Noise and Vibration

WEGEMT Association 25th School Small Craft Technology - Athens October 6-11 1997

Noise and Vibration Enrico Rizzuto

University of Genoa - DINAV (I)

Hosted by: National Technical University of Athens, Department of Naval Architecture and Marine Engineering

Small Craft Technology - Athens October 6-11 1997

Noise and Vibration E. Rizzuto

University of Genoa - DINAV (I) SUMMARY A growing attention has been devoted recently to acoustical problems on board of vessels, particularly in connection with the construction of an increasing number of high standard cruise vessels and pleasure crafts. Severe specifications on maximum noise levels on board, in particular in working, living, and resting areas are now common, reflecting into the need for a careful acoustical planning since the very first steps of the design. This approach implies in general a detailed analysis of the various elements of the acoustic circuit with identification and characterisation of the sources present on board, of receiving positions (location and required limits) and of transmission paths connecting them. In principle, actions on any of these elements are possible, but not always they all are practically feasible: in any case the strategy to be adopted should be chosen since the beginning, in order to have the maximum flexibility in the solutions: a posteriori actions are often incomplete, uneconomical, and can bring a considerable increase in weight. The present work aims at defining the physical meaning of the various elements of noise transmission and the way of acting on them.

Noise and Vibration Enrico Rizzuto University of Genoa - DINAV Page 3

1. INTRODUCTION The attention to environmental conditions in all the aspects of social living has continuously increased in the last decades: this has reflected in the enforcement of several laws and regulations aiming at improving the conditions of the working ambient and of living areas and, more in general, at reducing the impact, in its various aspects, of human activities on the environment. Among such aspects, noise pollution has been recognised to play an important role. Targets of rules and regulations in the noise field are in general: preventing hearing damage, improving safety (ensuring audibility of acoustic warnings), reducing communication interference (to enhance safety and efficiency), improving comfort (possibility of recreation, rest and sleep). The above mentioned concepts can be applied both to people directly involved in the considered activity (e.g. workers inside a plant) and to third parties (inhabitants of surrounding areas). Specific noise problems are present in the transportation industry, due to the presence of strong, moving sources and of comparatively flexible and light structures: this brings peculiarities in the treatment of the problem with regards both to people on board of the vehicle (crew and passengers) and to third parties. Typical examples in this field are aeroplanes and airports or cars and motorways, whose design is nowadays heavily conditioned by noise considerations. Coming to the marine field, rules and requirements have addressed the problems connected to health and operability of the crew in working areas and to the comfort of crew and passengers in living, recreation and resting spaces. Noise propagation from the vessel to the surrounding environment is considered only in a few, specific cases, like for fishing vessels (to avoid disturbances to fishing operations) and for naval units (to reduce detectability). In recent years, the expansion of the market for cruise vessels has stressed the need for a careful consideration of the aspect of comfort on board. A sign of this growing attention is represented by the introduction of comfort class notations by DNV and other Classification Societies: such notations are mainly based on the evaluation of the noise and vibration situation present on board.

2. ACOUSTIC PLANNING The answer to the request for higher performances as regards noise and vibration on board comes from a detailed analysis of the phenomena which are at the bases of noise and vibration generation, transmission and perception. The identification and characterisation of the elements of what could be defined the acoustic circuit (Fig.1) is therefore the first step of the study of the problem, the second being the implementation of actions that can reduce the amount of noise reaching the receiving position.

Fig.1 Scheme for Noise Transmission

SOURCE RECEIVER transmission

path


Such study takes the name of acoustic planning and should be undertaken since the very first phases of the vessels design, in order to be able to exploit all the possible solutions, without the constraints that are imposed by a design already developed or by an already completed construction.

