Tài liệu Kiểm soát tiếng ồn cn và âm học industrial noise control and acoustics

Industrial Noise Control and Acoustics Randall F. Barron Louisiana Tech University Ruston, Louisiana, U.S.A. Marcel Dekker, Inc. New York • Basel Copyright © 2001 by Marcel Dekker, Inc. All Rights Reserved. Copyright © 2003 Marcel Dekker, Inc. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress. ISBN: 0-8247-0701-X This book is printed on acid-free paper. Headquarters Marcel Dekker, Inc. 270 Madison Avenue, New York, NY 10016 tel: 212-696-9000; fax: 212-685-4540 Eastern Hemisphere Distribution Marcel Dekker AG Hutgasse 4, Postfach 812, CH-4001 Basel, Switzerland tel: 41-61-260-6300; fax: 41-61-260-6333 World Wide Web http://www.dekker.com The publisher offers discounts on this book when ordered in bulk quantities, For more information, write to Special Sales/Professional Marketing at the headquarters address above. Copyright # 2003 by Marcel Dekker, Inc. All Rights Reserved. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming, and recording, or by any information storage retrieval system, without permission in writing from the publisher. Current printing (last digit): 10 9 8 7 6 5 4 3 2 1 PRINTED IN THE UNITED STATES OF AMERICA Copyright © 2003 Marcel Dekker, Inc. Preface Since the Walsh-Healy Act of 1969 was amended to include restrictions on the noise exposure of workers, there has been much interest and motivation in industry to reduce noise emitted by machinery. In addition to concerns about air and water pollution by contaminants, efforts have also been directed toward control of environmental noise pollution. In response to these stimuli, faculty at many engineering schools have developed and introduced courses in noise control, usually at the senior design level. It is generally much more effective to design ‘‘quietness’’ into a product than to try to ‘‘fix’’ the noise problem in the field after the product has been put on the market. Because of this, many engineering designs in industry take into account the noise levels generated by a system. Industrial Noise Control and Acoustics was developed as a result of my 30 years of experience teaching senior-level undergraduate mechanical engineering courses in noise control, directing graduate student research projects, teaching continuing education courses on industrial noise control to practicing engineers, and consulting on various industrial projects in noise assessment and abatement. The book reflects this background, including problems for engineering students to gain experience in applying the principles presented in the text, and examples for practicing engineers to illustrate the material. Several engineering case studies are included to illustrate practical solutions of noise problems in industry. This book is Copyright © 2003 Marcel Dekker, Inc. designed to integrate the theory of acoustics with the practice of noise control engineering. I would like to express my most sincere appreciation to those students in my classes who asked questions and made suggestions that helped make the text more clear and understandable. My most heartfelt thanks are reserved for my wife, Shirley, for her support and encouragement during the months of book preparation, and especially during the years before I even considered writing this book. Randall F. Barron Copyright © 2003 Marcel Dekker, Inc. Contents Preface iii 1 Introduction 1.1 Noise Control 1.2 Historical Background 1.3 Principles of Noise Control 1.3.1 Noise Control at the Source 1.3.2 Noise Control in the Transmission Path 1.3.3 Noise Control at the Receiver References 1 1 3 7 8 9 9 10 2 Basics of Acoustics 2.1 Speed of Sound 2.2 Wavelength, Frequency, and Wave Number 2.3 Acoustic Pressure and Particle Velocity 2.4 Acoustic Intensity and Acoustic Energy Density 2.5 Spherical Waves 2.6 Directivity Factor and Directivity Index 2.7 Levels and the Decibel 2.8 Combination of Sound Sources 12 12 13 15 17 21 24 27 31 v Copyright © 2003 Marcel Dekker, Inc. 2.9 Octave Bands 2.10 Weighted Sound Levels Problems References 3 4 Acoustic Measurements 3.1 Sound Level Meters 3.2 Intensity Level Meters 3.3 Octave Band Filters 3.4 Acoustic Analyzers 3.5 Dosimeter 3.6 Measurement of Sound Power 3.6.1 Sound Power Measurement in a Reverberant Room 3.6.2 Sound Power Measurement in an Anechoic or Semi-Anechoic Room 3.6.3 Sound Power Survey Measurements 3.6.4 Measurement of the Directivity Factor 3.7 Noise Measurement Procedures Problems References Transmission of Sound 4.1 The Wave Equation 4.2 Complex Number Notation 4.3 Wave Equation Solution 4.4 Solution for Spherical Waves 4.5 Changes in Media with Normal Incidence 4.6 Changes in Media with Oblique Incidence 4.7 Sound Transmission Through a Wall 4.8 