Environmental Engineering

Noise Pollution

Book & Author

0Book: Environmental Engineering Handbook, Chapter 6

0Author: David H.F. Liu, Howard C. Roberts

NOISE SOURCES0 Noise is found almost everywhere, not just in factories.0 Thunder is perhaps the loudest natural sound we hear; it sometimes reaches the threshold of

discomfort. 0 Jet aircraft takeoffs are often louder to the listener. 0 Some industrial locations have even louder continuous noise. 0 Community noise is largely produced by transportation sources-most often airplanes and

highway vehicles. 0 Noise sources are also in public buildings and residences.Typical Range of Noise Levels0 Variation in noise levels is wide. In rural areas, ambient noise can be as low as 30 dB; even in

residential areas in or near cities, this low level is seldom achieved. 0 In urban areas, the noise level can be 70 dB or higher for eighteen hours of each day. Near

freeways, 90 to 100 dB levels are not unusual. 0 Many industries have high noise levels. 0 Heavy industries such as iron and steel production and fabricating and mining display high

levels; so do refineries and chemical plants, though in the latter few people are exposed to the highest levels of noise.

0 Automobile assembly plants, saw-mills and planing mills, furniture factories, textile mills, plastic factories, and the like often employ many people in buildings with high noise levels throughout.

0 Hearing impairment of such employees is probable unless corrective measures are taken.0 The construction industry often exposes its employees to hazardous noise levels and at the

same time adds greatly to community noise.

NOISE SOURCESTypical Range of Noise Levels0 Community noise may not be high enough to damage hearing (within buildings) and yet have

an unfavorable effect on general health.0 Transportation contributes largely to community noise.0 The public may suffer more than the employees-the crew and passengers of a jetliner do not

receive the high noise level found along the takeoff and approach paths. 0 The drivers of passenger cars often are less bothered

by their own noise than are their fellow drivers, and they are less annoyed than residents nearby for psychological reasons.

0 Noise levels high enough to be harmful in theirimmediate area are produced by many tools, toys, and other devices.

0 The dentist’s drill, the powder-powered stud-setting tool used in building, home workshop tools, and even hi-fi stereo headphones can damage the hearing of their users.

0 They are often overlooked because their noise is localized.

0 Some typical noise sources are listed in Table andare classified by origin.

NOISE SOURCESCharacteristics of Industrial Noise0 Industrial noise varies in loudness, frequency components, and uniformity. 0 It can be almost uniform in frequency response (white noise) and constant in level; large

rotating machines and places such as textile mills with many machines in simultaneous operation are often like this.

0 An automobile assembly line usually shows this steady noise with many momentary or impact noises superimposed on it.

0 Other industries show continuous background noise at relatively low levels with intermittently occurring periods of higher noise levels.

0 Such no uniform noises are likely to be more annoying and more fatiguing than steady noise, and they are more difficult to evaluate.

0 The terms used to describe them are sometimes ambiguous. 0 Usually the term intermittent refers to a noise which is on for several seconds or longer-

perhaps for several hours-then off for a comparable time.0 The term interrupted usually has approximately the same meaning except that it implies that

the off periods are shorter than the on periods. 0 Intermittent or interrupted noises can be measured with a standard sound level meter and a

clock or stopwatch.0 Sounds whose duration is only a fraction of a second are called impulsive, explosive, or

impact sounds. 0 The terms are often used interchangeably for pulses of differing character, alike only in that

they are short.

NOISE SOURCESCharacteristics of Industrial Noise0 They must be measured with instruments capable of following rapid changes or with

instruments which sample and hold peak values.0 The wave form of the noise can be modified appreciably by reflection before it reaches the ear,

but it is usually described as either single-spike pulses or rapidly damped sinusoidal wave forms.

0 Such wave forms can be evaluated fairly accurately by converting the time-pressure pattern into an energy spectrum and then performing a spectral analysis.

0 A more accurate evaluation of the effect of intermittent but steady-level noise is possible through computation based on the ratio of on-to-off times.

0 The ear cannot judge the intensity of extremely short noise pulses or impact noises since it seems to respond more to the energy contained in the pulse than to its maximum amplitude.

0 Pulses shorter than A second, therefore, do not sound as loud as continuous noise having the same sound pressure level; the difference is as much as 20 dB for a pulse 20 ms long. (See Table)

0 Thus, the ear can be exposed to higher sound pressures than the subject realizes from sensation alone; a short pulse with an actualsound pressure level of 155 to 160 dB might seem only at the threshold of discomfort, 130 to 135 dB for continuous noise.

NOISE SOURCESCharacteristics of Industrial Noise0 Interruptions in continuous noise provide brief rest periods which reduce fatigue and the

danger of permanent hearing impairment. 0 Intermittent periods of high noise during otherwise comfortable work sessions are annoying

and tend to cause carelessness and accidents.0 Industrial noises also vary in their frequency characteristics.0 Large, slow-moving machines generally produce low-frequency noises; high-speed machines

usually produce noise of higher frequency. 0 A machine such as a large motor-generator

produces noise over the entire audible frequency range; the rotational frequency is the lowest (1800 RPM produces 30 Hz) but higher frequencies from bearing noise (perhaps brush noise too), slot or tooth noise, wind noise, and the like are also present.

0 A few noise spectra are shown in Figure, in octave-band form. Curve No. 1 of a motor-generator set shows a nearly flat frequency response; it is a mixture of many frequencies from different parts of the machine.