3. RECEIVING POSITION The receiver is the end point of noise transmission: characterising it means essentially identifying a location and set limits for unwanted effects at that position. In general, unwanted effects are related to the perception of vibration and particularly of noise by the human body (there are cases, however, in which the limiting factor is represented by the operability of equipment very sensitive to vibrations). Concentrating the analysis on airborne noise, the sensitive element of the receiver is represented by the human ear, which can be said to perceive sounds according to the Noise Rating (NR) curves of Fig.2: in this (approximate) representation, each curve connects points of equal sensation. As it can be noted, the scales adopted in Fig.2 for both variables describing the noise signal (amplitude of sound pressure and frequency) are logarithmic (respectively with base 10 and 2: for more details on the units, see annex1). This corresponds quite closely to the sensitivity of the ear. The curves presented in Fig.2 can be used to assess the disturbance due to a noise signal with a given frequency content: the highest NR curve reached in all the considered bands gives the NR number associated to that signal. This evaluation tends to penalise the noises with a frequency content concentrated on a single band (the one characterised by the higher degree of disturbance, which gives the rating of the whole signal). If the evaluation of a frequency averaged disturbance is sought, in order to take into account the whole frequency content of the noise signal, than dB(A) are usually introduced (see annex 1 for definitions). A dB(A) level is related to the total energy of a noise signal perceived by the ear: it is obtained by filtering the signal with a filter (named A) which resembles the sensitivity of the ear. Opposite to NR numbers, dB(A) levels tend to underestimate the annoyance due to pure tones, because their energy content is somehow averaged in frequency: a very high contribution on one band can be counter balance in this approach by low levels in other bands, but this does not necessarily fit with the total human perception. Most of regulations are expressed in terms of dB(A) levels. This represents a

Fig.2 Noise Rating curves


compromise between the practical need for a clearly identifiable indicator of noise, representing the whole frequency range of interest, and the complicate human reaction to noise. The fulfilment of requests expressed in this way, therefore, not necessarily ensure to attain the goal of comfort. To give figures of the state of the art of noise control on board, the following tables are reported, describing DNV and RINA Comfort Class requirements regarding noise levels. The Comfort Class is a voluntary class notation specifying comfort criteria and it is issued when the fulfilment of the criteria has been verified by measurements. In the rules text a detailed description is included of procedures and standards to be followed for measurements. Noise and vibration, as well as climate (for DNV) are classified according to a rating which reflects different levels of comfort. According to DNV, a comfort rating number (crn) ranging from 1 to 3 is defined according to the following tables (see /1/ chapt.11).

Passenger Ships Passenger Accommodation

Noise levels in dB(A) Locations comfort rating number (crn) 1 2 3 Passenger top grade cabins 44 47 50 Passenger cabins, standard 49 52 55 Public spaces 55 58 62 Open deck recreation * # 65 65 70

* near ventilation inlet/outlet 5 dB(A) relaxation is allowed # measurements to be carried out at max Beaufort 3

Table 1 - DNV Comfort Class criteria for passenger ships /1/

High Speed, Light Craft Length above and below 50m

Noise levels in dB(A) Locations comfort rating number (crn) 50m and below above 50m 1 2 3 1 2 3 Passenger localities 65 70 75 60 65 68 Navigation Bridge 60 60 65 60 60 65 Service areas / shops / kiosk 68 73 78 65 65 70

Table 2 - DNV Comfort Class criteria for high speed light craft vessels


Yacht

Owner and Guest Areas Noise levels in dB(A)

Locations comfort rating number (crn) In harbour cond. Transit cond. 1 2 3 1 2 3 Sleeping rooms 35 40 45 - - - Lounges / Saloons 40 45 50 53 58 62 Outdoor Recreation Areas 50 55 60 75 80 85 Navigation Bridge - - - 60 60 65

Table 3 - DNV Comfort Class criteria for yachts

According to RINA, a noise merit coefficient (cmr) is calculated as the linear interpolation between two limiting dB(A) values: Lmin and Lmax, corresponding respectively to cmr=1 and cmr=0. No dB(A) values over Lmax are allowed.

Fast vessels Noise levels in dB(A) corresponding to limit values

of the noise merit coefficient (cmr) Locations cmr =1 (Lmin) cmr =0 (Lmax) 60m >60m 60m >60m Operational space 60 55 65 65 Public space (seating places) 65 60 75 70 Public space (other) 65 65 75 75

Table 4 - RINA noise merit coefficient criterion for fast vessels

4. SOURCES On board of marine vehicles several noise sources can significantly contribute to the sound pressure levels in the various areas of the vessel, their relative importance being dependent on the type of vessel and on the operating conditions, which can be very different. Generally speaking, sources can be classified on the basis of the type of emission they produce. A source can radiate into air, into water or directly in the ship structure: accordingly, airborne, waterborne or structure-borne noise sources are to be considered. The same source can have more than one type of emission: in that case it should be characterised with respect to any type of noise it produces.


