A guide for workers and employers
U.S. Department of Labor
Occupational Safety and Health Administration
|NOISE: ITS EFFECT ON HEALTH||2|
|NOISE CONTROL: BASIC CONCEPTS AND TERMS||4|
|APPLICATION OF NOISE CONTROL PRINCIPLES||11|
|A. Sound Behavior||12|
|B. Sound from Vibrating Plates||28|
|C. Sound Production in Air of Gases||44|
|D. Sound Production in Flowing Liquids||58|
|E. Sound Movement Indoors||62|
|F. Sound Movement in Ducts||72|
|G. Sound from Vibrating Machines||82|
|H. Sound Reduction in Enclosure Walls||94|
|NOISE CONTROL MEASURES||104|
|Changes in Machinery and Equipment||104|
|Enclosure of Machines||106|
|Control of Noise from Vibrating Surfaces||107|
|Sound Insulating Separate Rooms||109|
|Planning for Noise Control||110|
|HOW OSHA CAN HELP EMPLOYERS AND EMPLOYEES||112|
It can destroy hearing.
It can create physical and psychological stress.
And it can contribute to accidents by making it impossible to hear warning signals.
An estimated 14 million workers in the U.S. are exposed to hazardous noise.
Luckily, noise exposure can be controlled. No matter what the noise problems may be in a particular workplace, technology exists to reduce the hazard.
It may be possible to:
In the field of noise control, where there's a will, there's a way.
The Occupational Safety and Health Administration (OSHA) was established by the U.S. Congress to help eliminate job safety and health hazards such as noise.
This book is presented by OSHA for workers and employers interested in reducing workplace noise. OSHA believes that highly technical training is not generally necessary in order to understand the basic principles of noise control. Noise problems can often be solved by the workers and employers who are directly affected.
The book contains five major sections.
First, a brief overview of the effects of noise on human health
Second, a discussion of some of the key words and concepts involved in noise control.
Third, an explanation of specific principles of noise control which the reader can apply in his or her workplace.
Fourth, a discussion of particular techniques for controlling noise.
Fifth, a description of the ways OSHA can help employers and employees, including an explanation of the legal requirements for noise control which employers must follow.
OSHA hopes that the information in this book will be discussed by employers, workers, and union representatives. If more help is needed, contact the nearest OSHA area office listed in the back of the book.
Noise: Its effect on health
The ability to hear is one of our most precious gifts. Without it, it is very difficult to lead a full life either on or off the job.
Excessive noise can destroy the ability to hear, and may also put stress on other parts of the body, including the heart.
For most effects of noise, there is no cure, so that prevention of excessive noise exposure is the only way to avoid health damage.
The damage done by noise depends mainly on how loud it is and on the length of exposure. The frequency or pitch can also have some effect, since high-pitched sounds are more damaging than low-pitched ones.
Noise may tire out the inner ear, causing temporary hearing loss. After a period of time off, hearing may be restored. Some workers who suffer temporary hearing loss may find that by the time their hearing returns to normal, it is time for another work shift, so, in that sense, the problem is "permanent."
With continual noise exposure, the ear will lose its ability to recover from temporary hearing loss, and the damage will become permanent. Permanent hearing loss results from the destruction of cells in the inner ear-cells which can never be replaced or repaired. Such damage can be cause by long-term exposure to loud noise or, in some cases, by brief exposures to very loud noises.
Normally, workplace noise first affects the ability to hear high frequency (high-pitched) sounds.
This means that even though a person can still hear some noise, speech or other sounds may be unclear or distorted.
Workers with hearing impairment typically say "I can hear you, but I can't understand you." Distortion occurs especially when there are background noises or many people talking. As conversation becomes more difficult to understand, the person becomes isolated from family and friends. music and the sounds of nature become impossible to enjoy.
A hearing aid can make speech louder, but cannot make it clearer, and is rarely a satisfactory remedy for hearing loss.
Workers suffering from noise-induced hearing loss may also experience continual ringing in their ears, called "tinnitus." At this time there is no cure for tinnitus, although some doctors are experimenting with treatment.
Although research on the effects of noise is not complete, it appears that noise can cause quickened pulse rate, increased blood pressure and a narrowing of the blood vessels. Over a long period of time, these may place an added burden on the heart.
Noise may also put stress on other parts of the body by causing the abnormal secretion of hormones and tensing of muscles (see Figure 1).
Workers exposed to noise sometimes complain of nervousness, sleeplessness and fatigue. Excessive noise exposure also can reduce job performance and may cause high rates of absenteeism.
Figure 1. In addition to causing hearing loss by destroying the inner ear, noise apparently can put stress on other parts of the body by causing reactions such as those shown at left.
Figure 2. Severe destruction of the hair cells in the hearing organ. Top picture: normal hair cells, lower picture: hair cells destroyed by noise.
Noise control: Basic concepts and terms
There are a number of words and concepts which must be understood before beginning a discussion of noise control methods.
Sound is produced when a sound source sets the air nearest to it in wave motion. The motion spreads to air particles far from the sound source. Sound travels in air at a speed of about 340 meters per second. The rate of travel is greater in liquids and solids; for example, 1,500 m/s in water and 5,000 m/s in steel.
(Note: Measurements in this book are generally given in the metric system. To convert, one meter equals about 39.4 inches, and one millimeter equals 0.04 inches, and one kilogram equals about 2.2 pounds.)
The frequency of a sound wave refers to the number of vibrations per second, measured in units of hertz (Hz). Sound is found within a large frequency range; audible sound for young [continued on next page]
Figure 3. The sound source vibrates and affects air particles, which strike the ear drum.
