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Noise Control

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Noise Control NOISE CONTROL 1986 ISBN 8787355094 (ib.) ISBN 8787355 11 6 (hf.) 2nd edition 1 st impression Printed in Denmark by Ncerum Offset FOREWORD This booklet is adapted from a publication of the Swedish workers protection fund (Arbetarskyddsfonden) and is printed with the permission of the fund. Established by national legislation and funded via a small levy on the total wage bill, the fund is operated jointly by the employers and trade unions. Its object is to make industry and industrial employees aware of safety and environmental questions, and to encourage the provision of an improved and safer working environment throughout swedish industry. To this end it conducts research and collates industrial experience on a number of different subjects, of which the effects and reduction of noise is one. The results of its work are then distributed to interested parties in a straightforward and understandable form for the benefit of the non-experts in individual factories who have to deal with the problems at first hand, as well as those with previous knowledge. It is hoped that this booklet will spread the benefits of this work to a wider audience than was originally possible. CONTENTS INTRODUCTION .......................................................................................................... 1 NOISE AND MAN ....................................................................................................... 2 ACOUSTIC CONCEPTS ........................................................................................... 7 Sound .................................................................................................................... 7 Noise and tones .................................................................................................. 7 Frequency, Hertz ................................................................................................. 8 Infrasound and ultrasound ................................................................................ 8 Decibel, dB ........................................................................................................... 8 Sound level measurement ................................................................................. 9 Equivalent sound pressure level, LAeq,T .......................................................... 9 Octave band filters ........................................................................................... 11 Structure-borne and liquid-borne sound ...................................................... 11 Resonances ........................................................................................................ 11 Addition of noise from several sources ....................................................... 11 Attenuation by distance ................................................................................... 12 Sound insulation and sound reduction coefficient .................................... 13 Sound absorption and absorbents ................................................................ 13 GENERAL NOISE CONTROL MEASURES ......................................................... 14 Alteration of machines and equipment ........................................................ 14 Material handling ............................................................................................... 16 Enclosure of machines ..................................................................................... 16 Attenuation of structure-borne sound .......................................................... 17 Attenuation by using absorbents ................................................................... 18 Sound insulated rooms .................................................................................... 19 Noise control of new projects ........................................................................ 20 Planning the building ........................................................................................ 22 Noise reduction measures in rooms ............................................................. 23 Purchase and installation of machinery ....................................................... 24 A PROGRAM FOR NOISE CONTROL .................................................................. 25 METHODS OF NOISE REDUCTION ..................................................................... 28 Reduction at source ......................................................................................... 28 Changing methods of work ............................................................................ 28 Preventing propagation .................................................................................... 28 NOISE MEASUREMENT ......................................................................................... 30 The purpose of measurements ...................................................................... 31 Frequency weighting networks ....................................................................... 31 Time constants .................................................................................................. 31 Practical noise measurement procedure ..................................................... 35 Background noise ............................................................................................. 35 INSTRUMENTATION ................................................................................................ 37 Sound level meters and portable instruments ............................................ 37 Calibration .......................................................................................................... 46 Laboratory measurement and analysis systems ........................................ 46 PRACTICAL EXAMPLES OF NOISE CONTROL TECHNIQUES ................................................................... 51 Characteristics of sound ................................................................................. 52 Causes of noise generation ............................................................................ 67 Plates in vibration .... : ........................................................................................ 76 Damping of resonances ................................................................................... 86 Vibration isolation of machines ..................................................................... 94 Airborne sound insulation of walls and panels ........................................ 104 Room treatments by absorbents ................................................................. 112 Propagation in ducts ...................................................................................... 120 Noise caused by air movement ................................................................... 132 Noise in pipes carrying liquids .................................................................... 146 INDEX ....................................................................................................................... 150 INTRODUCTION Laws governing the safety and health of employed people exist in most coun­ tries of the world; the purpose being to create a suitable working environment and eliminate unsafe practices and processes. Work areas should be designed and laid out so that they are satisfactory both from the environmental and safety pOints of view. In this connection safety also implies that noise is kept at a level which is not likely to cause hearing damage. If this requirement is given due consideration at the planning stages of new production plant and factories, and when replacing existing equipment and plant, it is nearly always possible to reduce sound from machinery and installa­ tions. In existing factories and work areas, significant noise reduction can often be achieved by relatively simple methods. The safety organisation should be involved on questions of measurement and control of noise in the factory and take part in the planning of new or altered work methods and processes. A noise control program should involve the following: 1. The preparation of a noise map after making measurements in all areas. 2. The setting of target noise levels for all areas. 3. A description of all measures planned, a cost analysis, and the attenuation expected. 4. The setting of priorities within a plan to achieve the agreed targets, stating start and finish times. Noise and man The development of society has lead to more and more sound sources giving higher and higher noise levels. Noise is one of the most widely and most frequently experienced problems of the industrial working environment. Noise affects man both physically, psychologically and socially. Noise can damage hearing interfere with communication be annoying cause tiredness reduce efficiency Intense noise or long stays in a noisy environment can lead to permanent reduction of hearing sensitivity caused by damage to the sensory organs of the inner ear. This type of hearing damage can never be repaired. The risk of hearing damage increases both with the sound level and the time spent in the noisy environment, but the risk also depends on the characteristics of the sound. In addition, sensitivity to noise is extremely dependent on the individual. Some people may suffer hearing damage after only a short time, others can work for long periods, sometimes their whole working lives, in very noisy surroundings without suffering any demonstrable hearing damage. After spending a short period in intense noise
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