4.1 Characterisation of airborne and waterborne sources The emission of a source in a fluid can be described by the amplitude of the pressure fluctuation in a considered position (usually at a reference distance: 1m). The units are sound pressure levels, one for each frequency band, expressed in dB and referred to a given reference pressure (usual values: 20 10-6 Pascal and 10-6 Pascal respectively for air and water). Sources can be directional and an interaction can be present between the source and the walls of the room where it is placed: therefore, pressure level measurements made in a different ambient or position, even if at the same distance, can give different values. To overcome this problem, the total power emitted can be measured, which is not dependent on direction, distance and room characteristics. The strength of the source is expressed in this case in sound power levels (units: dB referred to 10-12 Watts). Both pressure and power levels can be A-filtered, as briefly mentioned in annex1, in order to obtain a single value indication of their direct impact on the human ear.

4.2 Characterisation of structureborne sources The characterisation of structureborne noise sources is more difficult, because the quantities that can be used to describe the phenomenon (forces and velocities) are vectors, instead of scalar variables (like pressure and power). Furthermore, the connections between the source and the surrounding environment are somehow stronger and have a direct influence on the power transmission (contrary to the case of fluidborne sources, for which the power emission can be considered as independent from the environment). If a single degree of freedom system is considered, the rate at which energy flows out of it can be expressed as (harmonic excitation):

= 1/2 Re [V F*]

V = complex velocity F = complex excitation force F*= complex conjugate of F

Where V and F are to be measured simultaneously. One of these two "active" quantities can be expressed as a function of the other one through the definitions of the mobility Y or of impedance Z (these two complex quantities express the relationship between the source and the environment at the contact points and can be calculated for different excitations, provided the system is in its linear field): Y = V/F = 1/2 |F|2 Re[Y*] Z = F/V = 1/2 |V|2 Re[Z*] The same relationship can be expressed in spectral terms for amplitude and phase in the case of a random excitation /4/. When considering the more realistic case of a source with Ns point connections, each of


them having 6 degrees of freedom, it can be shown that the following quantities are necessary to calculate the power transmission: 6N velocity spectra and 6N force spectra or: 6N velocity spectra and (6N)2 mobility spectra or: 6N force spectra and (6N) 2 impedance spectra. Even if, in suitable hypotheses, the total number of required data can be reduced, from what briefly above reported it can be concluded that: The complete description of the structure-borne noise radiated by a real source

would require an enormous amount of data and surveys that are technically possible, but very time-consuming

Such a description cannot be independent on the constraints of the source itself, because the forces and the velocities experienced at the contact points are mutually related through the constraints themselves (this relationship being expressed by the mobility or impedance matrices).

The problem of obtaining a source strength description before installation, to be used for comparison of different solutions or as an input for noise predictions is somehow tackled on a practical basis. Phase relationships between the variables are neglected and only a few components of forces, velocities or accelerations (the most significant ones: usually vertical and horizontal components) are surveyed at contact points. Such measurements are performed in standardised constraint conditions, as close as possible to those actually realised in the specific application. A structureborne sources is then usually characterised by measuring average traslational acceleration levels in 2-3 directions at the feet of the machine under investigation The choice of both the components to be evaluated and, particularly, of the constraints to be used in the experimental tests, is still matter of discussion and different methodologies have been developed by various Institutes on the bases of their own experience: care should be taken when using data from different origins.

4.3 Propulsion and Generator Diesel Engines Propulsion engines (usually Diesel engines) in small crafts are dominant sources of airborne and structureborne noise (at least in navigation and in manoeuvre). At harbour Diesel generation engines take over. Airborne emissions come from combustion in cylinders, turbochargers and valve gears, while structureborne contributions are generally related to the inertia forces of the reciprocating parts and gear train camshafts for valves or fuel injection pumps. Reduction gears can contribute both to the airborne and structureborne noise. For more details, see /2/, chapt.6.


