Noise is present with nearly every phase of paper product production, from the cutting of trees to converting the paper into a final product. Reduction of the noise of these operations is a challenge of tremendous magnitude. Placing this noise problem in proper perspective, it is seen that all of the major noise problems of the industry have received considerable attention and most have been solved to some extent.
While the noise levels of paper machines have been reduced up to 20 dBA, sound levels still exceed 90 dBA. Despite huge sum invested in research efforts, some noise problems remain totally unsolved, such as chain saw noise and single facer roll noise. Where noise control solutions exist, considerable time will be required for implementation on a large scale basis.
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The heart of the paper industry and the majority of the noise problems lie in the pulp and paper mills. An outstanding feature of paper mill noise environments is that employees are seldom exposed to noise sources continuously for an eight hour work day. Thus, with the use of control rooms, noise exposure can often be reduced to within the OSHA limits without reducing the sound level of all sources within a mill below 90 dBA.
There are mills in the United States which have reduced the noise exposure levels of all employees to within the OSHA guidelines by means of a combination of source control and operator enclosures. For older mills, however, it is not technically feasible to reduce sound levels to below the OSHA limits for all operations. In some mills, potential energy savings associated with noise control designs have been shown to exceed the total noise abatement costs.
Paper Machines:
The primary noise source of paper machines is suction noise due to high speed suction rolls. The function of the paper machine suction rolls is to mechanically extract, by means of a vacuum, water from the newly formed, very wet sheet of paper prior to its passage over the heated dryer drums.
Suction roll noise is generated by air-evacuated suction roll holes ‘popping’ as they come off the trailing packing strip of the suction box and are filled rapidly with atmospheric air. As these holes ‘pop’ into the atmosphere in rapid succession, they produce a whining noise.
Two methods are recognised for silencing of suction (both couch and press) rolls:
1. Special drill patterns.
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2. Silencers.
Three generations of drilling patterns have been developed for suction rolls to control noise. The special pattern is designed for pressure cancellation between holes and is the quietest water extraction method available.
The shoe silencer forces the air-evacuated holes to fill from the outside of the shell and the gap formed between the two rolls restricts the reentry of air to provide slow filling of the holes.
Even with quiet drill patterns and silencers, the noise levels or most paper machines will be above 90 dBA. Of the two treatments, rolls with silent drill patterns are the most effective and most costly.
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It is also observed that most paper mills in the United States and Europian Countries have replaced older rolls with straight drill patterns with new, quieter rolls.
In conjunction with control of paper machine noise levels, employee noise exposure may be reduced by the use of control rooms for machine tenders and ‘wet end’ operators. Operators will spend from 15 per cent (frequent grade changes and paper breaks) up to 80 per cent (all controls located in booth) in control booths, allowing compliance with the OSHA guidelines even though sound levels may be above 90 dBA.
Dryer hoods, generally installed to increase drying efficiency and as an energy conservation measure, will reduce sound levels adjacent to the dryer significantly. Typically, sound levels will range from 90-93 dBA with dryer hoods; however, the majority of this noise is carryover from the suction rolls and wet end operations.
An additional noise source on paper machines is pneumatic air jets used for calendar roll cooling. This noise may be reduced by replacement of the pneumatic system with one utilising low pressure air or by the use of pneumatic mufflers.
Winders:
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In the early days of high speed paper manufacturing, before consideration was given to noise pollution effects, it was found that front and back drums with small chevron angles resulted in efficient winding. These drum types are inherently noisy. Present-day concern for employee hearing loss has caused a reinvestigation of optimum groove angles and also of the necessity for the grooves altogether.
The noise generated by winders has been correlated to groove patterns in the drum.
The predominate frequencies are given by –
fi = min (rpm ÷ 60)
Where,
m = mode number
n = number of grooves.
The dominant spectrum peaks are identified as the harmonics of the drum groove spacing. The second harmonic generally provides the highest sound level. The primary mechanism of noise generation is localised high velocity air flow or ‘air pumping’ due to air which is trapped and compressed in the cavities between the stock and drum grooves. Noise reduction may be achieved by groove modification.
Based on the study, it was determined that where grooves are required on the front drum, the chevron angle could be increased to approximately 15°, thereby reducing noise levels while maintaining adequate friction forces. It is recommended that chevron grooves be used where roll tension requirements are critical and that 15° is the optimum chevron angle.
The friction of the front drum has been defined as building tension in the paper, minimising breaks and creating a tight roll. A new approach to winder noise abatement is the complete elimination of chevron grooves. A spiral groove is used on the first (back) drum and the second (front) drum is smooth.
To avoid costly replacement of rewinder drums, a technique has been developed to modify existing drums.
The procedure which has been followed with successful results is as follows:
1. Winder grooves are sandblasted.
2. Circumferential, spiral grooves on the front drum are masked to prevent filling.
3. The chevron grooves are overfilled with putty type plastic steel.
4. The epoxy is allowed 10-24 hours to harden. Curing is accelerated with higher temperatures, such as from heat lamps.
5. The epoxy fill is ground until the surface is flush with the metal drum face.
This treatment has been applied to winders with 67 pound liner board and speeds up to 6000 fpm. Epoxy-filled grooves have proven reliable in all cases; however, it should be recognised that either a poor bond or epoxy deterioration may result in delamination of the material while rotating at high speeds, creating a potential safety hazard.
Some rewinders have guards which would protect workers against possible delamination. It is recommended that guards be employed for all such applications.
As an alternative to the epoxy fill, metal weld fill may be utilised to eliminate existing chevron grooves.
A second noise problem associated with rewinders relates to trim blowers. Where the noise is generated only by the blower, a solution lies in the installation of a straight-through silencer along the trim duct. When air jets are utilised to assist the trim movement, no attractive solution appears to exist for noise reduction.