Figure 4. A pure tone is marked by a single column indicating the frequency and the sound level or intensity. Musical notes contain several tones of different frequencies and intensities.
persons is between about 20 Hz and 20,000 Hz.
The boundary between high and low frequencies is generally established at 1,000 Hz.
Sound may consist of a single pure tone, but in general it is made up of several tones of varying intensities.
It is customary to call any undesirable sound "noise." [sic] The disturbing effects of noise depend both on the intensity and the frequency of the tones. For example, higher frequencies are more disturbing than low ones. Pure tones are more disturbing than a sound made up of many tones.
Infrasound and ultrasound
Sound with frequencies below 20 Hz is called infrasound, and sound with more than 20,000 Hz is called ultrasound. There is some evidence that these sounds which cannot be heard can under certain conditions be hazardous to workers' health. This book deals only with noise which can be heard.
Figure 5. Noise is a disorderly mixture of tones at many frequencies.
Figure 6. At the same intensity, the noise from a truck is less disturbing than the sound of air blowing or suction because it is at a lower frequency.
Sound levels are measured in units of decibels (dB). If sound is intensified by 10 dB, it seems to the ears approximately as if the sound intensity has doubled. A reduction by 10 dB makes it seem as if the intensity has been reduced by half.
Noise level measurement
In measuring sound levels, instruments are used which resemble the human ear in sensitivity to noise composed of varying frequencies. The instruments measure the "A-weighted sound level" in units called dB(A).
Workplace noise measurements indicated the combined sound levels of tool noise from a number of sources (machinery and materials handling) and background noise (from ventilation systems, cooling compressors, circulation pumps, etc.).
In order to accurately identify all workplace noise problems, the noise from each source should also be measured separately. Measurements at various production rates may be useful in considering possible control measures. A number of manuals for noise measurements are commercially available.
Adding noise levels
Decibel levels for two or more sounds cannot simply be added. Figure 8 shows how the combined effect of two sounds depends on the difference in their levels. Two or more sounds of the same level combine to make a higher noise level.
It is common practice to divide the range of frequencies we can hear into eight octave bands. The sound level is then listed for each octave band. The top frequency in an octave band is always twice the bottom one. The octave band may be referred to by a center frequency. For example, 500 Hz is the center frequency for the octave band 354-708 Hz.
|number of decibels to add to higher of two levels|
Figure 8. A fan produces a sound level of 50 dB(A). Another fan produces 56 dB(A). The difference is 6 dB(A), and according to the diagram, 1 dB(A) will be added to the highest level. Operating together, the fans will result in a level of 57 dB(A).
The word "sound" usually means sound waves traveling in air. However, sound waves also travel in solids and liquids. These sound waves may be transmitted to air to make sound we can hear.
Each object or volume of air will "resonate," or strengthen a sound, at one or more particular frequencies. The frequency depends on the size and construction of the object or air volume.
Sound reduction by distance
Sound spreading in open air and measured at a certain distance from the source is reduced by about 6 dB for each doubling of that distance. Sound is reduced less when spreading inside a room. (See Figure 9.)
Sound transmission loss (TL)
When a wall is struck by sound, only a small portion of the sound is transmitted through the wall, while most of it is reflected. The wall's ability to block transmission is indicated by its transmission loss (TL) rating, measured in decibels. The TL of a wall does not vary regardless of how it is used.
Noise reduction (NR)
Noise reduction is the number of decibels of sound reduction actually achieved by a particular enclosure or barrier. This can be measured by comparing the noise level before and after installing an enclosure over a noise source. NR and TL are not necessarily the same.
Sound is absorbed when it strikes a porous material. Commercial sound-absorbing materials usually absorb 70 percent or more of the sound that strikes them.
Figure 9. If a small sound source produces a sound level of 90 dB at a distance of 1 meter, the sound level at a 2 meter distance is 84 dB, at 4 meters 78 dB, etc.
Figure 10. Part of the sound that strikes a wall is reflected, part is absorbed, and part is transmitted. The transmission loss (TL) of the wall is determined by the portion of the noise which is not transmitted through the wall.
Application of noise control principles
The following section explains how to apply basic noise control principles. In many cases, several principles must be applied and several control measures must be taken. Of course, these principles do not cover every possible noise problem.
The principles are discussed in eight sections:
A. Sound behavior
B. Sound from vibrating plates
C. Sound production in air or gases
D. Sound production in flowing liquids
E. Sound movement indoors
F. Sound movement in ducts
G. Sound from vibrating machines
H. Sound reduction in enclosure walls
A number of symbols are used throughout the drawings. For example, large black arrows indicate strong sound radiation and smaller ones show reduced radiation.
SOUND BEHAVIOR - CAUSES OF SOUND PRODUCTION - A1
Changes in force, pressure, or speed produce noise
Sound is always produced by changes in force, pressure, or speed. Great changes produce louder noises and small changes quieter ones. More noise is produced if a task is carried out with great force for a short time than with less force for a longer time.
In a box machine, cardboard is cut with a knife blade. The knife must cut very rapidly and with great force in order for the cut to be perpendicular to the strip Much noise results.
Using a blade which travels across the strip, the cardboard can be scored with minimal force for a longer time. Since the cardboard strip continues to move, the knife must travel at an angle in order for the cut to be perpendicular. The cutting is practically noise free.
SOUND BEHAVIOR - CAUSES OF SOUND PRODUCTION - A2
Airborne sound is usually caused by vibration in solids or turbulence in fluids
For example, vibrations of the strings in a musical instrument are transmitted through the bridge to the sound box. When the sound box vibrates, sound is transmitted to the air. A circulation pump produces pressure variations in the water in a heating system. The sound waves are transmitted through the pipes to the radiators, whose large metal surfaces transmit airborne sound.