Transmission Loss for Walls 4.8.1 Region I: Stiffness-Controlled Region 4.8.2 Resonant Frequency 4.8.3 Region II: Mass-Controlled Region 4.8.4 Critical Frequency 4.8.5 Region III: Damping-Controlled Region 4.9 Approximate Method for Estimating the TL 4.10 Transmission Loss for Composite Walls 4.10.1 Elements in Parallel 4.10.2 Composite Wall with Air Space 4.10.3 Two-Layer Laminate 4.10.4 Rib-Stiffened Panels Copyright © 2003 Marcel Dekker, Inc. 33 34 37 40 41 42 46 49 50 50 51 52 58 62 66 69 73 76 78 78 83 84 88 91 96 101 107 108 111 112 113 113 117 120 121 122 127 131 4.11 Sound Transmission Class 4.12 Absorption of Sound 4.13 Attenuation Coefficient Problems References 5 6 Noise 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 5.10 Sources Sound Transmission Indoors and Outdoors Fan Noise Electric Motor Noise Pump Noise Gas Compressor Noise Transformer Noise Cooling Tower Noise Noise from Gas Vents Appliance and Equipment Noise Valve Noise 5.10.1 Sources of Valve Noise 5.10.2 Noise Prediction for Gas Flows 5.10.3 Noise Prediction for Liquid Flows 5.11 Air Distribution System Noise 5.11.1 Noise Attenuation in Air Distribution Systems 5.11.2 Noise Generation in Air Distribution System Fittings 5.11.3 Noise Generation in Grilles 5.12 Traffic Noise 5.13 Train Noise 5.13.1 Railroad Car Noise 5.13.2 Locomotive Noise 5.13.3 Complete Train Noise Problems References Acoustic Criteria 6.1 The Human Ear 6.2 Hearing Loss 6.3 Industrial Noise Criteria 6.4 Speech Interference Level 6.5 Noise Criteria for Interior Spaces 6.6 Community Reaction to Environmental Noise 6.7 The Day-Night Level Copyright © 2003 Marcel Dekker, Inc. 134 139 143 153 160 162 162 164 169 171 173 177 178 182 185 186 186 188 190 192 193 195 198 207 211 211 213 214 217 222 225 226 229 231 235 238 243 247 6.8 6.9 7 8 6.7.1 EPA Criteria 6.7.2 Estimation of Community Reaction HUD Criteria Aircraft Noise Criteria 6.9.1 Perceived Noise Level 6.9.2 Noise Exposure Forecast Problems References 247 250 253 255 256 257 262 267 Room Acoustics 7.1 Surface Absorption Coefficients 7.1.1 Values for Surface Absorption Coefficients 7.1.2 Noise Reduction Coefficient 7.1.3 Mechanism of Acoustic Absorption 7.1.4 Average Absorption Coefficient 7.2 Steady-State Sound Level in a Room 7.3 Reverberation Time 7.4 Effect of Energy Absorption in the Air 7.4.1 Steady-State Sound Level with Absorption in the Air 7.4.2 Reverberation Time with Absorption in the Air 7.5 Noise from an Adjacent Room 7.5.1 Sound Source Covering One Wall 7.5.2 Sound Transmission from an Adjacent Room 7.6 Acoustic Enclosures 7.6.1 Small Acoustic Enclosures 7.6.2 Large Acoustic Enclosures 7.6.3 Design Practice for Enclosures 7.7 Acoustic Barriers 7.7.1 Barriers Located Outdoors 7.7.2 Barriers Located Indoors Problems References 269 269 269 270 271 274 274 281 289 Silencer Design 8.1 Silencer Design Requirements 8.2 Lumped Parameter Analysis 8.2.1 Acoustic Mass 8.2.2 Acoustic Compliance 8.2.3 Acoustic Resistance 8.2.4 Transfer Matrix 330 330 332 332 335 338 339 Copyright © 2003 Marcel Dekker, Inc. 289 291 293 293 295 299 300 304 311 312 313 317 321 328 8.3 The Helmholtz Resonator 8.3.1 Helmholtz Resonator System 8.3.2 Resonance for the Helmholtz Resonator 8.3.3 Acoustic Impedance for the Helmholtz Resonator 8.3.4 Half-Power Bandwidth 8.3.5 Sound Pressure Level Gain Side Branch Mufflers 8.4.1 Transmission Loss for a Side-Branch Muffler 8.4.2 Directed Design Procedure for Side-Branch Mufflers 8.4.3 Closed Tube as a Side-Branch Muffler 8.4.4 Open Tube (Orifice) as a Side Branch Expansion Chamber Mufflers 8.5.1 Transmission Loss for an Expansion Chamber Muffler 8.5.2 Design Procedure for Single-Expansion Chamber Mufflers 8.5.3 Double-Chamber Mufflers Dissipative Mufflers Evaluation of the Attenuation Coefficient 8.7.1 Estimation of the Attenuation Coefficient 8.7.2 Effective Density 8.7.3 Effective Elasticity Coefficient 8.7.4 Effective Specific Flow Resistance 8.7.5 Correction for Random Incidence End Effects Commercial Silencers Plenum Chambers Problems References 371 373 377 381 381 383 384 385 387 389 391 397 405 Vibration Isolation for Noise Control 9.1 Undamped Single-Degree-of-Freedom (SDOF) System 9.2 Damped Single-Degree-of-Freedom (SDOF) System 9.2.1 Critically Damped System 9.2.2 Over-Damped System 9.2.3 Under-Damped System 9.3 Damping Factors 9.4 Forced Vibration 9.5 Mechanical Impedance and Mobility 9.6 Transmissibility 9.7 Rotating Unbalance 406 407 410 411 412 412 413 419 424 427 431 8.4 8.5 8.6 8.7 8.8 8.9 9 Copyright © 2003 Marcel Dekker, Inc. 341 341 342 343 344 348 350 351 357 361 365 368 368 9.8 Displacement Excitation 9.9 Dynamic Vibration Isolator 9.10 Vibration Isolation Materials 9.10.1 Cork and Felt Resilient Materials 9.10.2 Rubber and Elastomer