SIST EN ISO 11688-1:2009

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Akustik - Richtlinien für die Konstruktion lärmarmer Maschinen und Geräte - Teil 1: Planung (ISO/TR 11688-1:1995)Acoustique - Pratique recommandée pour la conception de machines et d'équipements à bruit réduit - Partie 1: Planification (ISO/TR 11688-1:1995)Acoustics - Recommended practice for the design of low-noise machinery and equipment - Part 1: Planning (ISO/TR 11688-1:1995)21.020Characteristics and design of machines, apparatus, equipment17.140.20Emisija hrupa naprav in opremeNoise emitted by machines and equipmentICS:Ta slovenski standard je istoveten z:EN ISO 11688-1:2009SIST EN ISO 11688-1:2009en01-november-2009SIST EN ISO 11688-1:2009SLOVENSKI
STANDARDSIST EN ISO 11688-1:1999/AC:2004SIST EN ISO 11688-1:19991DGRPHãþD

EUROPEAN STANDARDNORME EUROPÉENNEEUROPÄISCHE NORMEN ISO 11688-1August 2009ICS 17.140.20; 21.020Supersedes EN ISO 11688-1:1998
English VersionAcoustics - Recommended practice for the design of low-noisemachinery and equipment - Part 1: Planning (ISO/TR 11688-1:1995)Acoustique - Pratique recommandée pour la conception demachines et d'équipements à bruit réduit - Partie 1:Planification (ISO/TR 11688-1:1995)Akustik - Richtlinien für die Konstruktion lärmarmerMaschinen und Geräte - Teil 1: Planung (ISO/TR 11688-1:1995)This European Standard was approved by CEN on 3 August 2009.CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this EuropeanStandard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such nationalstandards may be obtained on application to the CEN Management Centre or to any CEN member.This European Standard exists in three official versions (English, French, German). A version in any other language made by translationunder the responsibility of a CEN member into its own language and notified to the CEN Management Centre has the same status as theofficial versions.CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland,France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal,Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.EUROPEAN COMMITTEE FOR STANDARDIZATIONCOMITÉ EUROPÉEN DE NORMALISATIONEUROPÄISCHES KOMITEE FÜR NORMUNGManagement Centre:
Avenue Marnix 17,
B-1000 Brussels© 2009 CENAll rights of exploitation in any form and by any means reservedworldwide for CEN national Members.Ref. No. EN ISO 11688-1:2009: ESIST EN ISO 11688-1:2009

Relationship between this European Standard and the Essential Requirements of EU Directive 98/37/EC .4Annex ZB (informative)
Relationship between this European Standard and the Essential Requirements of EU Directive 2006/42/EC .5 SIST EN ISO 11688-1:2009

Annex ZA (informative)
Relationship between this European Standard and the Essential Requirements of EU Directive 98/37/EC This European Standard has been prepared under a mandate given to CEN by the European Commission and the European Free Trade Association to provide a means of conforming to Essential Requirements of the New Approach Directive 98/37/EC, amended by 98/79/EC on machinery. Once this standard is cited in the Official Journal of the European Communities under that Directive and has been implemented as a national standard in at least one Member State, compliance with the normative clauses of this standard confers, within the limits of the scope of this standard, a presumption of conformity with the relevant Essential Requirements of that Directive and associated EFTA regulations. WARNING - Other requirements and other EU Directives may be applicable to the product(s) falling within the scope of this standard. SIST EN ISO 11688-1:2009

Relationship between this European Standard and the Essential Requirements of EU Directive 2006/42/EC This European Standard has been prepared under a mandate given to CEN by the European Commission and the European Free Trade Association to provide a means of conforming to Essential Requirements of the New Approach Directive 2006/42/EC on machinery. Once this standard is cited in the Official Journal of the European Communities under that Directive and has been implemented as a national standard in at least one Member State, compliance with the normative clauses of this standard confers, within the limits of the scope of this standard, a presumption of conformity with the relevant Essential Requirements of that Directive and associated EFTA regulations. WARNING — Other requirements and other EU Directives may be applicable to the product(s) falling within the scope of this standard.
TECHNICAL REPORT IS0 TR 11688-l First edition 1995-03-I 5 Acoustics - Recommended practice for the design of low-noise machinery and equipment - Part 1: Planning Acoustique - Pratique recommandge pour la conception de machines et d’6quipements 2 bruit r6duit - Partie I: Planifica tion Reference number ISOfTR 17 688-1:1995(E) SIST EN ISO 11688-1:2009