4.4 Exhaust Ducts Exhaust ducts are sources of both air and structureborne noise: high pressure levels are measured around the duct and at duct outlet, while structureborne excitation occurs via the supports of the duct and of the silencer. For more details: /2/, chapt.9

4.5 Propeller and shaft Propellers can be a major source of noise, in navigation and in manoeuvre: the mechanism of noise production can be different, all more or less related to hydrodynamic effects on the blades. In normal working conditions of the propeller, irregularities in water velocity at the propeller disk, due to the hull wake and/or to shaft inclination, produce variations of the angle of attack of the blade profiles, with consequent fluctuations of the pressure field around the propeller. Such fluctuations represent a source of waterborne noise which acts directly on the hull surface in the stern region or can cause on the blade cavitation phenomena, which, in turn, radiate waterborne noise. These two different mechanism for waterborne noise generation (cavitation and hydrodynamic pressure fluctuations) are also responsible for the generation of varying forces and moments on the shaft (bearing forces), which are transmitted through the sealing and the bearings to the structure of the vessel. Finally, the detachment of vortices at the blades trailing edge can sometime excite resonances of the blades themselves, with a consequent noise emission (singing of the propeller). For more details see /2/ chapt.10 and /3/

Fig. 5 Cavitation on propeller blades /3/

Fig.4 Wake and thrust irregularities /3/

Fig.3: Noise from exhaust gas system /2/.


4.6 Waterjets A growing number of high speed crafts adopts water jet propulsion systems: the mechanisms of noise generation are somehow different from those of common propellers and have been studied only in recent times. According to the ISSC97 committee II.2 report, the main waterjet noise sources are related to impeller pressure fluctuations (tonal components) and cavitation, to vortices and cavitation at the inlet lip and to jet impingement effects (jet impact and pulsation of entrained bubbles).

Fig 6: Noise generating mechanisms in waterjets /4/

4.7 Compressors In small crafts, compressors can be present for air conditioning and refrigerating systems. The associated sound power levels are much lower than those of the previously considered sources, but this kind of devices are supposed to run also at harbour, in silent conditions for the vessel. This can create problems, also due to the fact that resting areas can be adjacent to the machinery space. The radiation is mainly in air, but protection against structureborne transmission should not be underestimate, particularly in case of large air conditioning units.

4.8 Pumps On cargo ships, large and heavy pumps are fitted for ballast and oil cargo. They can run for several hours when loading or unloading, giving annoyance in adjacent areas. In smaller units a problem could be represented (at harbour) by sewage pumps with automatic start-up or by other devices (e.g. pumps for fresh water production systems working on inverse osmosis, which requires high pressure). Once again, the source strength is comparatively low, but care should be taken to minimise possible unwanted effects.


5. TRANSMISSION PATHS It must be noted that, in general, several paths connect the source to the receiver, including transmission in solid media, in air and in water (in form of structure-borne, air-borne and waterborne noise, respectively). Moreover, conversions from acoustic energy into mechanical and vice-versa are possible, giving often rise to complicate paths: potentially, all of them are responsible for the noise level at the receiver and care should be taken not to neglect any possible contribution. If a cabin on board of a ship is taken as receiving position and a running machine in an adjacent area is the source, the main structureborne noise transmission path will be represented by the connection of the feet of the machine to the underneath basement, the deck structure, the cabin walls and the inner part of the cabin. Other structureborne noise paths can be represented by any minor connection (cables, pipes, etc) connecting the machine to the ship structure (flanking paths). Further, an air-borne coupling of the surface of the source with the basement and/or the cabin bulkhead can occur, as well as a pure airborne path through the ventilation system to the receiving position. See Fig.7 from /5/

Fig.7 Transmission paths /5/

Usually, actions against noise transmission are undertaken particularly in the initial part (close to the source) and on the last part (close to the receiving position) of the path. The first class of noise counter-measures is represented by devices (resilient mounting, flexible connections, enclosures, etc.) that tend to create discontinuities between the source and the surrounding environment, reflecting back to the source part of the emitted energy. Other noise counter-measures are placed in the proximity of the receiving position, with the aim of preventing the noise present in surrounding areas to reach the receiver. In particular, in the mentioned case of the cabin on board, the last part of a possible transmission path crosses the walls (floor, ceiling and perimetral bulkheads) of the cabin itself.


6 ANTI-NOISE ACTIONS ON SOURCES As stated above, in principle it is possible to reduce the amount of energy transmitted to the receiver acting both on the source and on the transmission paths. Direct actions on the noise sources can be very effective, particularly if undertaken at a very early stage (design or installation).

