NPC Hearing Noise Control-A Guide for Workers and Employers

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:

  • Use quieter work processes.
  • Alter or enclose equipment to reduce noise at the source.
  • Use sound-absorbing materials to prevent the spread of noise by isolating the source.

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.

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.

Hearing

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.

Other effects

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.

There are a number of words and concepts which must be understood before beginning a discussion of noise control methods.

Sound

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.)

Frequency (Hz)

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.

Noise

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.

Decibel (dB)

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.).

Figure 7.

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.

Octave band

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).

Sound transmission

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.

Resonance

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 absorption

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.

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.

Principle

Example

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.

Control Measure

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.

Example

Turbulent fluid flow within pipes produces sound which can be radiated from the pipes and even transmitted to the building structure.

Control measure

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 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.

Principle

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.

Example

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.

Example

High frequency noise travels directly from the high-speed riveting machine to the worker’s ears.

Control measure

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.

Principle

Example

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.

Control measure

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.

Principle

Example

The low frequency noise from roof fans in an industrial building disturbs residents of houses a quarter-mile away.

Control measure

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.

Principle

Example

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.

Control measure

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.

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.

Principle

Example

Too much noise is radiated from the control panel of a hydraulic system.

Control measure

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.

Example

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.

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.

Principle

Example

A belt drive provides a large amount of low frequency noise because of the vibration of the broad belt.

Control measure

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.

Principle

Example

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.

Control measure

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.

Principle

Example

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.

Control measure

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.

Principle

Example

The loudest noise from a pump system comes from the coupling guard which is made of sheet metal.

Control measure

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.

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:

  • Use quieter work processes.
  • Alter or enclose equipment to reduce noise at the source.
  • Use sound-absorbing materials to prevent the spread of noise by isolating the source.

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.

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.

Hearing

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.

Other effects

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.

There are a number of words and concepts which must be understood before beginning a discussion of noise control methods.

Sound

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.)

Frequency (Hz)

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.

Noise

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.

Decibel (dB)

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

NPC Hearing Noise Control-A Guide for Workers and Employers

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.).

Figure 7.

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.

Octave band

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).

Sound transmission

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.

Resonance

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 absorption

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.

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.

Principle

Example

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.

Control Measure

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.

Example

Turbulent fluid flow within pipes produces sound which can be radiated from the pipes and even transmitted to the building structure.

Control measure

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 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.

Principle

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.

Example

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.

Example

High frequency noise travels directly from the high-speed riveting machine to the worker’s ears.

Control measure

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.

Principle

Example

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.

Control measure

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.

Principle

Example

The low frequency noise from roof fans in an industrial building disturbs residents of houses a quarter-mile away.

Control measure

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.

Principle

Example

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.

Control measure

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.

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.

Principle

Example

Too much noise is radiated from the control panel of a hydraulic system.

Control measure

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.

Example

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.

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.

Principle

Example

A belt drive provides a large amount of low frequency noise because of the vibration of the broad belt.

Control measure

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.

Principle

Example

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.

Control measure

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.

Principle

Example

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.

Control measure

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.

Principle

Example

The loudest noise from a pump system comes from the coupling guard which is made of sheet metal.

Control measure

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.

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