IsO/TR 11688=1:1995(E) Contents Page I Scope . 1 2 References . 1 3 Definitions . 2 4 Methodical design and acoustic aspects . 4 5 Conceptual and detailed design . 5 6 Low-noise prototyping . 23 7 Final testing . 25 Annexes A Summary of design rules . 26 B Noise control requirements for design . 31 C Information to be reported . 34 D Bibliography . 36 0 IS0 1995 All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from the publisher. International Organization for Standardization Case Postale 56 l CH-1211 Geneve 20 l Switzerland Printed in Switzerland ii SIST EN ISO 11688-1:2009

0 IS0 ISO/TR 11688=1:1995(E) Foreword IS0 (the International Organization for Standardization) is a worldwide federation of national standards bodies (IS0 member bodies). The work of preparing International Standards is normally carried out through IS0 technical committees. Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organizations, govern- mental and non-governmental, in liaison with ISO, also take part in the work. IS0 collaborates closely with the Internation Electrotechnical Commission (IEC) on all matters of electrotechnical standardization. The main task of technical committees is to prepare International Standards, but in exceptional circumstances a technical committee may propose the publication of a Technical Report of one of the following types: - type 1, when the required support cannot be obtained for the publication of an International Standard, despite repeated efforts; - type 2, when the subject is still under technical development or where for any other reason there is the future but not immediate possibility of an agreement on an International Standard; - type 3, when a technical committee has collected data of a different kind from that which is normally published as an International Standard (“state of the art”, for example). Technical Reports of types 1 and 2 are subject to review within three years of publication, to decide whether they can be transformed into International Standards. Technical Reports of type 3 do not necessarily have to be reviewed until the data they provide are considered to be no longer valid or useful. lSO/TR 11688-1, which is a Technical Report of type 3, was prepared by Technical Committee lSO/rC 43, Acoustics, Subcommittee SC 1, Noise. IS0 11688 consists of the following parts, under the general title Acoustics - Recommended practice for the design of low-noise machinery and equipment: - Part “I: Planning [Technical Report] - Par? 2: Introduction into physics of low-noise design ,. III SIST EN ISO 11688-1:2009

ISO/TR 11688=1:1995(E) 0 IS0 Introduction This International Technical Report provides a guideline for the design of low-noise machinery. Most of the existing International Technical Reports prepared in ISO/TC 43/SC I specify methods for the measurement and/or evaluation of noise. The final objective of this International Technical Report, however, will be noise control in existing machinery and noise control at the design stage. It is important that non-acoustic engineers are engaged in noise control practice. It is of great importance for these engineers to have a basic knowledge of noise generation and propagation characteristics and to understand the basic principles of noise control measures. Hence, this International Technical Report also serves as an introduction into acoustical terms, and as a basis to the acquisition of further knowledge in noise control. It is strongly required to support the dissemination of the given here through standardisation. Such considerations have led to the preparation of Technical Reports in the area of noise control. design rules nternational SIST EN ISO 11688-1:2009

‘TECHNICAL REPORT @ IS0 ISO/TR 11688=1:1995(E1) Acoustics - Recommended practice for the design of low-noise machinery and equipment - Part 1: Planning 1 Scope This International Technical Report is an aid to understanding the basic concepts of noise control in machinery and equipment. The recommended practice presented here is intended to assist the designer at any design stage to control the noise of the final product. Methodical development of products was chosen as a basis for the structure of this document (see Clause 4). The list of design rules given in this International Technical Report is not exhaustive. Other technical measures for reducing noise at the design stage may be used if their efficacy is identical or higher. To solve problems going beyond the scope of this International Technical Report, the designer can refer to the bibliography in Annex D, which presents the general state of acoustic handbooks at the time of publication. Furthermore, reference is made to the numerous technical publications dealing with acoustical problems. 2 References IS0 3744:1994, Acoustics - Determination of sound power levels of noise sources using sound pressure - Engineering method in an essentially free field over a reflecting plane. IS0 3746:- ‘1, Acoustics - Determination of sound power levels of noise sources - Survey method employing an enveloping measurement surface over a reflecting plane. IS0 4871:- ‘), Acoustics - Declaration and verification of noise emission values of machinery and equipment. IS0 961 l:- ‘1, Acoustics - Characterization of sources of structure-borne sound with respect to the airborne sound radiation of connected structures - Measurement of velocity at the contact points of machinery when resiliently mounted. IS0 9614-l : 1994, Acoustics - Determination of sound power levels of noise sources using sound intensity - Part I: Measurement at discrete points. IS0 9614-Z:-? Acoustics - Determination of sound power levels of noise sources using sound intensity - Part 2: Measurement by scanning. 1) To be published. SIST EN ISO 11688-1:2009