A new economical and simple alternative is to replace the existing system with a Transvector air flow amplifier. These units are circular with an open centre. Air flow of approximately 20 times the pneumatic air consumed by the unit is drawn through the centre opening. It is considered that paper trim may also be sucked through the opening. The sound levels of a Transvector Model 903 are 86 dBA at 1½ feet.
Logging Operations:
Three primary noise sources associated with logging operations have high noise levels which present a risk of hearing damage to operators:
1. Chain saws.
2. Skidders.
3. Trucks.
Chain saws produce noise at an average noise level of 106 dBA at the operator’s ear. In normal cutting, in which the operator cuts, limbs and tops one tree at time with a chain saw duty cycle of 0.33, he is working in a noise environment which is generally acceptable, without fear of much loss of hearing.
When a chain saw operator is working outside this pattern, as are most on mechanised logging operations and thus not within the tolerance levels of [the OSHA] criteria, he exposes himself to the possibility of hearing damage and should wear some form of ear protection. Mufflers can be installed on some older chain saws; however, this often decreases performance and is not a feasible approach. Many newer saws have incorporated silencing and are somewhat quieter than older models.
Skidders were found to operate at an average noise level of 104 dBA at the operator’s ear. Due to their estimated duty cycle of 0.6, the operator works in an excessive noise environment and risks suffering damage to hearing over an extended period of time.
It is strongly recommended that skidders be equipped with proper mufflers and otherwise modified to reduce the noise level. The noise reduction due to exhaust muffling may be relatively little (perhaps 5 to 10 dB) since there are other noise sources (air intake and engine clatter) which also need silencing, but this small reduction will greatly extend the exposure times for equal risk of hearing damage.
If existing skidders are not silenced to meet the acceptable criterion of 90 dBA or lower, then ear protection should be worn. It has been found that the installation of Stemco silencers on several models of skidders reduced sound levels from the high 90’s to the low 80’s. The installation of glass fibre in cabs provided an additional 4 dBA reduction.
The sound levels of trucks may range from below 90 dBA to 100 dBA in the cab.
The following approaches to noise reduction may be considered:
1. Installation of a more effective muffler.
2. The use of air conditioning to allow the cab windows to be closed.
3. Sound absorptive materials on the ceiling of the cab.
4. Increasing the sound transmission loss of the firewall between the engine and cab.
Corrugated Box Manufacturing:
In a typical container plant, the two main sources of noise are the corrugator line and the hogger. Despite extensive research by major container manufacturers and the ‘fibre box association’, solutions to reduce the noise of the single-facer and double-facer machines have not been found. This looms as one of the major unsolved problems in the field of industrial noise control.
The use of acoustical enclosures for the single facer has been considered; however, critical temperature and humidity requirements have precluded their use, rendering this approach unfeasible within the present state-of-the-art.
Two approaches to noise control of corrugator lines not related to machine design may be considered to reduce employee noise exposure:
1. The installation of sound absorptive materials to reduce reverberant sound build-up.
2. The use of barriers, partial enclosures and enclosures to shield employees when not working directly with the machine.
The applicability of these solutions will vary from plant to plant. In some plants, reverberation is not a significant problem. Also, in many plants, employee work requirements are such that an employee could not spend time in a booth. In other plants, these approaches have been successfully undertaken. One barrier design is reported to have reduced sound level exposure from a single facer by 10 dBA.
Converting Machinery:
Paper converting operations may involve a wide variety of machinery, depending upon the final product being produced.
The following sections present noise control approaches for several converting operations:
Four items of noise generation were identified on 1/6 Bbl sack machines.
These sources are, in order of significance:
(i) Folding flap;
(ii) Gears;
(iii) Cutter area where sack is jerked open; and
(iv) Former at heal of machine.
The first two sources are dominant generators of noise at the operator location. The latter two sources are distant from or directed away from the operator.
The use of machine enclosures is not considered feasible from either and acoustical or production viewpoint for the following reasons:
1. Continuous adjustment of the machines would require the enclosures to be open a considerable portion of the time, reducing their acoustical effectiveness. The adjustment requirements are due primarily to per variations and cannot be controlled.
2. The machine would often be required to run during adjustment within the enclosure. The accessibility interference of the complex enclosure would present a potential safety hazard.
3. Paper dust build-up within the enclosure may present a fire hazard.
4. Discussions with the manufacturers of converting machines confirmed that attempts to enclose 1/6 Bbl machines have been unsuccessful from a production viewpoint, except possibly for convertors which produce their own consistent grade of paper integral with the converting process.
Precluding the use of enclosures, minor machine modification should be considered for noise reduction:
1. The folding flap may be enclosed or redesigned.
2. Every other gear may be replaced with nylon or other non-metallic material gear.
On smaller grocery bag machines, acoustical enclosures are practical.
The following reflects the experience of one bag manufacturer regarding the use of experimental enclosures:
Like so many people connected with paper bag machines, we felt that these guards would be a tremendous interference with the operation of the machine; that not being able to hear the various noises eminating from the bag machine operation would be a detriment to the adjustors.
We also thought that the operators would quickly get fed up with these covers and manage to break them off. However, they use them; they take care of them; they seem to like them and indeed, the sharpness of the noises eminating from guarded machines is muffled and much less harsh.
Sound levels for various types of envelope machines are given in Table 14.2.
A distinctive trend is observed of increasing sound level with increasing machine speed. The primary noise generating mechanism for most envelope machines is acoustic radiation from the envelope itself it is cut, folded and impacts stops and other machine elements. The only feasible approach to noise reduction is the use of localised enclosures over the noise producing areas.
To minimise adverse production influences, the enclosures may consist of hinged clear plastic panels for access and visibility. On some envelope machines, machine mechanisms may also produce significant noise and may also require treatment. Vacuum pumps also generate considerable noise (91-97 dBA).
While conventional silencers may be employed to solve this noise problem, the installation of a central vacuum system may be considered. This approach would not only reduce noise, but would also serve as a significant energy conservation measure in larger plants.