Vibration Isolators 9.10.3 Metal Spring Isolators 9.11 Effects of Vibration on Humans Problems References 10 Case Studies in Noise Control 10.1 Introduction 10.2 Folding Carton Packing Station Noise 10.2.1 Analysis 10.2.2 Control Approach Chosen 10.2.3 Cost 10.2.4 Pitfalls 10.3 Metal Cut-Off Saw Noise 10.3.1 Analysis 10.3.2 Control Approach Chosen 10.3.3 Cost 10.3.4 Pitfalls 10.4 Paper Machine Wet End 10.4.1 Analysis 10.4.2 Control Approach Chosen 10.4.3 Cost 10.4.4 Pitfalls 10.5 Air Scrap Handling Duct Noise 10.5.1 Analysis 10.5.2 Control Approach Chosen 10.5.3 Cost 10.5.4 Pitfalls 10.6 Air-Operated Hoist Motor 10.7 Blanking Press Noise 10.7.1 Analysis 10.7.2 Control Approach Chosen 10.7.3 Cost 10.7.4 Pitfalls 10.8 Noise in a Small Meeting Room 10.8.1 Analysis 10.8.2 Control Approach Chosen 10.8.3 Cost Copyright © 2003 Marcel Dekker, Inc. 436 439 446 446 450 457 464 469 474 475 475 476 476 479 479 480 480 480 481 482 482 482 483 487 487 488 488 488 491 492 492 492 494 495 497 497 497 498 499 502 503 10.8.4 Pitfalls Problems References 503 503 504 Appendix A Preferred Prefixes in SI 506 Appendix B Properties of Gases, Liquids, and Solids 507 Appendix C Plate Properties of Solids 509 Appendix D Surface Absorption Coefficients 510 Appendix E 514 Copyright © 2003 Marcel Dekker, Inc. Nomenclature 1 Introduction 1.1 NOISE CONTROL Concern about problems of noise in the workplace and in the living space has escalated since the amendment of the Walsh–Healy Act of 1969. This act created the first set of nationwide occupational noise regulations (Occupational Safety and Health Administration, 1983). There is a real danger of permanent hearing loss when a person is exposed to noise above a certain level. Most industries are strongly motivated to find an effective, economical solution to this problem. The noise level near airports has become serious enough for some people to move out of residential areas near airports. These areas were considered pleasant living areas before the airport was constructed, but environmental noise has changed this perception. The airport noise in the areas surrounding the airport is generally not dangerous to a person’s health, but the noise may be unpleasant and annoying. In the design of many appliances, such as dishwashers, the designer must be concerned about the noise generated by the appliance in operation; otherwise, prospective customers may decide to purchase other quieter models. It is important that noise control be addressed in the design stage for many mechanical devices. Copyright © 2003 Marcel Dekker, Inc. 2 Chapter 1 Lack of proper acoustic treatment in offices, apartments, and classrooms may interfere with the effective functioning of the people in the rooms. Even though the noise is not dangerous and not particularly annoying, if the person cannot communicate effectively, then the noise is undesirable. Much can be done to reduce the seriousness of noise problems. It is often not as simple as turning down the volume on the teenager’s stereo set, however. Effective silencers (mufflers) are available for trucks and automobiles, but there are other significant sources of noise, such as tire noise and wind noise, that are not affected by the installation of a silencer. Household appliances and other machines may be made quieter by proper treatment of vibrating surfaces, use of adequately sized piping and smoother channels for water flow, and including vibration isolation mounts. Obviously, the noise treatment must not interfere with the operation of the applicance or machine. This stipulation places limitations on the noise control procedure that can be used. In many instances, the quieter product can function as well as the noisier product, and the cost of reducing the potential noise during the design stage may be minor. Even if the reduction of noise is somewhat expensive, it is important to reduce the level of noise to an acceptable value. There are more than 1000 local ordinances that limit the community noise from industrial installations, and there are legal liabilities associated with hearing loss of workers in industry. The designer can no longer ignore noise when designing an industrial plant, an electrical generating system, or a commercial complex. In this book, we will consider some of the techniques that may be used by the engineer in reduction of noise from existing equipment and in design of a quieter product, in the case of new equipment. We will begin with an introduction to the basic concepts of acoustics and acoustic measurement. It is important for the engineer to understand the nomenclature and physical principles involved in sound transmission in order to suggest a rational procedure for noise reduction. We will