ship. The structure-bornesound attenuation achieved is of some15-20 dB, which means that the finalresult corresponds to the level of asolid-mounted two-stroke engine.

The above-mentioned vibration velocitylevels in the diesel engine feet can,with the aid of empirical formulas, beused to calculate the excitation velo-cities and, thus, the sound pressurelevels in the accommodation quarters.The shipyards, or their consultants,normally have these formulas at theirdisposal.

Noise Limits

Limits for the maximum sound press-ure level are either defined specificallybetween owner/shipyard and enginebuilder, or indirectly by referring to na-tional or international legislation on thesubject. Many owners refer to the SBG(See-Berufsgenossenshaft) specifica-tions or the IMO (International MaritimeOrganisation) recommendations. TheIMO noise (sound pressure) limits fordifferent ship spaces are listed in Table 3.

The appearance of national and inter-national standards for noise levels inships has, in general, resulted in a con-siderable reduction of the noise levelsin newly-built ships.

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1. Exhaust gas - distance 7 metres (re 2 x 10-5 Pa)2. Airborne - average and maximum (re 2 x 10-5 Pa)3. Structure-borne - engine feet, vertical (re 5 x 10-8 m/sec)

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Fig. 6b: ISO’s NR curves and noise levels for a 6S26MC engine.MCR: 2,400 kW at 250 r/min

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Accommodation − Structure-bornenoise excitations

For example, the introduction of a“floating floor” construction in the ac-commodation quarters has reducedthe effect of the structure-borne noiseexcitation. Today, depending on thenoise standard to be met, the noiselimit requirements for accommodationare between 45 dB(A) and 65 dB(A), orlower, similar to those required in pas-senger ships. These noise require-ments can, as a rule, be observed bytaking adequate noise-attenuating pre-cautions, e.g. the above-mentionedfloating floor construction.

Engine room − Airborne noise

On the other hand, it is apparent thatthe above-mentioned general noise

reductions have not been achieved inthe engine room itself, where the air-borne noise from the diesel enginedominates.

The reason for this is that the accept-able noise limits for periodically mannedengine rooms have, for many years,been set at around 110 dB(A), and theintroduction of stricter requirementshas not been realistic as the noiseemission from a diesel engine has in-creased over the years because of thehigher rated engines.

The unchanged noise limit thus in itselfseems to have constituted a seriouslimitation for the engine builders. How-ever, it is a recognised fact that a noiselevel of over 110 dB(A) can, in the longterm, cause permanent damage tohearing, and therefore this limit cannotbe expected to be eased, rather on thecontrary.

This means that as engine designers −even though our engines generate nomore noise than the engines of ourcompetitors − we must, in future en-gine designs, pay particular attentionto the airborne noise emitted by ourengines.

Bridge wing − Exhaust gas noise

On the bridge wing, where it is theexhaust gas noise that predominates,there are also certain limitations, as thebridge wing is regarded as a listeningpost. The requirement here, dependingon the noise standard to be met, is amaximum of 60-70 dB(A), which canalways be met by installing a suitableexhaust gas silencer.

Conclusion

Generally, the noise emitted by the en-gine’s exhaust gas, and the structure-borne noise excited by the engine, areso low that it is possible to keep withinthe noise requirements for the bridgewing and accommodation.

On the other hand, the airborne noiseemitted from the engine in the engineroom is so high that in some casesthere is a risk that the noise limits forthe engine room cannot be met, unlessadditional noise reduction measures areintroduced.

In future, therefore, it must be expectedthat it will be very important, from amarketing point of view, to develop anengine with reduced airborne soundlevels.

References

(1) Akustik & Buller,by Johnny Andersson,Stockholm 1974

(2) Ship Noise Criteria.Do they Reflect the Present Level ofNoise Reduction Technology?by J. Ødegaard, Ødegaard &Danneskiold-Samsøe aps, 1986

dB(A)Work spacesMachinery spaces (continuously manned) ∗∗ 90Machinery spaces (not continuously manned) ∗∗ 110Machinery control rooms 75Workshops 85Unspecified work spaces ∗∗ 90

Navigation spacesNavigating bridge and chartrooms 65Listening posts, including navigating bridge

wings and windows 70Radio rooms (with radio equipment operating

but not producing audio signals) 60Radar rooms 65

Accommodation spacesCabins and hospital 60Mess rooms 65Recreation rooms 65Open recreation areas 75Offices 65

∗∗ Ear protectors should be worn when the noise level is above 85 dB(A),and no individual’s daily exposure duration should exceed four hourscontinuously or eight hours in total.

Table 3: IMO noise limits (sound pressure level)

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