On a new high-speed tissue decorator, ringing of the roll was identified as a major noise source. A 3-6 dBA noise reduction was achieved by damping the roll. This consisted of filling the cylinder with expandofoam.
A toilet tissue winder perforates the tissue by the use of a square-edged blade striking an anvil with the paper between the blade and anvil. A 6 dBA decrease in noise at the operator position was achieved by the application of a 0.038″ layer of polyfibron beneath the blade.
The roll and gears were also damped on the same winder, resulting an overall noise reduction from 101 dBA to 91 dBA.
Paper cutting noise is generated by rotary blade cutters by the ‘snap’ induced into the paper by the impact of the cutter blade and by the near-in-stantaneous increase in paper speed of 30-50 per cent as it passes the blade. The primary noise mechanism is sound radiated from the paper itself.
While cutter mechanisms other than the rotary blade type are inherently quieter, they are not capable of high production speeds. The most practical approach to reducing cutter noise is to install an acoustical cover over the cutter blade, extending to also enclose the noise-radiating paper sections.
Stock Preparation Machinery:
The noise generated by refiners and defibrators is due primarily to pulsations induced into the stock by each passage of the disk breaker bars.
The pulsations occur at a frequency given by –
Where,
f = frequency, Hz
NR = number of breaker bars on rotor disk
NS = number of breaker bars on stator disk
K = highest common factor of NR and NS
rpm = rotational speed of rotating disk.
These stock pulsations transmit vibrations to the refiner housing and outlet piping which, in turn, radiate noise.
The sound levels radiated by each component of a refiner or defibrator may be estimated by the following relationship –
Lp = Lv – 20 log f + 150
Where,
Lp = sound pressure level, dB
Lv = vibration level, dB re 1.0 gram
f = frequency, Hz
Below the coincidence frequency, the part sound radiation will be 3-4 dB/octave less than predicted by the equation. The equation is valid only for frequencies with wavelengths less than the part dimension.
Using this type of analysis, one may determine which components of the refiner and piping system require acoustical treatment.
One method which may be used to reduce the noise of stock preparation machinery is to alter the disk breaker bar patterns such that the passage frequency is moved above the audible range. For example – the frequency will be 720 Hertz for a vertical disk refiner running at 1200 rpm with 24 breaker bars on each disk. If a disk is replaced with one having 25 bars, the frequency will be shifted to 18,000 Hz. This would be expected to result in a 5-10 dBA noise reduction. Similar modifications may be made to the bars of the shell or plug.
The most common method used to control the noise of pulp preparation areas in paper mills is the isolation of refiners from the paper machine wet end operators by means of a wall or enclosure.
Refiners may also be quieted by means of acoustical enclosures or lagging treatment. It is also reported that a lead vinyl fabric curtain reduces sound levels of two high-speed refiners 6 to 13 dBA.
On refining equipment in general, the noise level may be decreased at any intensity by increasing the bar angel between the rotor and stator. This is true whether the machine is a beater, low-consistency disk refiner or a high-consistency disk refiner. Unfortunately, any noise level reductions are generally achieved at the expense of fibre characteristics.
The highest vibration levels can generally be identified on the shell and piping. A noise reduction of 5 dBA or more may be achieved by lagging these components. When refiners are treated for noise reduction, it is also often necessary to silence the motor.
The sound levels of pulpers are generally below 90 dBA. In two cases where excessive sound levels were identified, simple solutions were found. In one instance the noise was generated by steamline piping and a valve; this problem was corrected by a lagging treatment. In another case, sound levels of 92 dBA were measured under full load conditions due to panel vibrations; a vibration damping was recommended.
Chippers:
Sound levels near a chipper are typically 115 dBA. It has been found feasible to reduce sound levels within a wood room to an average of 95 dBA and to achieve OSHA compliance by restricting employee exposure time to a maximum of four hours per day.
The following measures were employed:
1. Improve existing booth for operator.
2. Construct booth for assistant operator.
3. Install mirror systems for operator inspection.
4. Optimise log transfer.
5. Install inlet silencer on chipper.
6. Apply lagging treatment on chipper housing.
7. Install room absorption.
These solutions, with the exception of item 5, employ conventional techniques of noise control engineering. Thus, only the chipper inlet silencer will be described here.
The highest noise levels of the chipper are radiated from the inlet opening. To reduce the sound from the opening, it is recommended that a silencer be constructed on the conveyor chute adjacent to the tunnel.
The silencer should consist of two parts:
1. A cover constructed on 1½ psf minimum sheet metal, a minimum of 10′ along the length of the conveyor. The cover should be hinged and operable by a control motor. The cover may have windows for inspection by means of an operator mirror system.
2. Baffles secured to the conveyor sides, constructed of 1½ psf sheet metal. The baffles may be protected from log damage by deflection bars. The area under the baffles should be the minimum required for log passage. This area under the baffles should be covered with vinyl curtains.
3. The underneath side of the covers and front of the baffles should be faced with a 4″ thick glass fibre insulation, fuzzy side exposed, with a perforated sheet metal (40 per cent open area) or screen facing.
The above measures reduced the sound levels at the chipper inlet from 114 dBA to 100 dBA and from 107 to 95 dBA at a distance of approximately 50′.
Core Saws:
Core saws typically generate sound levels ranging from 105 to 113 dBA. Although the low exposure time to this noise generally does not result in exceedance of the OSHA guidelines, noise reduction can be achieved quite readily.
The following treatment for core saws may be considered:
1. A large stiffening collar may be installed, extending as near the base of the teeth as allowable by the core thickness. A layer of vibration damping material, 1/8″ thick, should be installed between the collar and blade.
2. As an alternative, the centre portion of the blade not utilised for cutting may be treated with a constrained layer damping treatment. This treatment should cover as large an area as possible (30 per cent is the minimum for effectiveness) and must not come into contact with the material being cut.