ISO/TR 11688=1:1995(E) 0 IS0 IS0 11200:-'1, Acoustics - Noise emitted by machinery and equipment - Guidelines for the use of basic standards for the determination of emission sound pressure levels at the work station and at other specified positions. IS0 11689:-l), Acoustics - Systematic collection and comparison of noise-emission data for machinery and equipment. 3 Definitions For the purpose of this International Technical Report the following definitions apply: 3.7 3.2 3.3 3.4 3.5 3.6 3.7 38 . 3.9 3.70 3.77 Airborne, liquid-borne and structure-borne noise: Sound propagating through air, a liquid or a solid structure, respectively. Active noise components: Components of machinery, which generate noise. In many cases these are the power converting devices generating mechanical work from power resources, such as electrical, mechanical or magnetic energy, hydraulic pressure, internal forces, or friction. Other noise “components” may be regions with non-steady flow and contact surfaces between moving parts. Psssive noise components: Components which transmit noise generated by the active components; they do not contain noise sources but can be dominating radiators of noise. Typical passive components are structural parts and covering panels of machinery. Periodic noise: A noise event which is periodically repeated. Typical gear wheels and piston machines. It is characteristic for periodic spectrum. sources of periodic noise are noise that it exhibits a line Tonal noise: Noise which is dominated by one or several clearly distinguishable tone(s). Broad band noise: Noise generated by either single shocks, i.e. short duration pressure pulses or impacts, or by turbulence in an air or fluid flow. The characteristics of broad band noise are that the frequency analysis shows a continuous spectrum over a large frequency range. force excitation: The excitation force is independent of the properties of the excited structure; an example of this is the effect of a light and flexible source on a relatively stiff and heavy structure. velocity excitation: The excitation velocity is independent of the properties of the excited structure; an example of this is a light and flexible structure excited by a relatively massive source. Quasi-static response: Response of the machine at frequencies below the lowest resonant . frequency. Resonant response: Response in a frequency range of distinct resonances. Multi-resonant response: Response in a frequency range with many resonances. SIST EN ISO 11688-1:2009

0 IS0 ISO/TR 11688-1:1995(E) Design process Noise control Y ::: Requirements concerning :c 5 1. Clarification of task noise behaviour from x ::: ::: .:. ::: X .:. ::: :: - clarifying standards, standards, authorities .:. X - :;: :.: requirements, state of 4 :i: the art. . . - regulations of clients, state of the art, j ::: ::: ::: competition, sales argument ::: .:* :a: - list of specifications :> :.: ::: 1 - own experience ::: : j: x :;: :*: .> :*: .:. 3: :2 - search for solution principles - comparison of different concepts - selection of concept 1 1 / t - choice of dimensions, material . . . - comparison (calculation and modelling) / -stk;tion y detailed L 4. Prototyping - measurements on prototype - evaluation of noise behaviour - comparison with references -1 I Clearance for series Acoustical experience and knowledge for comparison of different solutions - acoustical rules - rough formulae - diagnosis information - experience and examples - literature, drawings - acoustical modelling and FEM - acoustic devices - source strengths of partial sources (airborne, structure-borne, liquid-borne) . . . Noise measurement and noise reduction using the prototype - analysis and modification - acoustical diagnosis -final testing - comparison with the requirements Fig. 1: Stages of the design procedure; support of design process by noise control methods SIST EN ISO 11688-1:2009

