Industrial Noise Control and Acoustics

Randall F. Barron

• Publisher: CRC Press
• Year: 2002
• ISBN:
  9781135551681
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Civil Engineering
• Index: Technology & Engineering

Compiling strategies from more than 30 years of experience, this book provides numerous case studies that illustrate the implementation of noise control applications, as well as solutions to common dilemmas encountered in noise reduction processes. It offers methods for predicting the noise generation level of common systems such as fans, motors, c

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.

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 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 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 obser-vint 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 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=(RT)1/2(incorrect!) (1-1)

R is the gas constant for the gas and T is the absolute temperature of the gas. For air (gas constant R = 287 J/kg-K) at 15°C (288.2K or 59°F), 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=(γRT)1/2 (1-2)

where γ is the specific heat ratio for the gas. For air, γ = 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. 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 w...

Table of contents

• Cover
• Half Title
• Series Page
• Title Page
• Copyright Page
• Preface
• Table of Contents
• 1 Introduction
• 2 Basics of Acoustics
• 3 Acoustic Measurements
• 4 Transmission of Sound
• 5 Noise Sources
• 6 Acoustic Criteria
• 7 Room Acoustics
• 8 Silencer Design
• 9 Vibration Isolation for Noise Control
• 10 Case Studies in Noise Control
• Appendix A Preferred Prefixes in SI
• Appendix B Properties of Gases, Liquids, and Solids
• Appendix C Plate Properties of Solids
• Appendix D Surface Absorption Coefficients
• Appendix E Nomenclature
• Index