Barking Drums:
Sound levels of barking drums range from 98-103 dBA. A typical sound level spectrum is shown in Fig. 14.7. This sound spectrum is very similar to that measured when isolated logs are hit together, indicating that log impacts within the drum are the primary noise contributor.
The design of deadened (damped) drums has been considered, however, this would probably provide only a marginal noise reduction, since the logs generate the most noise. The use of cushioned suspensions and rubber rollers are reported to be somewhat effective.
With the lack of feasible design approaches for barking drum noise reduction, the only approach to reduce employee noise exposure is to isolate the drum in a separate room or outdoors. In planning a new mill, this may be quite easy; however, for existing installations, building a complete room around a barking drum would be costly and may not be practical for some wood room layouts.
Secondary noise to barking operations may be generated by logs impacting metal chute panels as they enter or exit the drum. This noise may be reduced by approximately 10 dBA by applying a constrained layer damping treatment to the back of the panels.
Power Plants:
Many pulp and paper mills have power plants which supply some or all of the plant’s steam and electrical requirements. Table 14.3 summarises common noise control treatments for various power plant noise sources.
A detailed employee noise exposure survey is essential in abating power plant noise. Despite the numerous items of noisy equipment in a power plant, it must be recognised that employee noise exposure to any item of equipment is generally extremely low.
In many plants where equipment is operated from control rooms, it will be found that the daily noise dosages of all employees are within the OSHA requirements. Where noise abatement is required, the following sections outline approaches which may be taken for common items of equipment.
With the current emphasis on noise control, most compressor manufacturers have developed new models with sound levels below 90 dBA. This has been achieved by designing acoustical enclosures integral with the unit. For noisy compressors which are already installed, the only approach to silencing is the installation of an acoustical enclosure or curtains. Ventilation should be provided to prevent heat build-up.
Fluid turbulence associated with PRV and control valves causes sound levels up to 105 dBA to be radiated from the valve body and downstream piping. Manufacturers have developed quiet valves which, when properly sized, can result in acceptable noise levels.
For control of noise from valves already installed, the easiest approach is to lag the valve and downstream piping with 1″ to 4″ of glass fibre with a 1.0 psf exterior wrap of sheet metal or lead-vinyl.
Outdoor Mobile Equipment:
Three items of construction equipment are commonly used in wood yards such as-
(i) Cranes;
(ii) Bulldozers; and
(iii) Front loaders.
The important noise sources of these types of equipment are, in order of importance:
1. Engine exhaust.
2. Engine casing.
3. Cooling fan.
4. Engine intake.
5. Hydraulics (loaders and dozers)
6. Power transmission system, gears (cranes).
The noise levels of cranes, dozers and loaders may range as high as 110 dBA, with a strong trend that older equipment is noisier. Recent emphasis on noise control by manufacturers in response to OSHA, GSA and EPA requirements has resulted in a new generation of equipment with sound levels below 90 dBA.
When new equipment is purchased, product specifications should always include requirements for sound levels at the operator’s position. It is not sufficient to specify that equipment ‘comply with OSHA’. Complete specifications should be provided to equipment suppliers.
The usage time of equipment should be carefully established to determine noise design goals. It is found that most equipment is used intermittently; thus, noise reduction to 90 dBA is not required.
The following guidelines should be considered for noise control:
1. All cabs should be lined on the interior with a sound absorptive material. Suitable materials are 2″ glass fibre faced with 20 per cent open perforated sheet metal and non-combustible open cell polyurethane foam with a protective facing.
2. When the equipment has an enclosed cab and excessive noise levels are due to open windows, the use of air conditioning should be considered.
3. The installation of an operator cab may be considered if available. Cabs for existing equipment are often offered by the equipment manufacturer.
4. Effective exhaust silencers should be installed on loaders and dozers. Retrofit mufflers are generally available from the equipment manufacturer.
5. Where crane engine compartments open into the cab, they should be isolated with a solid wall (24 GA minimum damped sheet metal) or a 1.0 psf lead loaded vinyl curtain. The curtain should extend from the roof to the floor and be sealed on all sides with Velcro to minimise sound leakage. The engine compartment may also be treated with a sound absorptive material.
6. All mechanical equipment should be properly maintained. Missing assembly bolts in engine housings should be replaced. Grease and oil applied regularly will eliminate high frequency noise.
7. Sheet metal vibrations should be dampened.
8. Engines should be turned off during long idle periods.
Maintenance, Clean-Up, Inspection and Mobile Personnel:
Employees whose work requirements involve mobility may be exposed to the noise of several hundred items of equipment in any work day. It is difficult to assess actual noise exposure for employees whose work locations vary.
An estimate of the average daily noise dose may be obtained by obtaining a spatial average sound level for each work area and estimating the percentage of an average work day an employee will spend in each area. The spatial average sound level may be obtained by measuring sound levels throughout a work area at 10 foot intervals and averaging these values.
For employees with mobile work patterns, the sources of noise exposure actually include every item in a mill generating noise.
It is generally not feasible to reduce the noise of all of these sources immediately for the following reasons:
1. State-of-the-art technology does not exist for the reduction of all sources in a mill to below 90 dBA.
2. Reduction of the noise from only a few sources would reduce the total noise exposure insignificantly. For example – the elimination of ½ of the noise sources (assuming equal sound intensity) would reduce the overall sound level by only 3 dBA.
3. Future technology should be relied upon, since all equipment will eventually wear out and may be replaced with quieter equipment. New noise abatement technology will allow much greater noise reduction per machine than 3 dBA.
4. Replacement of older equipment (where only mobile employees are exposed) for the sole purpose of existing noise control technology would be unrealistic. Since existing technology is continuously being improved, this approach would not only require the immediate replacement of all mill equipment, but would require additional future replacement with each advance in the state-of- the-art of noise control.