Turbulent fluid flow within pipes produces sound which can be radiated from the pipes and even transmitted to the building structure.
In addition to reducing the turbulence in the pipe, the pipe can be covered with sound absorbing material. The vibrations can be isolated from the wall or ceiling with flexible connecting mechanisms.
SOUND BEHAVIOR - CAUSES OF SOUND PRODUCTION - A3
Vibrations can produce sound after traveling great distances
Vibrations in solids and liquids can travel a great distance before producing airborne sound. Such vibrations can cause distant structures to resonate. The best solution is to stop the vibration as close to the source as possible.
Vibrations from an elevator are transmitted throughout a building.
The elevator drive can be isolated from the building structure.
SOUND BEHAVIOR - LOW AND HIGH FREQUENCIES - A4
The slower the repetition, the lower the frequency of the noise
The level of low frequency noise from a sound source is determined primarily by the rate at which the changes in force, pressure, and speed are repeated. The longer the time between changes, the lower the frequency of the noise generated. The level of noise depends on the amount of the change.
Two gears have the same pitch diameter but different numbers of teeth. If they rotate at the same speed, the gear with fewer teeth will produce a lower frequency noise.
SOUND BEHAVIOR - LOW AND HIGH FREQUENCIES - A5
High frequency sound is strongly directional and more easily reflected
When high frequency sound strikes a hard surface, it is reflected much like light from a mirror. High frequency sound does not travel around corners easily.
High frequency noise travels directly from the high-speed riveting machine to the worker's ears.
A sound-insulating hood, open toward the bottom of the machine, is constructed above the hammer. The hood is coated on the inside with sound-absorbing material. The upper portion of the opening is covered with safety glass. As sound starts towards the ears, the glass reflects it against the sound-absorbing walls. The sound level for the machine operator is thus reduced.
SOUND BEHAVIOR - LOW AND HIGH FREQUENCIES - A6
Low frequency noise travels around objects and through openings
Low frequency noise radiates at approximately the same level in all directions. It travels around corners and through holes, and then continues to travel in all directions. A shield has little effect unless it is very large.
Compressors and the diesel engines inside them both may produce strong low frequency noise, even if they are provided with effective mufflers at the intake and exhaust.
A complete enclosure of damped material lined with sound absorbant will help. The air and exhaust gases must pass through mufflers which are partly made of channels with sound-absorbing walls. Doors for inspection must close tightly.
SOUND BEHAVIOR - REDUCTION IN AIR - A7
High frequency sound is greatly reduced by passing through air
High frequency sound is reduced more effectively than low frequency sound by passing through air. In addition, it is easier to insulate and shield. If the noise source does not cause problems in its immediate vicinity, it may therefore be worthwhile to shift the sound toward higher frequencies.
The low frequency noise from roof fans in an industrial building disturbs residents of houses a quarter-mile away.
The rooftop fan is replaced by another one of similar capacity but with a larger number of fan blades. This produces less low frequency noise and more high frequency noise. The low frequency noise no longer causes disturbances, and the high frequency noise is adequately reduced by the distance.
SOUND BEHAVIOR - HOW DISTURBING? - A8
Low frequency noise is less disturbing
The Human ear is less sensitive to low frequency noise than to high frequency noise. If it is not possible to reduce the noise, it may be possible to change it so that more of it is at lower frequencies.
The diesel engine in a ship operates at 125 rpm and is directly connected to the propeller. The noise from the propeller is extremely disturbing on board.
Differential gear is installed between the motor and the propeller so that the motor can revolve at 75 rpm. The propeller is replaced by a larger one. The noise is shifted to a lower frequency, making it less disturbing.
SOUND FROM VIBRATING PLATES - SIZE AND THICKNESS - B1
Small vibrating surfaces give off less noise than large ones
An object with a small surface area may vibrate intensely without a great deal of noise radiation. The higher the frequencies, the smaller the surface must be to prevent disturbance. Since machines always will vibrate to some extent, noise control will be aided if the machines are kept as small as possible.
Too much noise is radiated from the control panel of a hydraulic system.
The panel is detached from the system itself, the vibrating surface is reduced, and therefore the noise level is decreased.
SOUND FROM VIBRATING PLATES - SIZE AND THICKNESS - B2
Densely perforated plates produce less noise
Large vibrating surfaces cannot always be avoided. The vibrating surface pumps air back and forth like the piston of a pump, causing sound radiation. If the panel is perforated, the "piston" leaks, and the pumping functions poorly. Alternatives to perforated plates include mesh, gratings and expanded metal.
The protective cover over the flywheel and belt drive of a press is a major noise source. The cover is made of solid sheet metal.
A new cover is made of perforated sheet metal and wire mesh. Sound radiation is reduced.
SOUND FROM VIBRATING PLATES - SIZE AND THICKNESS - B3
A long, narrow plate produces less sound than a square one
When a plate is set into vibration, excess air pressure forms on one side of the plate and then the other. Sound comes from both sides. The pressure difference balances out close to the edges, so the noise radiation there is slight. Therefore, a long, narrow plate radiates less sound.
A belt drive provides a large amount of low frequency noise because of the vibration of the broad belt.
The broad drive belt is replaced by narrower belts, separated by spacers. This reduces the noise problem.
SOUND FROM VIBRATING PLATES - SIZE AND THICKNESS - B4
Plates with free edges produce less low frequency noise
If a plate vibrates with free edges, pressure equalization takes place between the two sides of the plate, thus reducing sound emissions. Clamping the corners prevents pressure equalization and the sound emission is greater, especially at low frequencies. For example, speakers produce more bass if they are enclosed in a cabinet.