ISO/TR 11688-1:1995(E) 0 IS0 4 Methodical design and acoustic aspects Methodical design is an operational approach which makes use of information from a variety of disciplines, for example machine acoustics. This way a basis is set for achieving targets and making decisions in design and development. The design procedure can be divided into four phases (listed below) which are increasingly specific (see Fig. I). Increase of information from phase to phase makes it possible to sort alternative solutions with respect to specific design criteria such as low noise level. The phases of systematic design are: 1. Clarification of task: Make a list of requirements which is the controlling document for the whole design task. Include noise specifications in this list with reference to legislation, the state of the art, competitors’ products, client demand or the weighting of machine noise as a company sales argument. (See Annex B.) 2. Conceptional design: This phase of the design process concentrates mainly on achieving the desired objectives. Little information is available about the final product at this stage and the noise behaviour is often assessed by comparison to known designs. 3 . Design and detail: As the design and choice of individual components progress, quantitative estimates of noise behaviour can be made through the selection of design options. 4. Prototyping: Measurements on the prototype allow quantification of major noise sources and sound paths. This may indicate specific measures leading to design changes. Compliance with the requirements can be confirmed by measurements. The following procedure can be applied in each of the four phases described above. It is very important to follow the methodology of eliminating the most dominant noise problems in the earliest possible stage of design: The first step of the process is determining the major sources of noise in the machine and establishing a priority list or scheme (see 5.2). Once the major sources are recognised, a more detailed analysis of the noise mechanisms must be carried out (see 5.3). The next step is analysing and describing the direct radiation of noise from the sources to the receiving position(s), and the transmission through the structure to the radiating surfaces (see 5.4). The final step contributions is to analyse the radi to the sound pressure ation level from those surfaces and to determine the various at the receiving position(s). Evaluate which combination of noise control measures is optimal. In designing low-noise machinery one should try to identify the basic acoustic mechanisms involved by consideration of the causal chain (Fig. 2). All design processes have a recursive element. So at every phase a decision has to be made as to whether the next phase can be entered or whether previous steps shall be repeated. SIST EN ISO 11688-1:2009

0 IS0 ISO/TR 11688=1:1995(E) I Acoustical mechanism I I 1 Part of machinery 1 r- Generation I 1 Source 1 1 Transyission 1 I Radiation I 1 Path I 1 Surface 1 Fig. 2: Causal chain of noise generation An illustration of how the different noise mechanisms are connected is shown in Fig. 3. The first priority in noise control is to identify the source. Different types of sources are shown in the first and second ring with key words corresponding to the headlines of the following clauses. Once the source type is determined, transmission through the particular medium will take place as seen in the third ring. Finally the noise will radiate into free air or excite a structure. The figure can be used to show that every sound source has its own characteristics, its specific transmission path through the structure and excitation of the radiating surfaces. To control the noise from a machine with many different types of sources, it is necessary to analyse each noise source, transmission path and radiating surface on its own to be able to evaluate the relative importance. The next clause shows an example of such a machine. 5 Conceptual and detailed design 51 . General Since a design solution always comprises the choice of a physical operating principle and the choice of a functional system, it is possible to make the following general comments for the choice of design concepts. With a high degree of probability, the mode of operation with the lowest speed and acceleration will provide the best acoustic solution. For a given operational principle the noise from a machine can be reduced by altering the mass, stiffness and damping of the structure. Design parameters such as material, shape, position, number of elements, dimensions, structure and type of connections can have a large effect on the noise emission. If applied in the proper way such alterations may reduce the vibration and/or radiation of the machine. Steady flow of gases and liquids is quieter than unsteady flow. Both in the conceptual phase and in the detailed design, the procedure described in Clause 4 and elaborated further in the following clauses can be used for diagnosis and noise control measures. In the conceptual phase only rough estimates, common design rules or a comparison with existing solutions is possible. In the detailed design phase the results of detailed calculations, modelling and survey experiments can be applied. 5 SIST EN ISO 11688-1:2009

ISO/TR 11688=1:1995(E) 0 IS0 -%$.A -u!!Ak~v /Ii+ Fig 3: Basic model of noise generation in machines SIST EN ISO 11688-1:2009