Best Approach to Reduction of Noise Exposure:
Thus, the best approach to reduction of noise exposure for mobile employees is not to initiate noise control for every noise source in a mill, but rather to:
1. Establish initial priorities to abate noise from only equipment to which employees are exposed on a regular basis. The reduction of this equipment will also favourably affect noise exposure to maintenance and other mobile personnel.
2. When new equipment is purchased, specifications for noise control should be emphasised.
A second approach to noise exposure reduction is to streamline or minimise tasks requiring employees to work in noisy areas.
General Guidelines to Reduce Noise Exposure:
General guidelines to reduce noise exposure include:
1. Improve maintenance equipment and techniques.
2. Establish and encourage the use of work areas wherever possible.
3. Perform preventive maintenance, especially during shutdown, to reduce continuous maintenance.
4. Require breaks to be taken out of work areas.
1. Modernise or automate clean-up equipment.
2. Improve drainage systems.
3. Perform preventive maintenance to reduce leaks, etc.
4. Require breaks in quiet areas.
1. Utilise remote controls.
2. Establish minimum time inspection routes.
Sound screens are often discussed in conjunction with maintenance personnel. In a diffuse sound field, as exists in most mill areas, screens would be ineffective. Enclosures would be required. Where noisy equipment is located very near (generally less than 6′) a temporary work location, a barrier may be effective. Plywood or sheet metal barriers will generally provide a 5-10 dBA noise reduction.
Hearing protectors may be provided and used by an employee to limit noise exposures in lieu of feasible engineering controls if the employee’s exposure occurs on no more than one day per week.
This provision, if adopted, would allow hearing protection as a complete compliance measure for most maintenance personnel.
Supercalenders:
Supercalenders with speeds of 1000 fpm and above frequently generate sound levels in excess of 90 dBA at adjacent control panel operator locations. The chilled iron rolls of calenders and supercalenders should be dynamically balanced for operating speeds above 1500 fpm for minimum noise levels.
Additional noise reduction may be achieved by use of an acoustical enclosure for the control panel. Since only a relatively low noise reduction is generally required (supercalender noise levels seldom exceed 97-99 dBA), a three sided enclosure with a roof and considerable window area would generally provide sufficient noise reduction for most applications.
The use of barriers integral with the supercalender structure has been tried on an experimental basis; however, a significant noise reduction was not achieved.
Gears:
Geared systems may be extremely noisy. Gears consist of assemblies of toothed wheels used for the purpose of torque conversion, speed change or power distribution.
The main sources of noise in geared systems are:
1. Impact caused by tooth contacts.
2. Mechanical imbalance of the gear assembly.
3. Friction due to the contact motion of the tooth.
4. Variation of radial forces.
5. Air and oil pocketing.
Principles Used for Reducing Noise in Gear Systems:
Some of the principles used for reducing noise in gear systems are:
1. Selection of a suitable type of gear (for instance, a helical gear is quieter than a spur gear and a worm gear is still quieter, but is restricted to low speeds).
2. Accuracy of manufacturing (high accuracy in all gear parameters results in quieter gear systems).
3. Detuning (when the operational frequency of the gear assembly coincides with the natural frequency of the structural members, resonance takes place, amplifying the noise; to avoid resonance, the structural members are detuned to other frequencies by either stiffening or mass loading).
4. Damping (introduced by using gear material of high internal damping).
5. Vibration isolation.
6. Enclosing the gear assembly (with particular attention given to cooling and heat transfer requirements).
The use of rawhide, nylon and laminated phenolic gears is often proposed as method of noise reduction. Non-metallic gears should not be used for applications where the above relationship yields solutions of less than 1.0.
Roofing Plant Machinery:
Most items of machinery (loopers, festoons, coating rolls, etc.) are relatively quiet.
Generally, the only two noise problems in a roofing plant are:
1. Roll winders.
2. Cutters.
Two noise components were identified:
1. Steady-state noise generated by the gear drive and winding mechanism.
2. Impulse noise generated by various metal-to-metal impacts.
The steady state noises may range from below 90 dBA to 102 dBA and impact noises may reach 120 dB using the impulse scale.
The following approaches to noise reduction may be considered:
1. The drive mechanism may be enclosed.
2. Gears may be damped.
3. Lateral motion of the ways of the pull roll should be minimised.
4. The gear system should be maintained to eliminate all play between gears. The gear system should be set for proper backlash and should be adjusted or set such that it will not ‘bottom out’ when thin felt is run.
5. Metal-to-metal impact noise should be reduced by-
(a) Vibration damping of impacted parts.
(b) Substitution of plastic parts for metal components.
Sound levels of cutters may range up to 95 dBA. A complete vibration analysis of a cutter indicated that the primary source of noise was the felt itself.
With the identification of the felt as the noise radiating surface, only two design alternatives exist for noise reduction:
1. Modification of the cutting mechanism.
2. Enclosure of the vibrating felt.
The first alternative is not considered feasible from a production point of view. Investigation of enclosure designs was initiated utilising an experimental cardboard and glass fibre construction. Partial enclosures were first tried, but provided only a 1-3 dBA noise reduction.
A complete enclosure was found to provide satisfactory results, with a noise reduction of approximately 10 dBA being achieved. This approach appears to be the most feasible method of reducing cutter noise.
Air Noise:
The generation of air noise results from the creation of fluctuating pressures due to turbulence and shearing stresses as high velocity gas interacts with the ambient air or solid surfaces. Radiating sources called ‘eddies’ are formed with the high frequency noise being generated in the mixing shearing region and the lower frequency noise being generated downstream in the region of large scale turbulence.
Theoretically, as the pressure ratio between reservoir (line pressure) and ambient air is increased, the velocity of the air at the discharge nozzle increases. However, when a pressure ratio of approximately 1.9 is reached, the flow velocity through the nozzle becomes sonic, i.e., reaches the speed of sound and further increases in reservoir pressure do not significantly increase the flow velocity.