Bumps in the floor produce noise from the bottom and side plates of a cart when the cart is pushed. Sound is also emitted when material is slid down the cart walls. Pressure equalization only takes place at the top edges of the side plates.
The walls are replaced by new ones, constructed with a pipe frame. Plates are fastened with a gap between the plates and the frame. Pressure equalization takes place along all the edges, and the low frequency noise is reduced.
SOUND FROM VIBRATING PLATES - COLLISION AND IMPACT - B5
Light objects and low speed produce the least impact noise
When a plate is struck by an object, the plate vibrates and makes noise. The sound level is determined by the weight of the object and its striking speed. If the dropping height of an object is reduced from 5 meters (about 16 feet) to 5 centimeters (2 inches), the sound level drops about 20 dB.
Steel parts are transported from a machine to a storage bin. When the bin is empty, the dropping height is large and the noise is loud.
A hydraulic system is installed so that the conveyor belt can be raised and lowered. The belt ends in a drum equipped with rubber plates to break the fall of the parts. The drum is raised automatically.
SOUND FROM VIBRATING PLATES - INTERNAL DAMPING - B6
A damped surface gives off less sound
As vibration moves throughout a plate, it gradually decreases as it travels, but in most plates, this reduction is rather small. In such cases, the material is said to have low internal damping. Internal damping in steel, for example, is extremely poor. Good damping can be achieved by adding coatings or intermediate layers with better internal damping.
The loudest noise from a pump system comes from the coupling guard which is made of sheet metal.
The noise level is reduced by vibration isolating the guard or constructing it of damped metal. If the coupling creates a siren-type noise, the guard may need acoustical lining.
SOUND FROM VIBRATING PLATES - RESONANCE - B7
Resonance increases noise but it can be damped
Resonance greatly increases noise from a vibrating plate, but it can be suppressed or prevented by damping the plate. It may often be sufficient to damp only part of the surface, and, in some rare cases, damping of a single point is effective.
An automatic tooth cutter for circular saw blades produces intense resonance sound.
A urethane rubber coating clamped to the saw blade damps the resonance.
SOUND FROM VIBRATING PLATES - RESONANCE - B8
Resonance shifted to higher frequency is more easily damped
Large vibrating plates often have low frequency resonance which can be difficult to damp. If the plate is stiffened, the resonance shifts to higher frequency, which can be more easily damped.
The greatest low frequency sound from this machine comes from the side surfaces of the machine stand.
The side plates on the machine are stiffened with iron straps. A damped plate is installed over the braces.
SOUND PRODUCTION IN AIR OR
GASES - WIND TONES - C1
When air passes by an object at certain speeds, a strong pure tone, known as a Karman tone, can be produced. This can be prevented by making the object longer in the direction of flow, such as with a "tail," or by making the object's shape irregular.
Wind tones can be eliminated
At certain wind speeds, loud sounds can form around smoke stacks, causing disturbances.
A strip of sheet metal is mounted on the smoke stack in a spiral. The pitch of the spiral must not be constant. Regardless of the wind's direction, it encounters an irregular object.
SOUND PRODUCTION IN AIR OR GASES - WIND TONES - C2
Air flow past hollow openings should be avoided
When air or another gas blows across the edges of an opening to a hole, loud, pure tones are formed. This is how a wind instrument operates. The greater the volume of the hole and the smaller the number of openings, the lower the frequency of the tone will be.
When a cutter wheel revolves under no-load conditions, sound can arise from the track for holding the plane blade. An air stream is being chopped, creating a siren (pure tone) noise.
Minimizing the cavities by filling the empty space in the track with a rubber plate reduces the pumping action and the noise.
SOUND PRODUCTION IN AIR OR GASES - DUCTS - C3
Ducts without impediments produce the least amount of noise from turbulence
During flow in ducts or pipes there is always some turbulence against the duct walls. The noise from turbulence is increased if the flow must rapidly change direction, if the flow moves at a fast rate, and if objects blocking the flow are close together.
A branch of a steam line has three valves which produce a loud shrieking sound. The branch has two sharp bends which also produce a lot of noise.
A new branch is created with softer bends. Tubing pieces are placed between the valves, so that turbulence will be reduced or eliminated before the stream reaches the next valve.
SOUND PRODUCTION IN AIR OR GASES - DUCTS - C4
Undisturbed flow produces the smallest amount of exit noise
When a flowing gas mixes with a non-moving gas, noise may be produced, especially if the flow is disturbed before the outlet. A lower outflow speed will produce a lower sound level. For speeds below 325 feet/sec., reduction of the speed by half will mean that the sound will be about 15 dB weaker.
The exhaust air from a compressed air-driven grinding machine produces a loud noise. The air becomes turbulent while leaving the machine through the side handle.
A new handle is developed, filled with a porous sound-absorbing material between two fine-meshed gauzes. Passage through the porous materials breaks up the turbulence. The air stream leaving the handle is less disturbed, and the exhaust noise is weaker. A straight lined duct-type muffler may also be used.
SOUND PRODUCTION IN AIR OR GASES - DUCTS - C5
Jet noise can be reduced by using an extra air stream
The term "jet stream" applies at flow speeds in excess of 325 ft/sec. Turbulence outside the outlet is great. Reducing the outflow speed by half may decrease the noise level by as much as 20 dB. Since the noise level is determined by the speed of the jet stream in relation to the speed of the surrounding air, noise production can be greatly reduced by using an air stream with a lower speed outside the jet stream.
The cleaning of machine parts with compressed air after processing is often carried out with simple tubular mouthpieces. Very high exit speeds are required, and a strong high frequency noise develops.
The simple tubular mouthpiece can be replaced by mouthpieces which produce less noise, such as a dual flow mouthpiece. In this mouthpiece, part of the compressed air moves at a lower speed outside the central stream.