NOISE SOURCESCharacteristics of Industrial Noise0 Curve No. 2, for a large blower, shows a predominantly low-frequency noise pattern; its

maximum is around 100 or 120 Hz and can be caused by the mechanical vibration of large surfaces excited by magnetic forces. 0 Curve No. 3 is for a jet plane approaching land; it contains much high frequency energy and

sounds like a howl or scream, while the blower noise is a rumble. 0 Curve No. 4 describes the high-pitched noise caused by turbulence in a gas-reducing valve; it

is mechanically connected to pipes which readily radiate in the range of their natural frequencies of vibration.

0 Octave-band analyses have only rather broad resolution and are suited to investigate the audible sound characteristics; the mechanical vibrations causing the noise are best analyzed by a continuously variable instrument.

0 The radiating area of a source affects the amount of sound emitted; not only does the total amount of acoustic energy radiated increase roughly in proportion to the area in vibration, but a pipe or duct passing through a wall emits sound on both sides of the wall.

0 The vibration amplitude can be only a few micro-inches yet produce loud sounds. 0 If the natural frequency of an elastic member is near the frequency of the vibration, its

amplitude can become large unless the member is damped or the driving force isolated.

NOISE SOURCESINDUSTRIAL NOISE SOURCES0 In rotating and reciprocating machines, noise is produced through vibration caused by

imperfectly balanced parts; bearing noise, wind noise, and other noises also exist. 0 The amplitude of such noises varies with operating speed, usually increasing exponentially

with speed. 0 Noise frequencies cover a wide range since normally several harmonics of each fundamental

are produced.0 Electrical machines produce noise from magnetic as well as mechanical forces. 0 Alternating current machines convert electrical to mechanical energy by cyclically changing

magnetic forces which also cause vibration of the machine parts. 0 These magnetic forces change in magnitude and direction as the machine rotates and air gaps

and their magnetic reluctance change. 0 The noise frequencies thus produced are related both to line frequency and its harmonics and

to rotational speed. 0 The entire pattern is quite complicated. In nonrotating machines (transformers, magnetic

relays, and switches), the noise frequencies are the line frequency and its harmonics and the frequencies of vibration of small parts which are driven into vibration when their resonant frequencies are near some driving frequency.

0 In many machines, more noise is produced by the material being handled than by the machine. 0 In metal-cutting or grinding operations, much noise is produced by the cutting or abrading

process and is radiated from both work piece and machine.0 Belt and screw conveyors are sometimes serious noise sources; they are large-area sources;

their own parts vibrate and cause noise in operation, and the material they handle produces noise when it is stirred, dropped, or scraped along its path of motion.

NOISE SOURCESINDUSTRIAL NOISE SOURCES0 Vibration from conveyors is conducted into supports and building structure as well. 0 Feeding devices, as for automatic screw machines, often rattle loudly.0 Jiggers, shakers, screens, and other vibrating devices produce little audible noise in themselves

(partly because their operating frequency is so low), but the material they0 handle produces much higher frequency noise. 0 Ball mills, tumblers, and the like produce noise from the many impacts of shaken or lifted-

and-dropped pieces; their noise frequencies are often low, and much mechanical vibration is around them.

0 Industry uses many pneumatic tools. Some air motors are quite noisy, others less so. 0 Exhausting air is a major noisemaker, and the manner in which it is handled has much to do

with the noise produced. 0 Exhausting or venting any gas (in fact, any process which involves high velocity and pressure

changes) usually produces turbulence and noise. 0 In liquids, turbulent flow is noisy because of cavitation. 0 Turbulence noise in gas is usually predominantly high frequency; cavitation noise in liquids is

normally midrange to low frequency. 0 Both types of noise can span several octaves in frequency range.0 Gas and steam turbines produce high-frequency exhaust noise; steam turbines (for improved

efficiency) usually exhaust their steam into a condenser; gas turbines sometimes feed their exhausts to mufflers.

0 If such turbines are not enclosed, they can be extremely noisy; turbojet airplane engines are an example.

NOISE SOURCESINDUSTRIAL NOISE SOURCES0 Impact noises in industry are produced by many processes; materials handling, metal piercing,

metal forming, and metal fabrication are perhaps most important.0 Such noises vary widely because of machine design and location, energy involved in the

operation, and particularly because of the rate of exchange of energy.0 Not all industrial noises are within buildings; cooling towers, large fans or blowers,

transformer substations, external ducts and conveyor housings, materials handling and loading, and the like are outside sources of noise.

0 They often involve a large area and contribute to community noise.

0 Bucket unloaders, discharge chutes, and carshakers, such as those used for unloading ore, coal, and gravel, produce noise which is more annoying because of its lack of uniformity.

0 Figure summarizes a range of industrial plant noise levels at the operator’s position.

0 Table gives some industrial equipment noises sources.

NOISE SOURCESINDUSTRIAL NOISE SOURCES

THE EFFECTS OF NOISE0 Human response to noise displays a systematic qualitative pattern, but quantitative responses

vary from one individual to another because of age, health, temperament, and the like. 0 Even with the same individual, they vary from time to time because of change in health,

fatigue, and other factors. 0 Variation is greatest at low to moderately high sound levels; at high levels, almost everyone

feels discomfort.0 A detailed investigation of the physiological damage to human ears is difficult, but controlled

tests on animals indicate the probable type of physiological damage produced by excessively high noise levels.