When this critical pressure ratio of 1.9 is reached, the nozzle is said to be ‘choked’. It may be assumed, however, that at 80-100 psi, the air jets are generally in a condition of choked flow.
The velocity of the gas stream has the greatest influence on jet noise. Cutting the velocity in half would lower the sound power by 24 decibels. However, halving the area would account for a decrease of only 3 decibels.
Air discharges are observed throughout the plant and are considered a primary source of excessive sound levels in most areas. Most air discharges result from pneumatic exhaust of control systems and these air exhausts are either continuous or cyclic in nature.
Sound levels generated by air exhaust, where concentrated air flow is not required, may be silenced by the installation of pneumatic mufflers which diffuse the air stream.
The specification of an optimum pneumatic silencer for any application should be based upon:
1. Sound level reduction.
2. Low pressure drop.
3. Durability.
4. Non-clogging features for the air contaminants present.
5. Economy.
Mufflers should be periodically inspected for wear and effectiveness. If necessary an in-line filter system may be installed to prevent muffler clogging.
The following information should be provided to pneumatic muffler suppliers to assist in proper muffler selection:
1. Pipe thread sizes (N.P.T.).
2. Estimated or measured air flow (cfm).
3. Presence of air line contaminants (oil, excess moisture, etc.).
It should be pointed out that most air discharges have threaded outlets which would easily accept silencers. To insure that the silencers are not removed, they may be secured by means of welding or a set screw. Signs reminding employees that noise control devices are for their benefit may also help.
Air leaks from pneumatic systems are often found to be major contributors to the overall noise in many plants.
It is important that a programme be implemented including as inspection and maintenance procedures, performed on a regular basis, to identify air leaks such as due to the following:
1. Broken air hoses and cracked pipes.
2. Worn fittings and couplings.
3. Faulty valves.
4. Air-operated devices left on when not in use.
It should also be noted that such a maintenance programme would be a significant cost and energy conservation measure, as well as an important part of a noise abatement programme.
Acoustical Curtains:
Flexible curtains of mass-loaded vinyl have excellent sound transmission loss properties and may be used to block airborne sound.
Typical applications include:
1. Complete or partial machine enclosures.
2. Moveable walls to isolate noisy machine areas from other quieter areas.
3. Localised enclosures for machine parts.
Acoustical curtains are constructed of 0.5 to 1.0 psf lead-filled vinyl and may be installed with grommets or on sliding tracks. It is important that the curtains extend to the floor and be sealed at the edges with Velcro fasteners for maximum sound attenuation.
Employee Enclosures and Barriers:
Enclosures, partial enclosures and barriers are often considered as measures to reduce employee noise exposure from adjacent operations. These measures may be quite effective in some instances, however, may be ineffective or actually increase an operator’s noise exposure in other circumstances.
Two general guidelines should govern enclosure and barrier application:
1. An employee enclosure is effective only when an employee’s job tasks allow him to spend a significant portion of his workday in an enclosure.
2. An acoustical barrier is effective only when the receiver is in the direct field rather than reverberant field of a noise source.
If a four-sided enclosure is applicable, windows, ventilation and communication equipment should be installed. This type of booth can be used where the operator does not rely on audible detection techniques for operation.
This kind of enclosure is ideal for control rooms and for rest area locations where there are often employees present who are not directly involved in the area operations and are present for environmental benefits.
A three-sided or lean-to type enclosure is applicable where only a small noise reduction is required. Such an enclosure provides safety as well as production advantages. In operations which require that the operator be able to hear the machine, this type of enclosure may reduce sound levels to an acceptable level while permitting effective audible monitoring of the operation. The enclosure may have several openings or exits for accessibility and minimisation of potential entrapment hazards and can be equipped with windows as required.
An acoustical barrier, where applicable in an industrial environment, would seldom provide more than a 3-7 dBA noise reduction. To achieve this reduction, the barrier must completely block the line-of-sight path between a noise source and the observer. The barrier material must be without holes or openings and should be of 0.5 pound per square foot minimum weight.
The distance from a noise source where the reverberant sound field is equal to the direct sound field may be computed from-
For an observer located at this distance from a noise source, a barrier may provide a 3 dBA maximum noise reduction. A barrier would be ineffective where an observer is located a greater distance from the noise source.
Acoustic Maintenance:
Machine inefficiencies, wear and malfunction can result in significant increases in noise levels. The goal of normal plant maintenance is to keep machinery in proper operating condition for efficient production. By simply expanding the existing maintenance programme through noise awareness, it is possible to minimise the noise environment within the plant.
If noise criteria are not assigned a level of priority, a machine with noise-producing maintenance problems, but which is still operating at 100 per cent efficiency, would not justify maintenance attention. Because of this fact, acoustical maintenance may be met by the same opposition as its counterpart, preventive maintenance; cost savings are often hidden rather than direct.
Acoustical maintenance (AM) must be viewed as a separate discipline and an added responsibility to normal maintenance procedures. Noise control through maintenance would require an educational programme for maintenance personnel to create ‘noise awareness’ and outline engineering basics similar to those employed for preventive maintenance.
This will provide maintenance personnel with the technical background necessary to cope with the unique engineering features associated with noise reduction. An effective programme would involve creating a ‘noise awareness’ among all personnel and designating individuals whose primary duties are those related to plant-wide noise reduction. These duties may also include general energy conservation.
A training programme for maintenance personnel may follow the following outline:
I. Introduction to Noise Control:
1. Decibels.
2. OSHA.
3. Sound level meters.
4. Materials.
II. Machine Design and Noise Control:
1. Air sources.
2. Mechanical sources.
1. Inspection procedures.
2. Guidelines for installation.
The importance of noise control related to maintenance cannot be over-emphasised.
Organising an Acoustical Maintenance Programme:
To organise an acoustical maintenance programme, a maintenance engineer should be trained in noise control.