SOUND PRODUCTION IN AIR OR GASES - DUCTS - C6
Low frequency jet noise is easier to reduce if converted to high frequency
If the diameter of a gas outlet is large, the noise will peak at the low frequency. If the diameter is small the noise will peak at high frequency. The low frequency noise can be reduced by replacing a large outlet with several small ones. To some extent this will increase the high frequency noise, but this is more easily controlled.
Steam safety valves may discharge many times each day. Sound production during steam escape can produce high level, low frequency sound.
A diffuser is formed as a perforated cone. The holes produce many small jet streams and high frequency noise which is absorbed in the down-stream pack.
SOUND PRODUCTION IN AIR OR GASES - FANS - C7
Fans make less noise if placed in smooth, undisturbed flow streams
A fan produces turbulence in air, which causes noise. If turbulence is already present in the incoming air, the sound will be more intense. The same principle applies, for example, to propellers in water.
In one case the fan is located too close to a barrier, and in the other case too close to a sharp bend. The flow is disturbed and the noise at the outlet is intense.
The control vanes are moved farther from the fan so that the turbulence has time to die down. In the other case, the bend is made smoother, and the fan is moved away from the bend. Turning vanes could also be used.
SOUND PRODUCTION IN FLOWING LIQUIDS - PIPE SYSTEMS - D1
Rapid pressure changes produce more noise
Turbulence will form if the pressure in a liquid system drops rapidly. Gas is released in the form of bubbles and produces a roaring noise. The pressure drop can be produced by a large, rapid change in volume. Noise is avoided by a slow change in volume.
Control valves in liquid systems often have small valve seats, resulting in large flow speeds with large pressure changes. Twisted flow pathways and sharp edges produce intense turbulence. Sound radiates directly from valves and pipes, and solid sound is conducted to walls.
Control valves with larger cone diameters, straighter flow pathways, and more rounded edges are used.
SOUND PRODUCTION IN FLOWING LIQUIDS - PIPE SYSTEMS - D2
Large and rapid changes in pressure produce "cavitation" sound
Noise production takes place at control valves, at pump pistons, and at propellers when large and rapid pressure drops occur in liquids. This so-called "cavitation" noise is most common in hydraulic systems. Cavitation can be reduced by bringing about the pressure reduction in several smaller steps
In a hydraulic system, the full pump capacity is employed only in exceptional cases. The pressure is generally greatly reduced using a control valve. Cavitation can then arise, producing loud noise from the valve. The noise is conducted as solid-borne sound to connected machines and building structures
A pressure reducing insert is placed in the same pipe as the control valve. The inset has removable plates with different perforations. The plates are selected so that the inset will not produce a greater pressure drop than that required to prevent cavitation.
SOUND MOVEMENT INDOORS - PLACEMENT OF SOUND SOURCE - E1
Sound sources should not be placed near corners
The closer to reflecting surfaces a sound source is placed, the greater the noise it will radiate to a given distance. The worst placement is in corners near three surfaces. The best placement is away from the walls.
In an industrial shop, machines are placed in four rows with three aisles between them. This arrangement increases the noise from the machines in the two outermost rows.
The machines are placed together, two by two, away from the walls and new aisles are set up along the walls.
SOUND MOVEMENT INDOORS - ABSORPTION - E2
Thick, porous layers absorb both high and low frequency sound
Porous material through which air can be pressed often makes a good sound absorbant. Examples of such materials include felt, foam rubber, foamed plastic, textile fibers and a number of sintered metals and ceramic materials. If the pores are closed, the absorption is slight. Thin porous absorbants handle high tones. For good effects below 100 Hz, the thickness required may become impractical. Low frequency absorption is improved with the aid of an air gap behind the absorbant.
A workshop with intense low frequency noise is provided with absorbants that are effective for low tones. One part of the shop contains space for hanging absorption baffles, which provide good low frequency absorption and are easily installed. A traverse leaves no room for baffles in the other part of the shop. Instead, horizontal absorbant panels are installed above the traverse, 8 inches from the ceiling, to improve the low frequency absorption.
SOUND MOVEMENT INDOORS - ABSORPTION - E3
Cover layers with large perforations may be used without reducing absorption
For a variety of reasons, a covering material may be needed to protect a porous absorbant. This can be done without reducing the effectiveness of the absorbant if the cover material has a sufficient number of openings. The thicker the cover layer, the larger the number of perforations that will be required.
Sound-absorbing material is required on many wall and ceiling surfaces in a building. To provide a more attractive environment, it is desirable to have many absorbants with different appearances.
One material is used on all surfaces with varying thicknesses. Different covering materials provide the desired variation in appearance.
SOUND MOVEMENT INDOORS - ABSORPTION - E4
Panels on studs absorb low frequencies
Thin panels, fastened to a system of studs, absorb low frequencies. The absorption is effective in a narrow frequency range. This range is determined by the stiffness of the panels and the distance between the fastenings. If the panels are fastened to studs on a wall, the distance from the wall also has an effect. A panel with large internal damping absorbs in a larger frequency range. If a porous absorbing material is used at these low frequencies, it must be very thick.
Low frequency resonance in an engine room produced a very loud hum
near the walls and in the center of the room. When the revolution speed
was significantly changed, the hum disappeared completely.
The walls were coated with panels on studs to provide the greatest absorption in the range of the loudest tone. In order for the absorbant to continue to function even in the case of slight deviations from the normal rotation speed, a layer with good internal damping was used, which provided a more extensive range with good sound absorption. As a result, the resonance and the loud hum disappeared.