0 IS0 ISO/TR 116884:1995(E) 52 . Basic steps 5.2.1 Acoustical modelling and ranking The noise behaviour in machinery with different noise sources can be visualised by an acoustic model of the machine (see Fig. 2). To elaborate this model, the designer must first divide the machine into active and passive noise components. The active and passive noise components may have the capability of generating, transmitting and radiating airborne, liquid-borne and structure-borne noise. Therefore it is necessary to analyse the noise components for these three types of noise. The purpose of subdividing the noise components is the identification of the dominating noise sources, transmission paths and radiating surfaces. Then the designer must analyse along which paths noise can be propagated. Structure-borne, liquid-borne and airborne sound paths shall be considered. Furthermore, possible direct radiation of airborne sound from the individual active components must be considered. Finally the sound radiating surfaces of the machine must be identified. When the most important noise sources with their transmission paths are identified, an analysis of the process parameters must be carried out. The dominant noise contributions have to be controlled first. It is recommended to control the sources first before dealing with transmission paths and the radiating surfaces. Severe noise problems can be caused by the coincidence of driving frequencies and resonances in the active and passive components. General design rules: Divide machine into active and passive noise components; Locate airborne, liquid-borne and structure-borne noise sources; Locate the airborne, liquid-borne and structure-borne sound paths; Locate the sound radiating surfaces; Identify the strongest contributions (sources, transmission paths, radiating surfaces). 5.2.2 Example The purpose carried out. of this example is to demonstrate how acoustic modelling and noise source ranking can be Fig. 4 shows a hydrostatic power pack having active noise components such as: electric motor, hydraulic pump and a valve. They are all connected to the reservoir in a closed circuit. 7 SIST EN ISO 11688-1:2009

ISO/TR 11688=1:1995(E) 0 IS0 1) Relief valve 2) Outlet tube (12 mm) 3) Reduction fitting (25 mm - 12 mm) 4) Pump 5) Vibration-isolated flange 6) Mounting flange 7) Cover 8) Vibration-isolated coupling 9) Electric motor 10) Reservoir Fig. 4: Hydrostatic power pack The power pack has active noise sources representing airborne, structure-borne and liquid-borne noise sources. To visualise the transmission of noise from the different noise sources in the machine a block diagram, Fig. 5, is drawn which in graphical form illustrates the noise mechanisms of the power pack. A list of the noise sources, paths and surfaces is shown in Tables 1 to 3 SIST EN ISO 11688-1:2009

0 IS0 ISO/TR 11688-1:1995(E) Active Noise Components Passive Noise Components Reservoir, tubes and machine structure Key . . lo Airborne noise @ ‘/ “& Liquid - borne noise Structure-borne noise 0 Fig. 5: Acoustical model of power pack 9 SIST EN ISO 11688-1:2009

ISO/TR 11688-1:1995(E) 0 IS0 II Component Source Key I I A Airborne noise + Major contributor S Structure-borne noise - Minor contributor L Liquid-borne noise Component Source A S L Electric motor Electric motor Magnetic field Magnetic field Fan Fan -I- -I- Unbalance Unbalance Hydraulic pump Hydraulic pump Pumping Pumping + + + + Unbalance Unbalance Relief valve Relief valve Flow restriction Flow restriction Valve instabilitv Valve instability Table 1: Hydrostatic power pack; noise sources Table 2: Hydrostatic power pack; transmission paths Key I I A Airborne noise + Major contributor S Structure-borne noise - Minor contributor L Liquid-borne noise Component Path A S Electric motor Mounting points + Shaft Hydraulic pump Mounting points + Shaft Fluid connections Relief valve Mounting points Fluid connections . Coupling Coupling elements + Tubes Steel tubes Fluid Reservoir Mounting points + Plates Fluid Table 3: Hydrostatic power pack; radiating surfaces Key I I A Airborne noise + Major contributor S Structure-borne noise - Minor contributor L Liquid-borne noise 1 Component Radiating surface A S Electric motor Housing + Hydraulic pump Housing Tubes Walls Reservoir Walls + L IO SIST EN ISO 11688-1:2009