Reactions to Noise0 Specific physiological reactions begin at sound levels of 70 to 75 dB for a 1000 Hz pure tone. 0 At the threshold of such response, the observable reaction is slow but definite after a few

minutes. 0 These reactions are produced by other types of stimulation, so they can be considered as

reactions to general physiological stress. 0 First the peripheral blood vessels constrict with a consequent increase in blood flow to the

brain, a change in breathing rate, changes in muscle tension, and gastrointestinal motility and sometimes glandular reactions detectable in blood and urine.

0 Increased stimulation causes an increase in the reaction, often with a change in form. 0 These reactions are sometimes called N-reactions—nonauditory reactions. 0 If the stimulus continues for long, adaptation usually occurs with the individual no longer

conscious of the reaction, but with the effect continuing. 0 Auditory responses occur as well as these nonauditory or vegetative ones.

THE EFFECTS OF NOISEReactions to Noise0 If exposure is continued long enough, temporary threshold shifts (TTS) can occur, and a loss

of some hearing acuity usually results with increasing age. 0 Some workers refer to a “threshold of annoyance to intermittent noise” at 75 to 85 dB.0 At a slightly higher level, and especially for intermittent or impulsive noise, another non-

auditory response appears the startle effect. Pulse rate and blood pressure change, stored glucose is released from the liver into the bloodstream (to meet emergency needs for energy), and the production of adrenalin increases.

0 The body experiences a fear reaction. Usually psychological adaptation follows, but with changed physiological conditions.

0 At noise levels above 125 dB, electroencephalographic records show distorted brain waves and often interference with vision.

0 Most of these non-auditory reactions are involuntary; they are unknown to the subject and occur whether he is awake or sleeping.

0 They affect metabolism; and since body chemistry is involved, an unborn baby experiences the same reactions as its mother.

0 Sounds above 95 dB often cause direct reaction of the fetus without the brief delay required for the chemical transfer through the common bloodstream.

0 Most people find that under noisy conditions, more effort is required to maintain attention and that the onset of fatigue is quicker.

AUDITORY EFFECTS0 Within 0.02 to 0.05 seconds after exposure to sound above the 80 dB level, the middle-ear

muscles act to control the response of the ear. 0 After about fifteen minutes of exposure, some relaxation of these muscles usually occurs.

THE EFFECTS OF NOISEAUDITORY EFFECTS0 This involuntary response of the ear—the auditory reflex—provides limited protection against

high noise levels. 0 It cannot protect against unanticipated impulsive sounds; it is effective only against

frequencies below about 2000 Hz. And in any case, it provides only limited control over the entrance of noise. These muscles relax a few seconds after the noise ceases.

0 Following exposure to high-level noise, customarily a person has some temporary loss in hearing acuity and often a singing in the ears (tinnitus).

0 If it is not too great, this temporary loss disappears in a few hours. But if, for example, the TTS experienced in one work period has not been recovered at the start of the next work period, the effect accumulates; permanent hearing damage is almost certain if these conditions persist.

0 Important variables in the development of temporary and permanent hearing threshold changes include the following:

0 Sound level: Sound levels must exceed 60 to 80 dBA before the typical person experiences TTS.

0 Frequency distribution of sound: Sounds having most of their energy in the speech frequencies are more potent in causing a threshold shift than are sounds having most of their energy below the speech frequencies.

0 Duration of sound: The longer the sound lasts, the greater the amount of threshold shift.0 Temporal distribution of sound exposure: The number and length of quiet periods between

periods of sound influences the potentiality of threshold shift. Individual differences in tolerance of sound may vary among individuals.

0 Type of sound—steady-state, intermittent, impulse, or impact: The tolerance to peak sound pressure is reduced by increasing the duration of the sound.

THE EFFECTS OF NOISEACOUSTIC TRAUMA0 The outer and middle ear are rarely damaged by intense noise. However, explosive sounds can

rupture the tympanic membrane or dislocate the ossicular chain. 0 The permanent hearing loss that results from brief exposure to a very loud noise is termed

acoustic trauma. 0 Damage to the outer and middle ear may or may not accompany acoustic trauma.Damage-Risk Criteria0 A damage-risk criterion specifies the maximum allowable exposure to which a person can be

exposed if risk of hearing impairment is to be avoided. 0 The American Academy of Ophthalmology and Otolaryngology defines hearing impairment as

an average hearing threshold limit (HTL) in excess of 25 dB (ANSI–1969) at 500, 1000, and 2000 Hz. This limit is called the low fence.

0 Total impairment occurs when the average hearing threshold limit (HTL) exceeds 92 dB. 0 Presbycusis (age related hearing loss) is included in setting the 25 dB ANSI low fence. 0 Two criteria have been set to provide conditions under which nearly all workers can be

repeatedly exposed without adverse effect on their ability to hear and understand normal speech.

Psychological Effects of Noise PollutionSPEECH INTERFERENCE0 Noise can interfere with our ability to communicate. 0 Many noises that are not intense enough to cause hearing impairment can interfere with speech

communication. 0 The interference or masking effect is a complicated function of the distance between the

speaker and listener and the frequency components of the spoken words.

THE EFFECTS OF NOISEPsychological Effects of Noise PollutionSPEECH INTERFERENCE0 The speech interference level (SIL) is a measure of the difficulty in communication that is

expected with different background noise levels. 0 Now analysis talk in terms of A-weighted background noise levels and the quality of speech

communication (shown in figure).ANNOYANCE0 Annoyance by noise is a response to auditory experience.0 Annoyance has its base in the unpleasant nature of some

sounds, in the activities that are disturbed or disrupted bynoise, in the physiological reactions to noise, and in theresponses to the meaning of the messages carried by thenoise.