SOUND MOVEMENT INDOORS - ABSORPTION - E5
Sound shields may be combined with sound absorbing ceilings
High frequency noise can be reduced by using a shield. The shield is more effective the taller it is and the closer it is placed to the source. The effect of a shield is considerably reduced if the ceiling is not sound absorbant
In an auto plant with several assembly lines, the work on one line is noisier than the other. Grinding work on the bodies produces a shrieking, high frequency sound, disturbing everyone in the plant.
The other lines are protected from the grinding noise by means of shields on both sides of the line and sound absorbing baffles suspended above the open area.
SOUND MOVEMENT IN DUCTS - REACTIVE MUFFLERS - F1
All duct changes reduce sound transmission
With all changes in pathway, some sound energy is reflected back. In a duct this may apply to bends, branches, and changes in volume, shape, or wall materials.
An area is to be provided with mechanical ventilation. There is sufficient space for the fan, but not for the necessary muffler.
In place of a single inlet into the room, several smaller inlets are used. The sound reflection which takes place with all changes in volume and at each bend replaces the muffler.
SOUND MOVEMENT IN DUCTS - REACTIVE MUFFLERS - F2
Expansion chambers are useful for reducing low frequency noise
If a duct is provided with an expanded section or chamber, the low frequency pressure variations in the duct are reduced. The lower the frequency which must be reduced, the greater the space required in the chamber.
Using a jacket over the tip and a tubular outlet in the jacket, the high frequency noise given off by a jack hammer can be partially shielded. The low frequency noise in the exhaust air is effectively reduced. The enlarged are between the barrel and the jacket functions as an expansion chamber.
SOUND MOVEMENT IN DUCTS - REACTIVE MUFFLERS - F3
Reflection mufflers are effective in narrow frequency ranges
If noise is present in a limited frequency range, a reactive muffler may take up the least space. These are generally used at low frequencies. A large frequency range can be covered using several reactive chambers in succession. Perforated tubes are also employed in reflection dampers.
The muffler shown here is primarily used in large piston engines.
SOUND MOVEMENT IN DUCTS - PACKED MUFFLERS - F4
Absorption mufflers are effective over a broad range of frequencies
The simplest form of absorption muffler is a duct with sound-absorbing material on the walls. The thicker the material, the lower the frequency that can be reduced. For higher frequencies, the space between the absorbing walls must be made smaller. A large duct must therefore be subdivided into many smaller ones.
If a very large frequency range is to be reduced, it is generally necessary to employ absorption mufflers with thick and thin baffles.
SOUND MOVEMENT IF DUCTS - PACKED MUFFLERS - F5
Unused areas can be absorption chambers
The absorption chamber is simple muffler. One section of the duct is made up of a room whose walls are covered with a sound-absorbing material. When the sound is reflected against the chamber walls, sound energy is absorbed. To prevent the direct passage of high frequency, directed sound, the inlet and the outlet should not be located opposite one another. The greater the chamber volume and the thicker the absorbant used, the lower the frequency at which the muffler is effective.
The shape of the absorption chamber is of little significance. Unused rooms can be simple converted to absorption chambers.
SOUND FROM VIBRATING MACHINES - MACHINE MOUNTING - G1
Machines which vibrate should be mounted on heavy, rigid bases
Knocking on a thin door produces more sound than knocking on a thick wall. For the same reason, noise sources should be mounted on heavy or rigid bases.
A motor-driven oil pump is placed on the side wall of a hydraulic press. The vibrations are transmitted to all plates, which convert the solid-borne sound to loud airborne sound.
The oil system is removed from the press and installed in a frame on a heavy base. Sound transmission in the oil line is controlled with an accumulator.
SOUND FROM VIBRATING MACHINES - MACHINE MOUNTING - G2
Machines should be vibration isolated
Vibration isolation of machines can reduced the area of excessive noise as shown below. Either the machine or the working area can be isolated.
Vibration isolators are made of various materials and in various shapes.
SOUND FROM VIBRATING MACHINES - INSTALLATION - G3
Improperly selected springs can increase vibrations
A machine placed on springs has a so-called "fundamental frequency." Vibrations at or close to the fundamental frequency are greatly intensified. The machine may even break away from its fastenings. Vibrations with lower frequency that the fundamental frequency are not blocked. If the base is very heavy or very rigid, the fundamental frequency is determined entirely by the machine and base weights together with the rigidity of the spring. The lighter the machine and the more rigid the spring. the higher is the fundamental frequency. This reinforcement of vibrations can be avoided by using springs with good internal damping.
(Same fundamental frequency in the four examples.)
Two fans are used in the same building. Both are vibration isolate with steel springs which have very poor internal damping. The isolation functions well for both fans during constant operation, but one of the fans is started and stopped frequently. When this happens, the vibration frequency corresponds for a short time with the fundamental frequency, which produces serious disturbance.
On the fan with irregular operation, steel dampers are installed with pads which have good internal damping. The isolation is somewhat less, but the disturbance from starting and stopping disappears.
SOUND FROM VIBRATING MACHINES - INSTALLATION - G4
Isolating machines with low natural frequency may require a rigid floor
A machine and mount with low natural frequency are difficult to vibration isolate unless the floor is very rigid. As shown below, and extra heavy (stiff) or pile-reinforced floor might be necessary.
A company is planning a building where a need for freedom from vibration and noise is great. It should also be possible to remove and interchange the machines.
The building is constructed with large concrete plates on a pillar and beam system. The concrete plates which are expected to carry heavy machines are provided with strong reinforcements. If heavy machines are added later, the normal concrete plate is removed and replaced with a thicker one.