0 IS0 ISO/TR 11688-1:1995(E) A number of experiments were carried out on the power pack to identify the different sources, paths and radiating surfaces concerning noise emission. The main results are shown in Table 4 as sound power measurements in a reverberant room. All experiments were done under the same operating conditions. Table 4: Hvdrostatic Dower Dack: effect of noise control measures I Power ‘pack noise control 1500 rpm; 180 bar All transmission paths are present as in Fig. 5, A separate frame supporting the motor and the hydraulic pump is suspended by vibration isolators on the reservoir lid, The reduction in structure-borne sound transmission to the reservoir/machine structure results in a small reduction in sound power. The motor and pump frame is decoupled from the reservoir, The connection from the pump to the valve is made with a 2 m long hydraulic hose. This step gives a further reduction of 3 dB due to the reduction of structure-borne transmission to the reservoir, The reservoir is removed from the reverberation room, eliminating the airborne radiation from it, This does not result in a further noise reduction, leadidng to the conclusion that the reservoir was already sufficiently decoupled in step 3. The hydraulic pump is mounted on a conical flange on the electric motor, which included a vibration isolator, The fan is taken off the electric motor, and watercooling is provided, This results in a reduction of 1 dB, Finally the electric motor is encapsulated to reduce the airborne noise radiated from its surface, L WA in dB 90 89 86 86 85 81 The conclusions from the experiments were as follows: The sound pressure level of the airborne noise radiated from the surface of the hydrostatic pump alone was 9 dB less than the sound power from the complete pack; The major noise sources were structure-borne and liquid-borne contributions from the hydrostatic pump; The dominant structure-borne noise transmission paths were those between the pump and motor and pump and resevoir; The dominant radiating surfaces were those of the electric motor and the reservoir. The hydraulic pump used in this example is not typical of the equipment currently available. Replacing the hydraulic pump with one having less structure-borne and liquid-borne source strengths would have reduced the overall sound power level. 11 SIST EN ISO 11688-1:2009

ISO/TR 11688=1:1995(E) 0 IS0 53 . Control of noise sources 5.3.1 Airborne noise sources All streaming gases (e.g. air) can cause noise by turbulence, shock and pulsation. Turbulence Turbulence is a noise generating mechanism which has many different forms. Turbulence can create pure tone components in flows over a cylinder, such as a chimney pipe. Tones are also generated by flow over a cavity which is seen for instance in a flute or in cutters in woodworking machines. In channel flows, noise can be generated by sharp corners, struts or valves. Flows with high velocities at the nozzle exit or the tips of fans generate vortices due to the shear forces in the contact region between the air which is not moving near the nozzle and the exciting flow. This gives rise to broad band noise. The noise level and the spectrum of the noise depend on the flow velocity, the viscosity of the medium and the geometry of the nozzle. Reductions are achieved by lowering the flow velocity in the contact region. This can be done by lowering the pressure difference, by using larger diameters or by providing a bypass flow, e.g. in nozzles or tube exits. Noise sources are localised by analysing the flow system for possible obstacles. Reduction is effected by changing diameters of rods, by introducing spoilers on chimney stacks, by aerodynamic shaping or by reduction of flow velocity. A fan should be designed to operate with the tip speed as low as possible. Use variable speed instead of throttling. Too little clearance between rotor and housing can increase noise generation. Turbulence behind obstacles is avoided by removing obstacles, by minimising their number or by aerodynamic shaping (avoid sharp edges). Changing the geometry of nozzles or valves by using a branched or slit type will increase the frequency of the generated sound which makes sound absorption and isolation easier. Design rules to control turbulence in gases: Reduce operating pressure; Reduce pressure drops; Minimise flow speed; Optimise the jet outlet design to minimise velocity changes across a jet; Minimise tip speed of rotors; Avoid obstacles in the flow; Improve flow geometry. Shock and pulsation In piston machines, volume and pressure pulsations occur because of an uneven volume flow. Since these machines contain rotating components, pulsation occurs at frequencies proportional to the rotational frequency, generating tonall noise. Reductions are obtained by reducing the rotational speed, and in high pressure machines by reducing the operating pressure, if possible. 12 SIST EN ISO 11688-1:2009

0 IS0 ISO/TR 11688=1:1995(E) Shocks are generated by the fast release of a pressurised medium into a low pressure region. This happens during the opening and closing of valves and in high pressure pneumatic motors and pumps. Shock noise is reduced by slowing down the pressure-time variation either by reducing the pressure difference or by increasing the rise time. Quasi-stable shocks are generated in supersonic gas flows, for example in exhaust valves. These are reduced by reduction of the flow velocity. By designing throat area variations at the opening of valves in such a way that only slow temporal variation can occur, the noise can be minimised. Compression of trapped fluid in for instance piston or gear pumps should be avoided through equalisation channels. Single shocks from valves are broad band sources (generation of many frequencies). But shocks can occur periodically, for instance in high pressure pumps and motors, resulting in periodic noise with frequencies at the rotational frequency and multiples of this. Stable shocks generated in valve exhausts due to velocities exceeding the normal speed of sound in air cause intense broad band noise. This can be avoided by reducing the flow velocity. Design rules to control shock and pulsation in gases: Reduce speed of pressure change; Avoid obstacles near a rotor. 5.3.2 Liquid-borne noise sources Like air, liquids can also generate noise by turbulence, pulsation and shock. Therefore the same rules as those mentioned in 5.3.1 can be applied. Design rules to control liquid-borne noise sources: Reduce pressure drops; Minimise flow velocity; Avoid obstacles in the flow; Improve flow geometry; Reduce speed of pressure change. Cavitation Cavitation occurs in liquids when the static pressure drops below the vapour pressure. This may happen for instance in valves and pumps. In the region where the pressure is below the vapour pressure, cavitation bubbles grow. During recompression the bubbles implode, giving rise to high pressures. Since recompression often occurs by stagnation of flows on a surface, cavitation cannot only cause noise but can also be strongly erosive. Cavitation can be avoided, for example, by reducing the pressure drop per valve stage. Introducing more stages can lead to the desired total pressure drop. Cavitation is a broad band noise source. 13 SIST EN ISO 11688-1:2009