0 For example, a sound heard at night can be moreannoying than one heard by day, just as one that fluctuatescan be more annoying than one that does not.

0 A sound that resembles another unpleasant sound and is perhaps threatening can be especially annoying.

0 A sound that is mindlessly inflicted and will not be removed soon can be more annoying than one that is temporarily and regretfully inflicted.

0 A sound, the source of which is visible, can be more annoying than one with an invisible source.

0 A sound that is new can be less annoying. 0 A sound that is locally a political issue can have a particularly high or low annoyance.

THE EFFECTS OF NOISEPsychological Effects of Noise PollutionANNOYANCE0 The degree of annoyance and whether that annoyance leads to complaints, product rejection,

or action against an existing or anticipated noise source depend upon many factors. 0 Some of these factors have been identified, and their relative importance has been assessed. 0 Responses to aircraft noise have received the greatest attention. 0 Less information is available concerning responses to other noises, such as those of surface

transportation and industry and those from recreational activities. 0 Many of the noise rating or forecasting systems in existence were developed to predict

annoyance reactions.EFFECTS ON PERFORMANCE0 When a task requires the use of auditory signals, speech or non-speech, noise at any level

sufficient to mask or interfere with the perception of these signals interferes with the performance of the task.

0 Where mental or motor tasks do not involve auditory signals, the effects of noise on their performance are difficult to assess.

0 Human behavior is complicated, and discovering how different kinds of noises influence different kinds of people doing different kinds of tasks is difficult.

0 Nonetheless, the following general conclusions have emerged. 0 Steady noises without special meaning do not seem to interfere with human performance

unless the A-weighted noise level exceeds about 90 dBs. 0 Irregular bursts of noise (intrusive noise) are more disruptive than steady noises. 0 Even when the A-weighted sound levels of irregular bursts are below 90 dBs, they can

interfere with the performance of a task.

THE EFFECTS OF NOISEPsychological Effects of Noise PollutionEFFECTS ON PERFORMANCE0 High-frequency components of noise, above about 1000–2000 Hz, produce more interference

with performance than low-frequency components of noise.0 Noise does not seem to influence the overall rate of work, but high levels of noise can increase

the variability of the rate of work. 0 Noise pauses followed by compensating increases in the work rate can occur. 0 Noise is more likely to reduce the accuracy of work than to reduce the total quantity of work.

Complex tasks are more likely to be adversely influenced by noise than are simple tasks.

NOISE CONTROL AT THE SOURCESource-Path-Receiver Concept0 To solve a noise problem, one must find out something about what the noise is doing, where it

comes from, how it travels, and what can be done about it. 0 A straightforward approach is to examine the problem in terms of its three basic elements; that

is, sound arises from a source, travels over a path, and affects a receiver or listener.0 The source can be one or any number of mechanical devices that radiate noise or vibratory

energy. Such a situation occurs when several machines are operating at the same time.0 The most obvious transmission path by which noise travels is a direct line-of-sight air path

between the source and the listener. 0 For example, aircraft flyover noise reaches an observer on the ground by the direct line-of-

sight air path.0 Noise also travels along structural paths. Noise can travel from one point to another via any

one path or a combination of several paths. 0 Noise from a washing machine operating in one apartment can be transmitted to another

apartment along air passages such as open windows, doorways, corridors, or duct work. 0 Direct physical contact of the washing machine with the floor or walls sets these building

components into vibration. 0 This vibration is transmitted structurally throughout the building causing walls in other areas

to vibrate and to radiate noise.0 The receiver may be, for example, a single person, or a suburban community.0 The solution of a noise problem requires alteration or modification of any or all of the

following three basic elements:• Modifying the source to reduce its noise output•

NOISE CONTROL AT THE SOURCESource-Path-Receiver Concept• Altering or controlling the transmission path and the environment to reduce the noise level reaching the listener• Providing the receiver with personal protective equipment0 Modifying the source to reduce noise output involves noise-level specifications, process

substitution, machines substitution, and systems design.NOISE-LEVEL SPECIFICATIONS0 The best way of controlling noise at its source is to buy quieter machines. 0 Buying quieter machines is almost always more economical than trying to reduce noise by

modifying the machine after purchase. 0 Everyone profits from quieter machines: the employees’ hearing is better protected, work is

performed more efficiently, and the employer gains from increased production and product quality.

0 The purchase order should specify the maximum permissible noise levels for equipment as listed.

PROCESS SUBSTITUTION0 Substituting a quieter process, machine, or tool is another method of controlling noise. 0 Operations such as riveting, punching, shearing, and metal-forming are often performed by

impact when a slower energy application is equally effective. 0 Welding is a quieter substitute for riveting, drilling for punching, pressing or rolling for

forging, hot forming for cold forming, grinding of castings for chipping, and hydraulic and pneumatic equipment for mechanical equipment.

NOISE CONTROL AT THE SOURCEMACHINE SUBSTITUTION0 Noise reduction can be significant when belt drives are used instead of gears. 0 If using gears is necessary, rotating gears should be substituted for square gears; nylon gears

for metallic gears. 0 Other recommendations for reducing noise are described in the subsection on control of noise

sources by design and in next section.SYSTEMS DESIGN0 Besides engineering controls, noise reduction and isolation can be approached through

machine mounting or by architectural means. 0 If machines are laid out too closely, the operator may be exposed to a high dB level. However,

if machines are spaced adequately apart, noise levels can be within acceptable limits. 0 Noise can be confined within a restricted area by architectural means: building location and

arrangement, design, use of suitable building materials, and location of noise-producing and noise-sensitive areas.