SOUND FROM VIBRATING MACHINES - MACHINE MOUNTING - G5
A separate base layer provides the best solid-borne sound barrier
A good way to isolate very heavy machines with low natural frequency vibration is to place them on a concrete base plate which rests directly on the ground. Even more effective protection is achieved if the base plate is separated from the remainder of the building by means of a joint. If the ground has a clay layer, it may be necessary to place pilings beneath the plate.
Drive motors with gears and differentials connected to a paper-making machine cause both loud air noise and vibrations in the machines. They require only occasional maintenance which can generally be performed with the machines turned off. Therefore, the machines can be permitted to make large amounts of noise if the noise is prevented from entering the rest of the factory.
The engine room has it's own thick base plate which is in good contact with the solid ground. The large base plate is also vibration isolated with corrugated rubber mats. Sound is prevented from entering other rooms by means of a brick wall. Holes in the wall for the axles to pass through are sealed with mufflers.
SOUND FROM VIBRATING MACHINES - MACHINE MOUNTING - G6
Sound through solid connections can be blocked
Vibration isolation of a machine may be ineffective if sound is transferred through connections for oil, electricity, water, etc. These connections must be made very flexible. The machine movements will be reduced if a heavy base is selected, and more rigid springs can be used.
Cooling systems may be serious sources of noise as a result of intense pressure shocks in the liquid from compressors.
Compressors may be vibration isolated with steel springs. In addition, flexible connections should be used for all inlet and discharge pipes.
SOUND REDUCTION IN ENCLOSURE WALLS - TL - H1
The TL of a single wall is estimated from its surface weight
"Transmission loss" (TL) indicate a wall's ability to absorb sound-producing vibrations. TL is expressed in decibels (dB). The TL of a homogeneous single layer wall can be estimated by its surface weight, that is, kilograms per square meter or pounds per square inch.
A sand blast operation creates excess noise. A separate room is available, with a thin drape as a barrier.
A separate room is constructed for this operation. The blasting equipment is separated from other work areas with a drapery of lead-rubber fabric, which is heavy, but flexible.
SOUND REDUCTION IN ENCLOSURE WALLS - TL - H2
A single wall will provide poor sound isolation at a certain frequency
A single wall has a "resonant frequency" at which the TL will be less than the figure indicated by the weight per unit of surface. This "coincidence valley" will disappear if the wall has good internal damping.
Behind one end wall in a long factory room are a number of machines with an intense noise level peak around 1000 Hz. The end of the room is isolated with a wall of 25 mm chipboard and 6 mm glass. The isolation is ineffective since the chipboard has its coincidence valley at 1000 Hz.
The chipboard is replaced by two layers of 9 mm plasterboard. The isolation is improved by about 10 dB. The plasterboard weighs about the same as one 25 mm chipboard, but is less than one-fourth as rigid. The coincidence valley is located at 2500 Hz. (Absorptive material could also be added to the machines side of the wall.
SOUND REDUCTION IN ENCLOSURE WALLS - TL - H3
Rigidity and weight are both important in thick walls
In most single layer walls, the coincidence valley is close to 100 Hz for a thickness of about 20 cm. At higher frequencies, both increased weight and increased rigidity greater TL. A cast concrete wall has greater rigidity than a brick wall, and therefore provides greater TL if the two walls weights are equal.
Machines in a large open area in an industrial building create a noise hazard .
The are containing the machines is surrounded by a brick wall.
SOUND REDUCTION IN ENCLOSURE WALLS - TL - H4
Light double walls provide good TL
Two light walls separated by an air gap provide good TL, increasing with the distance between them up to about 15 cm. With sound-absorbing material in between, the TL further increases as the distance between increases. Double walls may provide the same TL as single walls that are five to ten times as heavy.
Extremely noisy machines disturb workers in two work areas separated by a thin wall.
The machines are brought together and placed in one end of the factory. This is separated off with a light double wall which provides 60 dB of insulation. A passageway is created with doors to the two quiet areas. The insulation between the areas is at least 35 dB even if one of the doors is open.
SOUND REDUCTION IN ENCLOSURE WALLS - TL - H5
Double walls should have few connections
A double wall provides the best TL if each layer is connected to heavy walls or if there are open joints on both ends. If the layers are fastened to shared studs, the TL is greatly reduced if the studs are close together. The thicker the layers, the farther apart the studs must be in order to avoid substantial reduction of TL.
The control room for a machine in a paper factory is noisy, and telephone conversation is practically impossible.
A well-insulated room is created with thin panels on common studs. The floor plate of the room is isolated from vibration from the floor of the factory.
Noise control measures
The following is a survey of noise control methods which have been applied with good results in various types of workplaces. Many noise sources produce airborne sound and sound from vibrating surfaces at the same time, so in many cases several noise control measures must be applied.
Changes in machinery and equipment
The machines or machine pars to be controlled must be identified. Methods of maintenance and servicing must be taken into account in noise control design. Attempts should be made to:
Designers should be encouraged to:
Noise of existing equipment can often be controlled as effectively as new equipment without complicated procedures. Common control measures include:
In a new plant, it is sometimes possible to make more extensive changes such as:
Various types of air and solid-borne noise control measures in a tool-making machine
Existing workplaces may be changed to prevent impact and collision during manual and mechanical materials handling.
If new transportation equipment is being purchased, consideration should be given to creating a system for quiet materials handling.
The following may be considered:
Enclosure of machines
If it is not possible to prevent noise, it may be necessary to enclose the machines.
Plates dropping from a great height off of a roller belt onto a stacking platform produce loud noise. By using a board whose height can be raised and lowered, the drop can be reduced and the noise decreased.
Enclosure of a hydraulic system requires muffled ventilation openings. Electric motors release both sound and heat, as do the pump and the oil tank.