6.1 Main Propulsion Plant Among the actions which can affect the design process since the beginning are e.g. the choice of type and location of the main propulsion engine and the relative position of it and of the exhaust gas system in respect to accommodation spaces. For larger units, (cruiser vessels) diesel electric systems are nowadays common, because they allow to separate the prime movers (diesel/generator sets) from the actuator (electric engine coupled with shaft and propeller). The diesel/generator sets, which are the most noisy component of the plant, can therefore be arranged in a more noise effective way, without the problems connected with the coupling to the shaft. In naval vessels with outstanding requirements in the limitation of noise radiation into water (e.g. mine-hunters), the engine can be structurally suspended below the bridge, in order to increase transmission losses toward the hull. High performances fast vessels are often propelled by gas turbines, which result in a lower structureborne noise generation (even if high frequency airborne noise generation must not be neglected). On small crafts no margins are present in general for this kind of actions, but, when designing the internal lay-out, it could be useful to separate as much as possible the resting areas from the machinery space.

6.2 Propulsion Systems The source strength of non-cavitating propellers as regards both induced pressures and bearing forces can be determined by means of suitable prediction codes. Theoretical prediction procedures are also available for cavitation noise, even if less accurate: they are generally based on the evaluation of the total cavitation volume and of its variations (Fig.5). The scheme adopted for the waterborne noise radiation due to cavitation is the one of a monopole source with the same volume variation. Experimental surveys are performed in model scale at the cavitation tunnel and in full scale, with flush mounted hydrophones and acoustic range. The above mentioned procedures are in general not applied to small vessels, for which designers approach the problem more on the basis of practical rules: keep as high as reasonably possible the clearance between blade tips and the hull to

limit the impact of radiated pressures reduce the shaft inclination to regularise water velocities at the propeller disk adopt skewed blades (even if extreme shapes are not typical of small crafts,

particularly in case of twin screw) to limit the effect of wake irregularities select comparatively high blade area ratios to limit cavitation unload the blade at tip and at hub to prevent edge vortices (and inherent cavitation)


For more details, see /1/,chapt.9 and /2/ chapt.10. Normal practise is to avoid singing of propellers by modification of the trailing edge. As regards waterjet, recommended anti-noise actions /4/ are the adoption of skewed blades impellers, a careful hydrodynamic design of the inlet and of the rotor-stator system (to limit interactions).

6.3 Machinery Not so much can be done by naval architects as regards other machinery, but trying to select them on the basis of low noise emission characteristics. However, reliable data are seldom available from manufacturers, particularly for a comparitive evaluation of different solutions: past experience in similar situations is generally followed.

7. ANTI-NOISE ACTIONS ON TRANSMISSION PATHS The current state of the art does not usually allow to solve noise problems only by reducing the emission of sources, which can result to be unfeasible for technical and/or economical reasons. The solution to noise problems is then sought also by means of actions on transmission paths. When dealing with anti-noise measures, two strategies are available to act on transmission paths: isolation and dissipation.

7.1 Isolation The first strategy consists in interposing in the transmission path as many discontinuities as possible, in order to reflect back (towards the source) a significant amount of energy, preventing it to proceed along the path. Discontinuities can be of a geometrical nature, (e.g. an abrupt change in section) or can be represented by changes in the material properties along the transmission path.

Fig. 8: Isolation of a structureborne noise source /2/, chapt.22


7.1.1 Resilient mounting Typical examples of such concept are resilient mountings interposed between the feet of a machine and the basement underneath. In this way, a rigid body and a stiff basement are connected by a (comparatively) elastic element which decouples them from each other. In minor ships (e.g. tugs) the whole superstructure can be isolated from the hull by means of resilient mountings. The working principle of elastic mounting can be related to behaviour of a single degree of freedom system (Fig. 9). The modulus of transmissibility TR (=ratio between the force transmitted to the basement PT and the exciting force P) is reported in Fig. 10 As it can be seen an increasing isolation effect is present over the natural frequency of the system. The effect of damping is to reduce the peak in the curve, but also to decrease the negative slope of the curve

Fig. 9: Single degree of freedom and inherent transmissibility.

In a more realistic case, the decrease in the transmissibility will be reduced by the flexibility of the machine and of the foundation, as well as by the inertial effects in the elastic element. Additional resonances are present (see the sketch presented in Fig. 10 for the case of zero damping.), thus reducing the actual attainable isolation.