0 Sound control for ceilings in offices must also be planned at the architectural stage.0 Holes should not be placed back to back immediately next to each other. Electrical boxes

should be staggered, at least one stud space. 0 A non-hardening, non-skinning, resilient caulking material should be used to seal all cutouts,

such as around electrical and telephone outlets. 0 Also, all intersections with the adjoining structure, such as under-floor and ceiling runner

tracks, around the perimeter where the assembly meets the floor, ceiling, and partitions, should be sealed.

0 Using center-of-gravity mounting whenever feasible prevents translational modes of vibration from coupling to rotational modes.

NOISE CONTROL AT THE SOURCEControl of Noise Source by Design0 This section describes controlling the noise source by design including reducing impact forces,

reducing speeds and pressures, reducing frictional resistance, reducing the radiating area, reducing noise leakage, and isolating and damping vibrating elements.

REDUCING IMPACT FORCES0 Many machines and items of equipment are designed with parts that strike forcefully against

other parts, producing noise. 0 Often, this striking action or impact is essential to the machine’s function. A familiar example

is the typewriter-its keys must strike the ribbon and paper to leave an inked impression. 0 But the force of the key also produces noise as the impact falls on the ribbon, paper, and

platen.0 Several steps can reduce noise from impact forces. The particular remedy is determined by the

nature of the machine. 0 Not all of the following steps are practical for every machine and every impact-produced

noise. However, applying even one suggested measure can often reduce the noise appreciably.0 Some of the more obvious design modifications follow. Figure shows the application of some

of these measures.0 Reduce the weight, size, or height of fall of the impacting mass.0 Cushion the impact by inserting a layer of shock-absorbing material between the impacting

surfaces. (For example, insert several sheets of paper in the typewriter behind the top sheet to absorb some of the noise-producing impact of the keys.)

0 In some situations, inserting a layer of shock-absorbing material behind each of the impacting heads or objects reduces the transmission of impact energy to other parts of the machine.

NOISE CONTROL AT THE SOURCE0 Whenever practical, one of the impact heads or surfaces should be made of nonmetallic material toreduce resonance (ringing) of the heads.

0 Substitute the application of a small impact force over along time period for a large force over a short period to achieve the same result.

0 Smooth out the acceleration of moving parts by applying accelerating forces gradually.

0 Avoid high, jerky acceleration or jerky motion.0 Minimize overshoot, backlash, and loose play in

cams, followers, gears, linkages, and other parts. 0 To achieve this measure, reduce the operational speed of the machine, make better

adjustments, or use spring-loaded restraints or guides. 0 Machines that are well made, with parts machined to close tolerances, generally produce a

minimum of impact noise.REDUCING SPEEDS AND PRESSURES0 Reducing the speed of rotating and moving parts in machines and mechanical systems results

in smoother operation and lower noise output. 0 Likewise, reducing pressure and flow velocities in air, gas, and liquid circulation systems

lessens turbulence, resulting in decreased noise radiation.0 The following suggestions can be incorporated in design:0 Operate fans, impellers, rotors, turbines, and blowers at the lowest blade tip speeds that still

meet job needs.

NOISE CONTROL AT THE SOURCEREDUCING SPEEDS AND PRESSURES0 Use large-diameter, low-speed fans rather than small-diameter, high-speed units for quiet

operation. In short, maximize diameter and minimize tip speed.0 All other factors being equal, centrifugal squirrel-cage type fans are less noisy than vane axial

or propeller type fans.0 Figure shows these two types of fans.0 In air ventilation systems, reducing the speed of the air flow by 50% can lower the noise

output by 10 to 20dB, or roughly one-quarter to one-half of the original loudness.

0 Air speeds less than 3 m/s measured at a supplyor return grille produce a level of noise that usually is unnoticeable in residential or office areas.

0 To reduce air speed in a given system operate lower motor or blower speeds, install more ventilating grilles, or increase the cross-sectional area of the existing grilles.

REDUCING FRICTIONAL RESISTANCE0 Reducing friction between rotating, sliding,

or moving parts in mechanical systems frequently results in smoother operation and lower noise output.

0 Similarly, reducing flow resistance in fluid distribution systems results in less noise radiation

NOISE CONTROL AT THE SOURCEREDUCING FRICTIONAL RESISTANCE0 Four of the more important factors that should be checked to reduce frictional resistance in

moving parts are the following (See figure):Alignment: Proper alignment of all rotating, moving, or contacting parts results in less noise output. Good axial and directional alignment in pulley systems, gear trains, shaft coupling, power transmission systems, and bearing and axle alignment are fundamental requirements for low noise output.Polish: Highly polished and smooth surfaces between sliding, meshing, or contacting parts are required for quiet operation, particularly where bearings, gears, cams, rails, and guides are concerned.Balance: Static and dynamic balancing of rotating parts reduces frictional resistance and vibration, resulting in lower noise output.(see figure on left)Eccentricity (out-of-roundness): Off-centering of rotating parts such as pulleys, gears, rotors, and shaft and bearing alignment causes vibration and noise. Likewise, out of- roundness of wheels, rollers, and gears causes uneven wear resulting in flat spots that generate vibration and noise.

NOISE CONTROL AT THE SOURCEREDUCING FRICTIONAL RESISTANCE0 The key to effective noise control in fluid systems is streamline flow. This fact holds true for

all their systems including air flow in ducts or vacuum cleaners and water flow in plumbing systems. Streamline flow is simply smooth, non-turbulent, low-friction flow.