examine methods for predicting the noise generated by several common engineering systems, such as fans, motors, compressors, and cooling towers. This information is required in the design stage of any noise control project. Information about the characteristics of the noise source can allow the design of equipment that is quieter in operation through adjustment of the machine speed or some other parameter. How quiet should the machine be? This question may be answered by consideration of some of the design criteria for noise, including the OSHA, EPA, and HUD regulations, for example. We will also consider some of the Copyright © 2003 Marcel Dekker, Inc. Introduction 3 criteria for noise transmitted outdoors and indoors, so that the anticipated community response to the noise may be evaluated. A study of the noise control techniques applicable to rooms will be made. These procedures include the use of acoustic treatment of the walls of the room and the use of barriers and enclosures. It is important to determine if acoustic treatment of the walls will be effective or if the offending noise source must be enclosed to reduce the noise to an acceptable level. The acoustic design principles for silencers or mufflers will be outlined. Specific design techniques for several muffler types will be presented. Some noise problems are associated with excessive vibration of portions of the machine or transmission of machine vibration to the supporting structure. We will consider some of the techniques for vibration isolation to reduce noise radiated from machinery. The application of commercially available vibration isolators will be discussed. Finally, several case studies will be presented in which the noise control principles are applied to specific pieces of equipment. The noise reduction achieved by the treatment will be presented, along with any pitfalls or caveats associated with the noise control procedure. 1.2 HISTORICAL BACKGROUND Because of its connection with music, acoustics has been a field of interest for many centuries (Hunt, 1978). The Greek philosopher Pythagoras (who also stated the Pythagorean theorem of triangles) is credited with conducting the first studies on the physical origin of musical sounds around 550 BC (Rayleigh, 1945). He discovered that when two strings on a musical instrument are struck, the shorter one will emit a higher pitched sound than the longer one. He found that if the shorter string were half the length of the longer one, the shorter string would produce a musical note that was 1 octave higher in pitch than the note produced by the longer string: an octave difference in frequency (or pitch) means that the upper or higher frequency is two times that of the lower frequency. For example, the frequency of the note ‘‘middle C’’ is 262.6 Hz (cycles/sec), and the frequency of the ‘‘C’’ 1 octave higher is 523.2 Hz. Today, we may make measurements of the sound generated over standard octave bands or frequency ranges encompassing one octave. The knowledge of the frequency distribution of the noise generated by machinery is important in deciding which noise control procedure will be most effective. The Greek philosopher Crysippus (240 BC) suggested that sound was generated by vibration of parts of the musical instrument (the strings, for example). He was aware that sound was transmitted by means of vibration Copyright © 2003 Marcel Dekker, Inc. 4 Chapter 1 of the air or other fluid, and that this motion caused the sensation of ‘‘hearing’’ when the waves strike a person’s ear. Credit is usually given to the Franciscan friar, Marin Mersenne (1588– 1648) for the first published analysis of the vibration of strings (Mersenne, 1636). He measured the vibrational frequency of an audible tone (84 Hz) from a long string; he was also aware that the frequency ratio for two musical notes an octave apart was 2:1. In 1638 Galileo Galilei (1939) published a discussion on the vibration of strings in which he developed quantitative relationships between the frequency of vibration of the string, the length of the string, its tension, and the density of the string. Galileo observed that when a set of pendulums of different lengths were set in motion, the oscillation produced a pattern which was pleasant to watch if the frequencies of the different pendulums were related by certain ratios, such as 2:1, 3:2, and 5:4 or octave, perfect fifth, and major third on the musical scale. On the other hand, if the frequencies were not related by simple integer ratios, the resulting pattern appeared chaotic and jumbled. He made the analogy between vibrations of strings in a musical instrument and the oscillating pendulums by observint that, if the frequencies of vibration of the strings were related by certain ratios, the sound would be pleasant or ‘‘musical.’’ If the frequencies were not related by simple integer ratios, the resulting sound would be discordant and considered to be ‘‘noise.’’ In 1713 the English mathematician Brook Taylor (who also invented the Taylor series) first worked out the mathematical solution of the shape of a vibrating string. His equation could be used to derive a formula for the frequency of vibration of the string that was in perfect agreement with the experimental work of Galileo and Mersenne. The general problem of the shape of the wave in a string was fully solved using partial derivatives by the young French mathematician Joseph Louis Lagrange (1759). There are some great blunders along the scientific route to the development of modern acoustic science. The French philosopher Gassendi (1592–1655) insisted that sound was propagated by the emission of small invisible particles from the vibrating surface. He claimed that these particles moved through the air and struck the ear to produce the sensation of sound. Otto von Guericke (1602–1686) said that he doubted sound was transmitted by the vibratory motion of air, because sound was transmitted better when the air was still than when there was a breeze. Around the mid-1600s, he placed a bell in a vacuum jar and rang the bell. He claimed that he could hear the bell ringing inside the container when the air had been evacuated from the container. From this observation, von Guericke concluded that the air was not necessary for the transmission of sound. He did not recognize that the sound was being transmitted through the solid support structure of Copyright © 2003 Marcel Dekker, Inc. Introduction 5 the bell. This story emphasized that we must be careful to consider all paths that noise may take, if we are to reduce noise effectively. In 1660 Robert Boyle (who discovered Boyle’s law for gases) repeated the experiment of von Guericke with a more efficient vacuum pump and more careful attention to the support. He observed a pronounced decrease in the intensity of the sound emitted from a ticking watch in the vacuum chamber as the air was pumped out. He correctly concluded that the air was definitely involved as a medium for sound transmission, although the air was not the only path that sound could take. Sir Isaac Newton (1687) compared the transmission of sound and the motion of waves on the surface of water. By analogy with the vibration of a pendulum, Newton developed an expression for the speed of sound based on the assumption that the sound wave was transmitted isothermally, when in fact sound is transmitted adiabatically for small-amplitude sound waves. His incorrect expression for the speed of sound in a gas was: c ¼ ðRTÞ1=2 ðincorrect!Þ ð1-1Þ R is the gas constant for the gas and T is the absolute temperature of the gas. For air (gas constant R ¼ 287 J/kg-K) at 158C (288.2K or 598F), Newton’s equation would predict the speed of sound to be 288 m/s (944ft/ sec), whereas the experimental value for the speed of sound at this temperature is 340 m/s (1116 ft/sec). Newton’s expression was about 16% in error, compared with the experimental data. This was not a bad order of magnitude difference at the time; however, later more accurate measurements of the speed of sound consistently produced values larger than that predicted by Newton’s relationship. It wasn’t until 1816 that the French astronomer and mathematician Pierre Simon Laplace suggested that sound was actually transmitted adiabatically because of the high frequency of the sound waves. Laplace proposed the correct expression for the speed of sound in a gas: c ¼ ð
RTÞ1=2 ð1-2) where
is the specific heat ratio for the gas. For air,
¼ 1:40. In 1877 John William Strutt Rayleigh published a two-volume work, The Theory of Sound, which placed the field of acoustics on a firm scientific foundation. Rayleigh also published 128 papers on acoustics between 1870 and 1919. Between 1898 and 1900 Wallace Clement Sabine (1922) published a series of papers on reverberation of sound in rooms in which he laid the foundations of architectural acoustics. He also served as acoustic consultant for several projects, including the Boston Symphony Hall and the chamber of the House of Representatives in the Rhode Island State Capitol Building. Copyright © 2003 Marcel Dekker, Inc. 6 Chapter 1 Sabine initially tried several optical devices, such as photographing a sensitive manometric gas flame, for measuring the sound intensity, but these measurements were not consistent. He found that the human ear, along with a suitable electrical timepiece, gave sensitive and accurate measurements of the duration of audible sound in the room. One of the early acoustic ‘‘instruments’’ was a stethoscope developed by the French physician Rene Laennee. He used the stethoscope for clinical purposes in 1819. In 1827 Sir Charles Wheatstone, a British physicist who invented the famous Wheatstone bridge, developed an instrument similar to the stethoscope, which he called a ‘‘microphone.’’ Following the invention of the triode vacuum tube in 1907 and the initial development of radio broadcasting in the 1920s, electric microphones and loudspeakers were produced. These developments were followed by the production of sensitive instruments designed to measure sound pressure levels and other acoustic quantities with a greater accuracy than could be achieved by the human ear. Research was conducted during the 1920s on the concepts of subjective loudness and the response of the human ear to sound. Between 1930 and 1940, noise control principles began to be applied to buildings, automobiles, aircraft and ships. Also, during this time, researchers began to investigate the physical processes involved in sound absorption by porous acoustic materials. With the advent of World War II, there was a renewed emphasis on solving problems in speech communication in noisy environments, such as in tanks and aircraft (Beranek, 1960). The concern for this problem area was so critical that the National Defense Research Committee (which later became the Office of Scientific Research and Development) established two laboratories at Harvard University. The Psycho-Acoustic Laboratory was involved in studies on sound control techniques in combat vehicles, and the Electro-Acoustic Laboratory conducted research on communication equipment for operation in a noisy environment and acoustic materials for noise control. After World War II ended, research in noise control and acoustics was continued at several other universities. Noise problems in architecture and in industry were addressed in the post-war period. Research was directed toward solution of residential, workplace, and transportation noise problems. The amendment of the Walsh–Healy Act in 1969 gave rise to even more intense noise control activity in industry. This law required that the noise exposure of workers in the industrial environment be limited to a specific value (90 dBA for an 8hour period). If this level of noise exposure could not be prevented, the law required that the workers be provided with and trained in the use of personal hearing protection devices. Copyright © 2003 Marcel Dekker, Inc. Introduction 7 F IGURE 1-1 Three components of a general noise system: source of noise, path of the noise, and the receiver. The path may be direct from the source to the receiver, or the path may be indirect. 1.3 PRINCIPLES OF NOISE CONTROL There are three basic elements in any noise control system, as illustrated in Fig. 1-1: 1. The source of the sound 2. The path through which the sound travels 3. The receiver of the sound (Faulkner, 1976). In many situations, of course, there are several sources of sound, various paths for the sound, and more than one receiver, but the basic principles of noise control would be the same as for the more simple case. The objective of most noise control programs is to reduce the noise at the receiver. This may be accomplished by making modifications to the source, the path, or the receiver, or to any combination of these elements. The source of noise or undesirable sound is a vibrating surface, such as a panel in an item of machinery, or small eddies with fluctuating velocities in a fluid stream, such as the eddies in a jet stream leaving an air vent pipe. The path for the sound may be the air between the source and receiver, as is the case for machinery noise transmitted directly to the operator’s ears. The path may also be indirect, such as sound being reflected by a wall to a person in the room. Solid surfaces, such as piping between a vibrating pump and another machine element, may also serve as the path for the noise propagation. It is important that the acoustic engineer identify all possible acoustic paths when considering a solution for a noise problem. Copyright © 2003 Marcel Dekker, Inc. 8 Chapter 1 The receiver in the noise control system is usually the human ear, although the receiver could be sensitive equipment that would suffer impaired operation if exposed to excessively intense sound. It is important that the acoustic designer specify the ‘‘failure mode’’ for the receiver in any noise control project. The purpose of the noise control procedure may be to prevent hearing loss for personnel, to allow effective face-to-face communication or telephone conversation, or to reduce noise so that neighbors of the facility will not become intensely annoyed with the sound emitted by the plant. The engineering approach is often different in each of these cases. 