Fig. 10: Transmissibility curve including flexibility of foundation and machine /2/ chapt.11


From what above mentioned the following practical rules can be derived: The first resonance frequency of the resilient mounting should be kept well lower

than the lowest significant exciting frequency. Above the first resonance frequency, damping effects tend to increase

transmissibility (isolators should have mainly an elastic behaviour) The stiffness of the foundation and in the machinery to be isolated improves the

mounting performances 7.1.2 Elastic connections The same concept of isolation applies to minor connections of the source to the structure: care must be taken to avoid bridge effects for noise propagation trough these flanking paths (e.g. pipes, ducts, even large electric cables)

Fig. 13 flexible pipes /6/:

(a) anular type (b) spiral type (c) flexible bend

Fig. 11: Elastic supports for a pipe passing through a wall /5/

Fig. 12 Elastic supports for a pipe fixed along a wall /5/


Fig. 14: Detail of the elastic mounting for a small component /6/. 7.1.3 Enclosures As regards isolation from airborne noise, an often adopted tool is represented by enclosures: the engine (or the machinery: pump, compressor, etc) is enclosed in an additional casing which reduces the sound power radiated in the room. Such measure mainly influences the sound pressure levels in the machinery space, but it can improve the situation also in an adjacent space. Care must be taken of the ventilating system of the machinery, which must ensure suitable running conditions, but must be studied not to limit the effectiveness of the enclosure.

Fig 15: Diesel-generator set with enclosure /2/, chapt. 19


7.1.4 Isolation of a receiving room The same isolation concept above mentioned can be applied also on the last part of the transmission path, close to the receiving position. In this way, the energy transmitted from the structure into the cabin is reduced. If the case of a cabin is considered, layers of a de-coupling material (rock wool, in general) can be interposed between the structural part of the deck and the inner surface of the floor such arrangement is known as floating floor. Similarly, walls and ceiling of the cabin can be isolated from the structure by means of elastic mountings. In Fig 16 the main parameters affecting the isolation of the cabin are presented. The utmost attention should be devoted to the connections of the inner part of the cabin (ceiling, walls and floor) with the load-carrying structures (decks, bulkheads), in order to avoid bridge effects (Fig. 16)

detail: side (porthole)

Fig. 16a: Isolation of a cabin key points- /2/ chapt.20

general view


detail: floor (corner) detail: ceiling

Fig. 16b: Isolation of a cabin key points- /2/ chapt.20

7.2 Dissipation The second technique for limiting noise propagation is to drain energy from the structure or the fluid and to dissipate it into heat. To this aim viscoelastic and phono-absorbing materials are adopted, specially designed to damp mechanical and acoustical energy respectively. 7.2.1 Damping layers In a cabin, this technique is e.g. represented by the adoption of a layer of viscoelastic material in the floor. The main role of such layer is to participate to the wall dynamic deflection storing strain energy and then to dissipate it into heat. The most common arrangement for viscoelastic materials is in the constrained layer configuration, with a thin (1-2 mm) damping layer with a (comparatively) rigid element on top (constraining layer) which acts as a base for the inner floor of the cabin. The constraining layer can be a metal plate or also made of a mixture of concrete and rubber latex (this latter solution being heavier than the former one and less indicated for small units). On fast ferry boats, the concept has been adopted also in the side plates of the vessel to damp the noise induced by the turbulent flow around the hull (constraining layers used: steel or aluminium plates).


7.2.2 Phono-absorbing materials To keep under control the sound pressure levels in a machinery space, phono-absorbing walls are commonly adopted. They are generally realised with glass or mineral wool with perforated covers (Fig. xx). Care should be taken of any opening in the walls, which, if not properly designed can affect the performances of the treatment. The same concept can be applied in cabin ceilings: if a phono-absorbing layer is placed on the upper part, it will provide an effective insulation from airborne noise coming from outside and also will contribute to reduce the sound pressure level in the cabin by draining energy from the interior (these effects will be added to the isolation from structureborne noise attained by the elastic mounting of the ceiling itself).