Control of noise from vibrating surfaces
Vibration in machines often results from slippage or loosened bolts. In such cases, the disturbance can be reduced by repair or replacement.
In the case of heavily vibrating machines, a separate machine base may be used as well as a separating joint to prevent the spread of sound. Here, two joints are used for separation.
All joints are equipped with double 10mm layers of cellular plastic projecting shields prior to concrete pouring.
After pouring, clean or burn out the joints, inspect, and reclean if necessary. No pieces of stone or the like should be present in the joints.
Seal the joint with a piece of rubber tubing, etc., which is pressed down. Then close off the surface with elastic sealing compound.
Damping with absorbants
In a workplace with hard materials on the ceiling, walls, and floor, almost all of the sound which strikes the surfaces is reflected. The sound level goes down at first as you move away from the machine, but after a certain point it remains practically unchanged. A better sound environment can be obtained by coating the ceilings and walls with effective sound-absorbing material.
How sound levels vary at different distances from a sound source before and after application of absorbent materials to the entire ceiling surface.
Sound insulating separate rooms
With automation of machines and processes, remote control from a separate room may become desirable. Some control measures may include:
Sound disturbances in an operating room or shop office may be caused by direct transmission (leakage through the door opening, etc.) from the machine or by radiation from the common floor.
In some cases, a noise hazard will be created or made worse by a lack of maintenance. Parts may become loose, creating more noise because of improper operation or scraping against other parts. Grinding noises may also occur as the result of inadequate lubrication.
It is especially important to provide proper maintenance of noise control devices which are added or built into machinery. If a muffler becomes loose or worn out, for example, it should be fixed or replaced as soon as possible.
Planning for noise control
Noise control should be taken into account from the beginning of the planning process for a new building:
Example of noise control measures which can be carried out in an industrial building to avoid the spread of noise.
How OSHA can help employers and employees
Legal limits on noise
Under the Occupational Safety and Health Act, every employer is legally responsible for providing a workplace free of such hazards as excessive noise.
OSHA regulations require every employer to limit workers' exposure to 90 "decibels," of dB(A), averaged over an 8-hour period. There are shorter time limits for higher noise levels (see chart). Measurements must be made under normal working conditions.
If noise exposure rises above these levels, the employer must use "engineering controls"-changes in the physical work environment-or "administrative controls"-limits on individual employees' exposure time-in order to comply with the law. While such controls are being implemented, workers must be provided with personal protective equipment, such as earmuffs or plugs, to protect their hearing.
Personal protective devices are generally not a good permanent solution for a number of reasons. They may cause infections or discomfort. They may not work effectively because of the difficulty in getting acceptable fit for each individual. In some cases-particularly when noise is intermittent and below 85 dB(A)- they may make communication more difficult, which can contribute to accidents and make jobs more difficult to perform. If ear protection equipment is not effective or comfortable, the worker should discuss the problem with his or her employer. Other devices may be more satisfactory, and perhaps permanent noise control measures may be installed more quickly.
If sound levels are above the OSHA limits, it is a serious violation of OSHA standards if the employer does not maintain an adequate hearing conservation program. The program must include periodic
|Hours of exposure||Sound level, dB(A)|
|¼ or less||115
OSHA standards establish limits on workplace noise exposure for given time periods. The limit for average exposure during an 8-hour shift is 90 decibels, or dB(a). Exposure to impulse noise should never exceed 140 decibels.
hearing tests (audiograms) for each overexposed employee and retesting or referral to a qualified physician if an employee's hearing ability is reduced by 20 decibels at any frequency
When insert ear plugs or custom-molded hearing protection devices other than self-fitted, malleable plugs are being used, OSHA requires that individual employee fitting be conducted by a trained person, and that employees be instructed in the care and use of the devices.
More information about OSHA requirements and procedures for enforcing noise standards is contained in OSHA's Industrial Hygiene Field Operation Manual.
If an employer fails to comply with OSHA standards, and the problem cannot be resolved at the workplace, workers should contact the nearest OSHA are office.
Consultation for employers
In all states, free consultation services are available to help employers identify job safety and health hazards, such as excessive noise exposure, and to recommend solutions. All information gathered by the consultant, including the employer's identity, is completely confidential and is not made available to OSHA enforcement personnel - with one very rare exception. If a consultant observes a serious violation of OSHA standards and the employer fails to correct it within the time period recommended by the consultant, OSHA or the responsible state agency will be notified.
Employers may on an anonymous basis contact the nearest OSHA office to find out how to take advantage of the free consultation services.
Loans for small businesses
The Small Business Administration (SBA), in cooperation with OSHA, provides loans to small businesses which are "likely to suffer substantial economic injury" in meeting OSHA safety and health standards.
Small business employers can obtain more information from the nearest SBA office or may write for a copy of SBA Loans for OSHA Compliance (OSHA 2005) from OSHA Office of Publications, Room S-1212, Department of Labor, Washington, D.C. 20210.
Workers' rights under OSHA
With its small staff of inspectors, OSHA obviously cannot prevent every hazard in every workplace. It is essential that workers and employers cooperate on job safety and health programs. In particular, OSHA encourages the formation of workplace health and safety committees. Health and safety committees can keep on top of job hazards such as noise, day in and day out. And if properly set up, they can make sure both employers and workers are involved in eliminating hazards.
(For more information on health and safety committees and the workers' rights described below, contact the nearest OSHA office.)
Whenever possible, safety and health problems should be resolved at the workplace. If they can't be, here are some ways workers can use the laws:
OSHA can seek an order from a federal court.
Workers' right to insist on job safety and health
Section 11(c) of OSHA's law says an employer cannot punish or discriminate against an employee for job safety and health activities, such as:
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