0 The rule of thumb for quiet operation is to use a low-speed, large-diameter system to meet a specified flow capacity requirement A system designed for quiet operation employs the following features:

Low fluid speed: Low fluid speeds avoid turbulence, one of the main causes of noise. Smooth boundary surfaces: Duct or pipe systems with smooth interior walls, edges, and joints generate less turbulence and noise than systems with rough or jagged walls or joints.Simple layout: A well-designed duct or pipe system with a minimum of branches, turns, fittings, and connectors is less noisy than a complicated layout.Long-radius turns: Changes in flow direction should be gradual and smooth. A recommendation is for turns to have a curve radius equal to about five times the pipe diameter or major cross-sectional dimension of the duct.Flared sections: Flaring of intake and exhaust openings, particularly in a duct system, tends to reduce flow speeds at these locations, often with substantial reductions in noise output.Streamlined transition in flow path: Changes in flow path dimensions or cross-sectional areas should be gradual and smooth with tapered or flared transition sections to avoid turbulence. A good rule of thumb is to keep the cross-sectional area of the flow path as large and uniform as possible throughout the system.Minimal obstacles: The greater the number of obstacles in the flow path, the more tortuous, turbulent, and noisy the flow. All other required and functional devices in the path, such as structural supports, deflectors, and control dampers, should be as small and streamlined as

NOISE CONTROL AT THE SOURCEREDUCING FRICTIONAL RESISTANCEpossible to smooth out the flow patterns.0 Design of quiet flow system is shown.

NOISE CONTROL AT THE SOURCEREDUCING RADIATING AREA0 Generally speaking, the larger the vibrating part or surface, the greater the noise output. 0 The rule of thumb for quiet machine design is to minimize the effective radiating surface areas

of the parts without impairing their operation or structural strength. 0 This design includes making parts smaller, removing excess material, or cutting openings,

slots, or perforations in the parts. For example, replacing a large, vibrating sheet-metal safety guard on a machine with a guard made of wire mesh or metal webbing might substantially reduce noise because of the reduced surface area of the part (figure).

NOISE CONTROL AT THE SOURCEREDUCING NOISE LEAKAGE0 In many cases, machine cabinets can be made into effective soundproof enclosures through

simple design changes and the application of some sound-absorbing treatment.0 Adopting some of the following recommendations may lead to substantial reductions in noise

output: Caulk all unnecessary holes or cracks, particularly at joints. Seal electrical or plumbing penetrations of the housing or cabinet with rubber gaskets or a

suitable non setting caulk. If practical, cover all other functional or required openings or ports that radiate noise with lids

or shields edged with soft rubber gaskets to effect an airtight seal. Equip other openings required for exhaust, cooling, or ventilation purposes with mufflers or

acoustically lined ducts. Direct openings away from the operator and other people.ISOLATING AND DAMPING VIBRATING ELEMENTS0 In all but the simplest machines, the vibrational energy from a specific moving part is

transmitted through the machine structure, forcing other component parts and surfaces to vibrate and radiate sound—often with greater intensity than that generated by the originating source itself.

0 Generally, vibration problems have two parts: 0 First, energy transmission must be prevented between the source and surfaces that radiate the

energy. 0 Second, the energy must be dissipated or attenuated somewhere in the structure.0 The first part of the problem is solved by isolation. The second part is solved by damping.

NOISE CONTROL AT THE SOURCEISOLATING AND DAMPING VIBRATING ELEMENTS0 The most effective method of vibration isolation involves the resilient mounting of the

vibrating component on the most massive and structurally rigid part of the machine.0 All attachments or connections to the vibrating part, in the form of pipes, conduits, and shaft

couplers, must have flexible or resilient connectors or couplers. For example, pipe connections to a pump that is resiliently mounted on the structural frame of a machine should be made of resilient tubing and mounted as close to the pump as possible.

0 Resilient pipe supports or hangers may also be required to avoid bypassing the isolated system (see Figure).

0 Damping material or structures are those that have some viscous properties.

0 They tend to bend or distort slightly, thus consuming part of the noise energy in molecular motion.

0 The use of spring mounts on motors and laminated galvanized steel and plastic in air-conditioning ducts are examples.

0 When the vibrating noise source is not amenable to isolation, as in ventilation ducts, cabinet panels, and covers, then damping materials can be used to reduce the noise.

0 The type of material best suited for a particular vibration problem depends on several factors such as size, mass, vibrational frequency, and operational function of the vibrating structure.

0 Generally the following guidelines should be observed in the selection and use of such materials to maximize vibration damping efficiency (see Figure):

NOISE CONTROL AT THE SOURCEISOLATING AND DAMPING VIBRATING ELEMENTS0 Damping materials should be applied to

those sections of a vibrating surface where the most flexing, bending, or motion occurs.

0 These areas are usually the thinnest sections.

0 For a single layer of damping material, the stiffness and mass of the material should be comparable to that of the vibrating surface to which it is applied.

0 Therefore, single-layer damping materials should be about two or three times as thick as the vibrating surface to which they are

applied.0 Sandwich materials (laminates) made up of metal sheets bonded to mastic (sheet metal

viscoelastic composites) are more effective vibration dampers than single-layer materials; the thickness of the sheet-metal constraining layer and the viscoelastic layer should each be about one-third the thickness of the vibrating surface to which they are applied.