1.3.1. Noise Control at the Source Modifications at the source of sound are usually considered to be the best solution for a noise control problem. Components of a machine may be modified to effect a significant change in noise emission. For example, in a machine used to manufacture paper bags, by replacing the impact blade mechanism used to cut off the individual bags from the paper roll with a rolling cutter blade, a severe noise problem was alleviated. Noise at the source may indicate other problems, such as a need for maintenance. For example, excessive noise from a roller bearing in a machine may indicate wear failure in one of the rollers in the bearing. Replacement of the defective bearing may solve the noise problem, in addition to preventing further mechanical damage to the machine. There may be areas, such as panel coverings, that vibrate excessively on a machine. These panels are efficient sound radiators at wavelengths on the order of the dimensions of the panel. The noise generated by large vibrating panels can be reduced by applying damping material to the panel surface or by uncoupling the panel from the vibrating force, if possible. Making the panel stiffer by increasing the panel thickness or reducing the panel dimensions or using stiffening ribs may also reduce the amplitude of vibration. In most cases, reducing the amplitude of vibratory motion of elements in a machine will reduce the noise generated by the machine element. In some cases, using two units with the same combined capacity as one larger unit may reduce the overall source noise. To determine whether this approach is feasible, the engineer would need information about the relationship of the machine capacity (power rating, flow rate capacity, etc.) and the sound power level for the generated noise from the machine. This information is presented in Chapter 5 for several noise sources. A change in the process may also be used to reduce noise. Instead of using an air jet to remove debris from a manufactured part, rotating clean- Copyright © 2003 Marcel Dekker, Inc. Introduction 9 ing brushes may be used. A centrifugal fan may replace a propellor-type fan to reduce the fan noise. 1.3.2 Noise Control in the Transmission Path Modifying the path through which the noise is propagated is often used when modification of the noise source is not possible, not practical, or not economically feasible. For noise sources located outdoors, one simple approach for noise control would be to move the sound source farther away from the receiver, i.e. make the noise path longer. For noise sources located outdoors or indoors, the transmission path may be modified by placing a wall or barrier between the source and receiver. Reduction of traffic noise from vehicles on freeways passing near residential areas and hospitals has been achieved by installation of acoustic barriers along the roadway. The use of a barrier will not be effective in noise reduction indoors when the sound transmitted directly from the source to receiver is much less significant than the sound transmitted indirectly to the receiver through reflections on the room surfaces. For this case, the noise may be reduced by applying acoustic absorbing materials on the walls of the room or by placing additional acoustic absorbing surfaces in the room. A very effective, although sometimes expensive, noise control procedure is to enclose the sound source in an acoustic enclosure or enclose the receiver in a personnel booth. The noise from metal cut-off saws has been reduced to acceptable levels by enclosing the saw in an acoustically treated box. Provision was made to introduce stock material to the saw through openings in the enclosure without allowing a significant amount of noise to be transmitted through the openings. If the equipment or process can be remotely operated, a personnel booth is usually an effective solution in reducing the workers’ noise exposure. An air-conditioned control booth is also more comfortable for the operator of a paper machine than working in the hot, humid area surrounding the wet end of the paper machine, for example. The exhaust noise from engines, fans, and turbines is often controlled by using mufflers or silencers in the exhaust line for the device. The muffler acts to reflect acoustic energy back to the noise source (the engine, for example) or to dissipate the acoustic energy as it is transmitted through the muffler. 1.3.3 Noise Control at the Receiver The human ear is the usual ‘‘receiver’’ for noise, and there is a limited amount of modification that can be done for the person’s ear. One possible Copyright © 2003 Marcel Dekker, Inc.