Following this training, his duties would be as follows:
1. The engineer would perform periodic inspections to identify noise problems related to maintenance.
2. He would report noise problems to the maintenance department to schedule for repair.
3. The engineer would specify and order any acoustical materials not in stock.
4. He would consult with maintenance personnel if technical questions should arise regarding implementation.
5. He should inspect the repairs upon completion.
6. The engineer should maintain records regarding noise control inspections and repairs.
The first task of the engineer is to document sound levels of each item of machinery when in good maintenance condition. This data serves as a baseline to identify when excessive noise is present during future surveys.
Following the development of baseline data, noise survey inspections should be performed periodically (typically at one month intervals) and machine problems which cause measured sound levels to be 2-3 dBA higher than the baseline data should be identified and reported for repair. In addition, careful visual inspection should be made of each machine during each inspection.
Particular attention should be given to:
1. Alignments and adjustments.
2. Vibration and impact treatments.
3. Air systems.
4. Lubrication.
5. Machine dynamics.
6. Acoustical installations.
Fire Codes:
Manufacturers of acoustical materials and the Insurance services have adopted a system using the flame spread classification (FSC) to rate combustible and non-combustible materials.
The engineer should insure that acoustical installations meet fire code standards or plant safety may be jeopardised and insurance rates may be raised.
Generally, use of foam in a building, especially vertical applications, would change a non-combustible metal building rating (NC-2) to a combustible frame building rating (wood). A similar condition would occur where combustible spray-on acoustical treatment is utilised. The rating would change from a metal building to a frame building.
In some cases acoustical materials have not been tested due to the fact that their classification is governed by the system for which they are used. For this reason, it is difficult to rate some acoustical materials, such as trowel-on dampening, since they are a part of an integral system and the industrial machines for which they are used have not been considered for classification at all. There are acoustical systems, such as composite panels of perforated sheet metal, foam or glass bonded to sheet metal, which are commonly used and still have not been listed.
Long Term Noise Abatement:
In the specification and purchase of all new items of equipment, requirements for reduced product noise should be included as part of the specification.
The manufacturer shall submit sound measurements for supplied equipment in accordance with this specification. In addition, where the sound measurements exceed the values stated below, the manufacturer shall advise on silencing provisions and additional costs to meet this standard.
The manufacturer shall be responsible for the supplied equipment, including any subassemblies. The manufacturer shall test the equipment.
Where no acceptable sound test exists or where manufacturer’s standard is different from the above, the manufacturer shall submit the method of testing.
The manufacturer shall submit octave sound power data for equipment.
In addition, the following information is required:
If the source is highly directional, such as a cooling tower, measurements shall be submitted at several locations.
If manufacturer’s data submitted exceeds 85 dBA, the quotation shall include the additional cost and silencing provisions necessary to meet this value.
Manufacturer’s Responsibility:
1. It is the manufacturer’s responsibility to engage an independent consultant, as required, in order to meet the noise requirements stated in this specification.
2. The manufacturer shall not ship any equipment which exceeds the manufacturer’s values promulgated in this specification without the purchaser’s written authorisation.
3. If the manufacturer must perform tests at the purchaser’s facility, it shall be stated in the quotation.
Control of Noise Source by Design:
(1) Reduce Impact Forces:
Many machines and items of equipment are designed with parts that strike forcefully against other parts, producing noise. Often, this striking action or impact is essential to the machine’s function.
Several steps can be taken to reduce noise from impact forces. The particular remedy to be applied will be determined by the nature of the machine in question. Not all of the steps listed below are practical for every machine and for every impact-produced noise. But application of even one suggested measure can often reduce the noise appreciably.
Some of the more obvious design modifications are as follows:
1. Reduce the weight, size or height of fall of the impacting mass.
2. Cushion the impact by inserting a layer of shock-absorbing material between the impacting surfaces. In some situations, you could insert a layer of shock-absorbing material behind each of the impacting heads or objects to reduce the transmission of impact energy to other parts of the machine.
3. Whenever practical, one of the impact heads or surfaces should be made of non-metallic material to reduce resonance (ringing) of the heads.
4. Substitute the application of a small impact force over a long time period for a large force over a short period to achieve the same result.
5. Smooth out acceleration of moving parts by applying accelerating forces gradually. Avoid high, jerky acceleration or jerky motion.
6. Minimise overshoot, backlash, and loose play in cams, followers, gears, link-ages and other parts. This can be achieved by reducing the operational speed of the machine, better adjustment, or by using spring-loaded restraints or guides. Machines that are well made, with parts machined to close tolerances, generally produce a minimum of such impact noise.
(2) Reduce Speeds and Pressures:
Reducing the speed of rotating and moving parts in machines and mechanical systems results in smoother operation and lower noise output. Likewise, reducing pressure and flow velocities in air, gas and liquid circulation systems lessens turbulence, resulting in decreased noise radiation.
Some specific suggestions that may be incorporated in design are the following:
1. Fans, impellers, rotors, turbines and blowers should be operated at the lowest bladetip speeds that will still meet job needs. Use large-diameter, low-speed fans rather than small-diameter, high-speed units for quiet operation. In short, maximise diameter and minimise tip speed.
2. All other factors being equal, centrifugal squirrel-cage type fans are less noisy than vane axial or propeller type fans.
3. In air ventilation systems, a 50 per cent reduction in the speed of the air flow may lower the noise output by 10 to 20 dB or roughly one-quarter to one-half of the original loudness. Air speeds less than 3 m/s measured at a supply or return grille produce a level of noise that usually is unnoticeable in residential or office areas.
In a given system, reduction of air speed can be achieved by operating at lower motor or blower speeds, installing a greater number of ventilating grilles or increasing the cross-sectional area of the existing grilles.
(3) Reduce Frictional Resistance:
Reducing friction between rotating, sliding or moving parts in mechanical systems frequently results in smoother operation and lower noise output. Similarly, reducing flow resistance in fluid distribution systems results in less noise radiation.