8. CONCLUSIONS In this work, an overview of the main topics concerning acoustic planning onboard of marine vessels has been given. The main sources have been classified according to the type of emission and concepts regarding their characterisation have been provided. A discussion of noise generating mechanisms of each source has been included. The receiver position has been characterised, by discussing the units adopted for noise perception by the human ear and reporting typical values of noise levels attainable in passengers vessels and small crafts. General concepts about noise transmission paths have been discussed with reference to marine vehicles. Finally, noise countermeasures that can be applied on board have been discussed, with an explanation of the main working principles. The presentation is intended to be at a general level and a constant reference is made throughout the notes to more analytic texts, focussed on specific items of noise and vibration on board. In particular, a complete and deep coverage of the subject is contained in reference /2/ and related bibliography.

Fig. 17 Scheme of sound absorption measures

in engine control rooms /2/, chapt.19


REFERENCES & BIBLIOGRAPHY /1/ Notes of the XXIII WEGEMT school, Full scale measurements of the performances

of ships and platforms held in Genoa 22nd-26th September 1997 /2/ Notes of the XVI WEGEMT school, Noise, vibration and shock on board ships

held in Genoa 23rd-28th March 1992 /3/ DNV Vibration control in ships 1985 A.S Veritec Hovik Norway /4/ ISSC97 Report of committee II.2 Dynamic response) /5/ Nippon Kaiji Kyokai Guide to ship noise control 1986 NKK Tokyo Japan /6/ C.M. Harris, Handbook of noise control, 1979 McGraw-Hill Inc. /7/ Cremer, L. & Heckl, M. Structure-Borne Sound 1988. Springer-Verlag

INDEX

1. INTRODUCTION...........................................................................................................3 2. ACOUSTIC PLANNING ...............................................................................................3 3. RECEIVING POSITION................................................................................................4 4. SOURCES.......................................................................................................................6

4.1 Characterisation of airborne and waterborne sources ..............................................7 4.2 Characterisation of structureborne sources...............................................................7 4.3 Propulsion and Generator Diesel Engines................................................................8 4.4 Exhaust Ducts ...........................................................................................................9 4.5 Propeller and shaft ....................................................................................................9 4.6 Waterjets .................................................................................................................10 4.7 Compressors............................................................................................................10 4.8 Pumps......................................................................................................................10

5. TRANSMISSION PATHS ...........................................................................................11 6 ANTI-NOISE ACTIONS ON SOURCES ....................................................................12

6.1 Main Propulsion Plant ............................................................................................12 6.2 Propulsion Systems.................................................................................................12 6.3 Machinery................................................................................................................13

7. ANTI-NOISE ACTIONS ON TRANSMISSION PATHS..........................................13 7.1 Isolation...................................................................................................................13

7.1.2 Elastic connections ..........................................................................................15 7.1.3 Enclosures ........................................................................................................16 7.1.4 Isolation of a receiving room...........................................................................17

7.2 Dissipation...............................................................................................................18 7.2.1 Damping layers ................................................................................................18

8. CONCLUSIONS...........................................................................................................19 REFERENCES & BIBLIOGRAPHY ..............................................................................20 INDEX...............................................................................................................................20 ANNEX 1: NOISE SIGNAL DESCRIPTION.................................................................21

Amplitude description...................................................................................................21 Frequency description ...................................................................................................22 A-weighted levels..........................................................................................................23


ANNEX 1: NOISE SIGNAL DESCRIPTION All noise signals are represented by fluctuations of a physical variable (sound pressure, acceleration, velocity, etc.) in the time domain. According to usual techniques for signal analysis, such fluctuations are decomposed into sinusoidal components, which are identified (spectrum of the signal) in terms of frequency and of root mean square (for a sine: rms = 0.707 the amplitude).

Amplitude description The amplitude of noise elementary components has ranges of several orders of magnitude (the human ear can perceive sound pressures from about 20 to 107 Pa): to deal with such wide ranges, a logarithmic scale with base 10 is usually adopted in order to compress the representation. Moreover, such a choice fits quite well with the human ear sensitivity: roughly the same increase in annoyance is perceived when passing from a 200 Pa noise to a 2000 one as from 20000 to 200000 Pa. For all the physical quantities used for noise description levels in decibels units are adopted, which are defined as the logarithm (to the base 10) of a ratio of quantities proportional to power:

Where Po is a given reference value, expressed in the same units as the variable to be measured. In case of pressure, acceleration and velocity, power is proportional to the square of the amplitude A of the signal, so:

The following units are the most used for noise and vibration description:

variable Levels [units and reference] Sound pressure LP [dB re 20 Pa] Acceleration La [dB re 10-5 m/s2] Velocity Lv [dB re 10-8 m/s] Force Lf [dB re 10-6 N] Power LW [dB re 10-12 W]

Table A-1

o10Pre P

Plog10][dBLo

=

o102

o

2

10Are AAlog20

AAlog10][dBL

o==


Frequency description Spectral analysis is easily performed by means of FFT analysers. The frequency range of interest is subdivided into equally spaced intervals (bands) and the average power associated with the components included in each interval is evaluated (Power Spectral Density function: PSD). The resolution in the PSD (i.e. the width of the frequency bands) depends on the analyser capabilities and on the acquisition parameters adopted for the sampling of the signal. In principle a very thin resolution in frequency can be achieved, with a very detailed description of the power content in frequency. In the following figure, for example, the most irregular curve corresponds to a PSD function with a resolution of 3.125 Hz (800 spectral lines in the range 5-2500 Hz), which is not at all to be considered as an extreme value.

Fig.A-1: comparison among different frequency resolutions

adopted for the analysis of the same signal

This type of analysis of a signal is called narrow band analysis and it is performed when dealing with particular problems of noise source identification. The frequency resolution featured by the human ear is not linear: the ear can not detect two frequencies separated by a fixed difference at high frequencies as well as it can at low frequencies. Its sensitivity to frequency is actually logarithmic: if a given variation in the perception of the tone of the sound is associated with a shift in frequency e.g. from 100 to 200 Hz, the same variation will be perceived when passing from 200 to 400 Hz, from 400 to 1600 and so forth. This type of sensitivity was earlier discovered by musicians, who introduced the term octave in relation with a doubling in frequency. What above suggested the introduction of a description of noise signals in terms of bands with a width increasing with frequency. Two systems have been adopted and internationally accepted: one based on bandwidth of an octave and the second one with a slightly higher resolution, based on bands of 1/3 of an octave.(see the figure above).

1 101 102 103 104

Freq. [Hz]

10dB


Each band is identified by a centre-band frequency which is given by the geometric average (i.e. square root of the product) of the extremes of the band itself. Centres of octave band are, in the range of audible frequencies [Hz]: (16) 31.5 63 125 250 500 1000 2000 4000 8000 16000 In the third octave description, the above frequencies are again centres of band, but two additional bands are defined between each couple of consecutive values: the complete series of centre-band frequencies is: 20 25 31.5 40 50 63 80 100 125 160 200 250 315 400 500 630 800 1000 1250 1600 2000 2500 3150 4000 5000 6300 8000 10000 12500 16000 20000. From what above, it can be concluded that octave and 1/3 octave band representations of a noise signal are somehow closer to the reaction of the human ear and are considerably more synthetic (11 and 31 figures respectively in the audible range). For these reasons, they are quite widely diffused in setting specifications for noise limitation

A-weighted levels A further step in the noise representation is the adoption of filters (in particular the one named A) which approximate the variation with frequency of the threshold and of the perception of human hearing: to be audible (or to give the same level of annoyance) a noise must feature a higher pressure value at low frequencies than at high ones. A widely diffused technique to account for this is to subtract to the (third or full) octave band levels of the signal a value in dB dependent on frequency (this corresponds to a tuning of the reference levels adopted in the dB representation to the curves of sensibility of the human ear). The filtered noise components are then somehow representative of the annoyance related to each band and are added to each other (in terms of power) to express a total annoyance, due to the whole signal (expressed in dB(A)).

Where dBi and AFi are respectively the level of the signal and the value of the A-filter in the ith band The use of dB(A) (usually for sound pressure level) represents a step forward in the simplification of the noise representation: a single number characterises the whole signal. For its simplicity, such a representation has been adopted in most regulations [see e.g. chapt. 3]

Frequency[H z]

A-Filter[dB]

50 -30.263 -26.280 -22.5

100 -19.1125 -16.1160 -13.4200 -10.9250 -8.6315 -6.6400 -4.8500 -3.2630 -1.9800 -0.8

1000 0.01250 0.61600 1.02000 1.22500 1.33150 1.24000 1.05000 0.56300 -0.18000 -1.1

10000 -2.5

Table A-2: Values of A-filter

=

i

10AFdB

10weightedA

ii

1010log[dB(A)]L

Noise and Vibration

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