0 IS0 Design rules to control cavitation: Reduce pressure drop; Reduce flow velocity; Increase static pressure; Improve flow geometry to avoid cavitation; Do not allow flow velocities exceeding I,5 m/s; Keep suction lines short; Place the reservoir higher than the pump inlet; Use components with low flow resistance, e.g. sieves, valves etc. 5.3.3 Structure-borne noise sources Impact Impact noise is one of the most dominant noise sources in machinery. Many noise generating mechanisms can be treated as periodic impacts. The most important parameters in impact noise are the mass and speed of the impacting bodies and the duration of the impact. The frequency analysis of one impact noise event shows that it is broad band noise dominated by high frequencies because of the short duration of the impact. Periodic impacts generate periodic noise. The spectrum shows the impact frequency and multiples of it. Design rules to control impact noise: Increase impact time; Decrease impact velocity; Minimise the mass of the free impacting body; Increase the mass of the fixed body; Avoid loose components with alternating loads. Tooth meshing Tooth meshing as a special form of impact noise occurs for example in gearboxes and chain drives. Important parameters are the period of contact of the contacting elements, the force-time-variation during contact and the stiffness of the contacting elements (teeth). Tooth defects can cause an additional force variation and thereby increase the noise. Tooth meshing mostly results in the generation of pure tones (multiples of the meshing frequency). Measures to influence tooth meshing are geometrical changes of teeth and contacting surfaces (profile relief at the tips and the ends of teeth, helical gears) to increase the contact ratios, improvement of the accuracy of gearing and adjustment or increase of the number of teeth. The number of teeth of the wheels in contact with each other should be chosen so that the same pair of teeth meet as seldom as possible (for instance by using prime numbers). Bending in teeth and shafts has to be taken into consideration with 14 SIST EN ISO 11688-1:2009

0 IS0 ISO/TR 11688=1:1995(E) respect to geometrica for all possible loads. I changes. Gear wheel tooth profiles can be optimised for a limited load range, but not In case of low loads (e.g. in gears of household appliances), plastics can be used as material for gears. At high specific loads, a change of material has no significant effect on noise generation. Design rules to control structure-borne noise caused by tooth meshing: Increase contact time; Use helical gears; Increase number of teeth; Improve quality (alignment, tooth accuracy); Use plastics for low loads. Rolling Noise generated by rolling is a result of roughness or irregularities in the contact region of the rolling surfaces. Rolling noise is encountered in roller and ball bearings, in conveyer systems, rail and road vehicles. Rolling noise also depends on the flexibility in the contact region. The frequency content of rolling noise is broad band. When there are periodic elements in the excitation (e.g. in roller bearings), which is often the case, there may be tonal components as well. Design rules to control rolling noise: Maintain smooth rolling surfaces; Use proper lubrication; Use precision roller bearings; Minimise tolerances in housing (fit of bearing); Use friction bearings; Increase flexibility in the contact area. Inertia The acceleration of mass induces forces which can result in noise generation by a variety of effects as for example impact, rolling, friction or pulsation. Inertia forces are induced by oscillating masses or by unbalanced or rotating parts. In some cases (e.g. a crank mechanism) inertia forces can cause excitation of parts of the machine structure with multiples of the rotation frequency. Beware of rolling noise if roller bearings are carrying inertia forces. Inertia forces can be reduced by balancing, reducing the speed of revolution, the accelerated masses or the acceleration itself. In some cases single-plane balancing of disc-shaped rotors is sufficient, in all other cases dynamic balancing is necessary. Design rules to control structure-borne noise caused by inertia: Minimise inertia forces by balancing of rotors or counterbalancing of moving masses; 15 SIST EN ISO 11688-1:2009