0 Ducts and panels can be purchased fabricated as laminates.Control of Noise Source by Redress

0 The best way to solve noise problems is with good design of the source. 0 However, frequently an existing source is a noise problem either because of age, abuse, or

poor design.0 Then the problem must be redressed or corrected as it exists. The following sections identify

measures for redressing or correcting the source.

NOISE CONTROL AT THE SOURCEBALANCING ROTATING PARTS0 One of the main sources of machinery noise is structural vibration caused by the rotation of

poorly balanced parts, such as fans, fly wheels, pulleys, cams, and shafts.0 Measures used to correct this condition involve adding counterweights to the rotating unit or

removing some weight from the unit. 0 A familiar noise caused by imbalance is in the high-speed spin cycle of washing machines.0 The imbalance results from clothes being unevenly distributed in the tub. By redistributing the

clothes, balance is achieved, and the noise ceases. 0 This same principle of balance can be applied to furnace fans and other common sources of

such noise.REDUCING FRICTIONAL RESISTANCE0 A well-designed machine that has been poorly maintained can become a serious source of

noise. 0 General cleaning and lubrication of all rotating, sliding, or meshing parts at contact points go a

long way toward fixing the problem.APPLYING DAMPING MATERIALS0 Since a vibrating body or surface radiates noise, applying any material that reduces or restrains

the vibrational motion of that body decreases its noise output. 0 Three basic types of redress vibration damping materials are available:0 Liquid mastics, which are applied with a spray gun and harden into relatively solid materials,

the most common being automobile undercoating0 Pads of rubber, felt, plastic foam, leaded vinyls, adhesive tapes, or fibrous blankets, which are

glued to the vibrating surface0 Sheet metal viscoelastic laminates or composites, which are bonded to the vibrating surface

NOISE CONTROL AT THE SOURCESEALING NOISE LEAKS0 Small holes in an otherwise noise-tight structure can reduce the effectiveness of the noise

control measures. As shown in Figure, if the designed transmission loss of an acoustical enclosure is 40 dB, an opening that comprises only 0.1% of the surface area reduces the effectiveness of the enclosure by 10 dB.

PERFORMING ROUTINE MAINTENANCE0 The noise of a worn muffler is familiar. Likewise, studies of automobile tire noise in relation

to pavement roughness show that maintenance of the pavement surface is essential to keep noise at minimum levels.

0 Normal road wear can yield noise increases on the order of 6 dBA. Faulty installation and maintenance can result in excessive vibration.

0 Equipment should be checked periodically.0 Gradual increases in vibration should be examined in

routine maintenance; sudden increases call for action.0 Increased vibration in machinery can be caused by the

following:• Rotational imbalance which requires rebalancing• Misalignment of couplings or bearings• Eccentric journals • Defective or damaged gears• Bent shafts • Mechanical looseness• Faulty drive belts • Rubbing parts and resonant conditions0 Rapid increases in vibration can be traced to a variety of

causes, such as lack of lubrication, overload, or misalignment.

NOISE CONTROL IN THE TRANSMISSION PATH0 This noise control can be done in several ways: (a) absorbing the sound along the path, (b) deflecting the sound in some other direction by placing a reflecting barrier in its path, or (c) containing the sound by placing the source inside a sound-insulating box or enclosure. Acoustical Separation0 Using the absorptive capacity of the atmosphere, as well as divergence, is a simple,

economical method of reducing the noise level. 0 Air absorbs high-frequency sounds more effectively than it absorbs low-frequency sounds. 0 However, if enough distance is available, even low-frequency sounds are absorbed

appreciably.ABSORBENT MATERIALS0 Noise, like light, bounces from one hard surface to another.0 In noise control work, this bouncing is

called reverberation. 0 If a soft, spongy material is placed on the

walls, floors, and ceiling, the reflected sound is diffused and soaked up (absorbed).

ACOUSTICAL LININGS0 Figure shows various types of duct lining baffles

and silencers0 Lining the inside surfaces of ducts, pipe chases,

or electrical channels with sound-absorbing materials can effectively reduce the noise transmitted through such passageways..

NOISE CONTROL IN THE TRANSMISSION PATHACOUSTICAL LININGS0 In typical duct installations, noise reductions of 10 dB/m for an acoustical lining 2.5 cm thick

are well within reason for high-frequency noise. 0 A comparable degree of noise reduction for the lower frequency sounds is more difficult to

achieve because it usually requires at least a doubling of the thickness and length of the acoustical treatment.

Physical BarriersBARRIERS AND PANELS0 Placing barriers, screens, or deflectors in the noise path is an effective way of reducing noise

transmission, provided that the barriers are large enough in size and depending upon whether the noise is high-frequency or low-frequency.

0 High-frequency noise is reduced more effectively than low-frequency noise.0 The effectiveness of a barrier depends on its location, its height, and its length. Figure shows

that the noise can follow five different paths.0 First, noise follows a direct path to receivers who can see the source well over the top of the

barrier. The barrier does not block their line of sight (L/S) and therefore provides no attenuation. No matter how absorptive the barrier is, it cannot pull the sound downward and absorb it.

0 Second, noise follows a diffracted path to receivers in the shadow zone of the barrier. The noise that passes just over the top edge of the barrier is diffracted (bent) down into the apparent shadow as shown. The larger the angle of diffraction, the more the barrier attenuates the noise in this shadow zone. Less energy is diffracted through large angles than through smaller

angles.

NOISE CONTROL IN THE TRANSMISSION PATHPhysical BarriersBARRIERS AND PANELS0 Third, in the shadow zone, the noise transmitted directly through the barrier is significant in

some cases. For example, with large angles of diffraction, the diffracted noise may be less than the transmitted noise. In this case, the transmitted noise compromises the performance of the barrier. It can be reduced with a heavier barrier. The allowable amount of transmitted noise depends on the total barrier attenuation needed.

0 The fourth path shown in Figure is the reflected path. After reflection, noise concerns only a receiver on the opposite side of the source. For this reason, acoustical absorption on the face of the barrier can sometimes reduce this reflected noise; however, this treatment does not benefit receivers in the shadow zone.

Transmission Loss0 When the position of the noise source is close to the barrier, the diffracted noise is less

important than the transmitted noise. 0 If the barrier is a wall panel that is sealed at the edges, the transmitted noise is the only

concern.0 The ratio of the sound energy incident on one surface of a panel to the energy radiated from

the opposite surface is called the sound transmission loss (TL). The actual energy loss is partially reflected and partially absorbed.

ENCLOSURES0 Sometimes enclosing a noisy machine in a separate room or box is more practical and

economical than quieting it by altering its design, operation, or component parts. 0 The walls of the enclosure should be massive and airtight to contain the sound.

NOISE CONTROL IN THE TRANSMISSION PATH0 Absorbent lining on the interior surfaces of the enclosure can reduce the reverberant buildup of noise within it. Structural contact between the noise source and the enclosure must be avoided, or the source vibration can be transmitted to the enclosure walls and thus short-circuit the isolation. Figure shows the incorrect and correct enclosure barrier systems.

NOISE CONTROL IN THE TRANSMISSION PATHIsolators and Silencers

VIBRATION ISOLATORS AND FLEXIBLE COUPLERS0 If the noise transmission path is structure-borne in character, vibration isolators in the form of

resilient mountings, flexible couplers, or structural breaks or discontinuities should be interposed between the noise source and receiver. For example, string mounts placed under a machine can prevent the

floor from vibration, or an

expansion joint cut along the

outer edge of a floor in a

mechanical equipment room

can reduce the amount of

vibration transmitted to the

structural frame or walls of a

building. These measures are shown in Figures.

NOISE CONTROL IN THE TRANSMISSION PATHIsolators and Silencers

VIBRATION ISOLATORS AND FLEXIBLE COUPLERS

NOISE CONTROL IN THE TRANSMISSION PATHIsolators and SilencersMUFFLERS AND SILENCERS0 No distinction exists between mufflers and

silencers. 0 They are often used interchangeably. 0 They are in effect acoustic filters and are used to

reduce fluid flow noise. 0 Figure on right shows six basic types of silencers.0 Reactive silencers are used for low-frequency

applications.0 Figure below shows a typical muffler that is

designed to attenuate sound waves with minimal back pressure. It includes a cylindrical-type unit. The outer portion of the through-pipe conduit contains a number of cavities where noise suppression occurs. A porous packing is sometimesused to increase efficiency. Airflow to the cavities is regulated by the size and number of holes from the center section. Mufflers are effective for high- and middle-frequencynoise control.

PROTECTING THE RECEIVERWork Schedules0 The amount of continuous exposure to high noise levels must be limited. For hearing

protection, scheduling noisy operation for short intervals of time each day over several days is preferable to a continuous eight-hour run for a day or two.

0 In industrial or construction operations, an intermittent work schedule benefits not only the operator of the noisy equipment but also other workers in the vicinity.

0 If an intermittent schedule is not possible, workers should have relief time during the day. 0 They should take their relief time at a low-noise-level location and should not trade this time

for more pay, paid vacation, or an early out at the end of the day.0 Inherently noisy operations, such as street repair, municipal trash collection, factory operation,

and aircraft traffic, should be curtailed at night and early morning to avoid disturbing the sleep

of the community.Equipment and

SheltersEar Protection0 Molded and pliable earplugs, cup-type

protectors, and helmets are commercially available as hearing protectors.

0 Such devices provide noise reductions from 15 to 35 dB (as shown in Figure).

0 Earplugs are effective only if they are properly fitted by medical personnel. Maximum protection can be obtained when both plugs and muffs are used. Only muffs with certification that stipulates the attenuation should be used.

PROTECTING THE RECEIVEREquipment and SheltersIndividual Enclosures or Noise Shelters0 In an industrial plant, large areas where the noise level is too high for efficient work often

exist. Any kind of office work is impractical at such noise levels, yet in plant offices are needed.

0 Noise shelters provide an effective solution for these problems. They may be fully enclosed rooms with separate heating and ventilating systems, which protect from dust and odors as well as noise.

0 When easy access to a noise shelter is needed, the labyrinth principle can be applied.

0 This principle is useful in isolating areas where specialized work is done (such as, inspection and final adjustment) and where both people and work move continually in and out. Figure shows how this principle is constructed.

PROTECTING THE RECEIVEREquipment and SheltersOther Possibilities0 When the noise within a confined area is too high to allow workers into it even with personal

protection devices, the operation might be automated. 0 An automated process is supervised from observation posts, that is, from remote control

stations where workers are adequately protected.0 A remote-control post receives information via a closed circuit television, or it can be a highly

insulated area within the department. 0 Mechanical devices handle the production procedures under operator or computer control. 0 For example, rolling mills are controlled from soundproof cabins. The same is true of

workshops for assembling and testing engines. 0 The noise of such operations cannot normally be stifled at the source.0 Noise-cancelling microphones and shielded microphones keep electrical communication

operable at high noise levels.