0 IS0 Minimise accelerated masses; Improve steadiness of motion. Friction. self-excitation Mechanisms where friction causes stick-slip phenomena are potential noise sources. The variations in force encountered here act as an impact type which can excite the resonances of the structure and assume the form of self-excitation of resonances. Friction created noise as seen in the squeaking of brake discs, hinges etc. is very dependent on material selection and lubrication. In principle sliding generates broad band noise, but because of the excitation of resonances of the structure there are often strong tonal components in the generated sound. Design rules to control structure-borne noise caused by friction and self-excitation: Control the friction by proper material selection; Control the friction by proper lubrication; Increase damping of the structure which can be self-excited. Maanetic fields Magnetic fields are used for example in electric motors to generate driving forces for rotation. The non- uniform variation of moment during one revolution resulting in force variations on the bearings and the static parts of the motor causes vibrations. Magnetically induced noise is load dependent. It can be dominant in the case of a good thermal design of the electric motor and if low-noise bearings are applied. For drives with variable speed controlled by converters, high frequency noise may be generated. Transformer noise consists of twice the mains frequency (50 Hz) and multiples of this up to about 600 Hz. Structure-borne noise generated in the transformer core by magnetic phenomena (e.g. magnetostriction which depends on material selection) is transmitted by the cooling medium and the mounting points and is radiated by the housing. The windings of electrical transformers have to be fixed carefully to avoid vibrations causing low frequency sound. Design rules to control structure-borne noise caused by magnetic fields: Choose number of slots to avoid excitation of resonances in stator and rotor; Avoid that slots are parallel to poles; Minimise tolerances in shape and position of the magnetic core to obtain a good degree of symmetry of the magnetic field; Optimise the shape of poles; Consider magnetically induced noise caused by converters of drives with variable speed; Select transformer core material to reduce structure-borne noise generation. 16 SIST EN ISO 11688-1:2009

0 IS0 ISO/TR 11688~1:1995(k) 54 . Noise transmission 5.4.1 Airborne noise transmission Airborne noise generated in parts of the machine is transmitted to the environment. There are several means to control this transmission: acoustic enclosures; acoustic screens; silencers; sound absorption. The physical phenomena used in these noise control measures are reflection and absorption. Acoustic enclosures . These are closed sound insulating covers. Even small openings must be sealed. Covers are usually made of thin sheet metal to provide reflection of the noise. To improve the noise reduction of the enclosure a sound absorbent lining with porous material inside is necessary (thickness depends on the lowest frequency of interest). The basic design of machines is either completed by enclosures, or existing machine covers are modified to act as acoustic enclosures. If openings are necessary (ventilation, flow of material, cables etc.) they should be equipped with silencers. Openings for maintenance purposes shall be closed carefully during operation. To avoid structure-borne noise transmission into the cover sheets vibration isolation at the mounting points should be used (see 5.4.3), Design rules to control airborne noise transmission by enclosures: Enclose noise sources totally, even small gaps or holes (e.g. slits, joints) are important and must be sealed; Use solid sheets (sound insulating material) for the outer shell of the enclosure; Use absorbent material inside; Use silencers at openings for ventilation, cables, pipes, transport of material etc.; Avoid rigid connections between enclosure and machine; minimise number of mounting points; Enclosure of components can be effective. Screens Screens can be mounted near small machine components with high noise emission. Their efficiency is much lower than that of enclosures and highly dependent on direction and distance. They are, however, useful for achieving a noise reduction within a restricted area (operator position). Their effect is restricted to frequencies for which the length and width of the screen are at least equal to or larger than the wavelength of the airborne sound. Ii SIST EN ISO 11688-1:2009

ISO/TR 11688=1:1995(E) 0 IS0 Design rules to control airborne noise transmission by means of a screen: Use solid sheets (sound insulating material) for the screen; Use screens for operator positions; The side of the screen facing the machine should be supplied with a sound-absorbent cover. Silencers Silencers are components which prevent the transmission of airborne sound via openings. Absorption silencers are of the type “porously lined channel”. They are frequently combined with enclosures and fans in order to ensure heat removal without reducing the efficiency of the enclosures. The working pri
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