Some of the more important factors that should be checked to reduce frictional resistance in moving parts are the following:
1. Alignment:
Proper alignment of all rotating, moving, or contacting parts results in less noise output. Good axial and directional alignment in pulley systems, gear trains, shaft coupling, power transmission systems, and bearing and axle alignment are fundamental requirements for low noise output.
2. Polish:
Highly polished and smooth surfaces between sliding, meshing, or contacting parts are required for quiet operation, particularly where bearings, gears, cams, rails and guides are concerned.
3. Balance:
Static and dynamic balancing of rotating parts reduces frictional resistance and vibration, resulting in lower noise output.
4. Eccentricity (Out-of-Roundness):
Off-centering of rotating parts such as pulleys, gears, rotors and shaft/bearing alignment causes vibration and noise. Likewise, out-of-roundness of wheels, rollers and gears causes uneven wear, resulting in flat spots that generate vibration and noise.
The key to effective noise control in fluid systems is streamline flow. This holds true regardless of whether one is concerned with air flow in ducts or vacuum cleaners, or with water flow in plumbing systems. Streamline flow is simply smooth, non-turbulent, low-friction flow.
The two most important factors that determine whether flow will be streamline or turbulent are the speed of the fluid and the cross-sectional area of the flow path, that is, the pipe or duct diameter. The rule of thumb for quiet operation is to use a low-speed, large-diameter system to meet a specified flow capacity requirement. However, even such a system can inadvertently generate noise if certain aerodynamic design features are overlooked or ignored.
A system designed for quiet operation will employ the following features:
1. Low Fluid Speed:
Low fluid speeds avoid turbulence, which is one of the main causes of noise.
2. Smooth Boundary Surfaces:
Duct or pipe systems with smooth interior walls, edges and joints generate less turbulence and noise than systems with rough or jagged walls or joints.
3. Simple Layout:
A well-designed duct or pipe system with a minimum of branches, turns, fittings and connectors is substantially less noisy than a complicated layout.
4. Long-Radius Turns:
Changes in flow direction should be made gradually and smoothly. It has been suggested that turns should be made with a curve radius equal to about five times the pipe diameter or major cross-sectional dimension of the duct.
5. Flared Sections:
Flaring of intake and exhaust openings, particularly in a duct system, tends to reduce flow speeds at these locations, often with substantial reductions in noise output.
6. Streamline Transition in Flow Path:
Changes in flow path dimensions or cross-sectional areas should be made gradually and smoothly with tapered or flared transition sections to avoid turbulence. A good rule of thumb is to keep the cross-sectional area of the flow path as large and as uniform as possible throughout the system.
7. Remove Unnecessary Obstacles:
The greater the number of obstacles in the flow path, the more tortuous, turbulent, and hence noisier, the flow. All other required and functional devices in the path, such as structural supports, deflectors, and control dampers, should be made as small and as streamlined as possible to smooth out the flow patterns.
Generally speaking, the larger the vibrating part or surface, the greater the noise output. The rule of thumb for quiet machine design is to minimise the effective radiating surface areas of the parts without impairing their operation or structural strength.
This can be done by making parts smaller, removing excess material or by cutting openings, slots or perforations in the parts. For example – replacing a large, vibrating sheet-metal safety guard on a machine with a guard made of wire mesh or metal webbing might result in a substantial reduction in noise because of the drastic reduction in surface area of the part.
In many cases, machine cabinets can be made into rather effective soundproof enclosures through simple design changes and the application of some sound-absorbing treatment.
Substantial reductions in noise output may be achieved by adopting some of the following recommendations:
1. All unnecessary holes or cracks, particularly at joints, should be caulked.
2. All electrical or plumbing penetrations of the housing or cabinet should be sealed with rubber gaskets or a suitable non-setting caulk.
3. If practical, all other functional or required openings or ports that radiate noise should be covered with lids or shields edged with soft rubber gaskets to affect an airtight seal.
4. Other openings required for exhaust, cooling, or ventilation purposes should be equipped with mufflers or acoustically lined ducts.
5. Openings should be directed away from the operator and other people.
(6) Isolate and Damper Vibrating Elements:
In all but the simplest machines, the vibrational energy from a specific moving part is transmitted through the machine structure, forcing other component parts and surfaces to vibrate and radiate sound—often with greater intensity than that generated by the originating source itself.
Generally, vibration problems can be considered in two parts. First, we must prevent energy transmission between the source and surfaces that radiate the energy. Second, we must dissipate or attenuate the energy somewhere in the structure. The first part of the problem is solved by isolation. The second part is solved by damping.
The most effective method of vibration isolation involves the resilient mounting of the vibrating component on the most massive and structurally rigid part of the machine. All attachments or connections to the vibrating part, in the form of pipes, conduits, and shaft couplers, must be made with flexible or resilient connectors or couplers.
For example – pipe connections to a pump that is resiliently mounted on the structural frame of a machine should be made of resilient tubing and be mounted as close to the pump as possible. Resilient pipe supports or hangers may also be required to avoid bypassing the isolated system.
Damping material or structures are those that have some viscous properties. They tend to bend or distort slightly, thus consuming part of the noise energy in molecular motion. The use of spring mounts on motors and laminated galvanised steel and plastic in air-conditioning ducts are two examples.
When the vibrating noise source is not amenable to isolation, as, for example, in ventilation ducts, cabinet panels, and covers, then damping materials can be used to reduce the noise.
The type of material best suited for a particular vibration problem depends on a number of factors such as size, mass, vibrational frequency, and operational function of the vibrating structure.
Generally speaking, the following guidelines should be observed in the selection and use of such materials to maximise vibration damping efficiency:
1. Damping materials should be applied to those sections of a vibrating surface where the most flexing, bending, or motion occurs. These usually are the thinnest sections.
2. For a single layer of damping material, the stiffness and mass of the material should be comparable to that of the vibrating surface to which it is applied. This means that single-layer damping materials should be about two or three times as thick as the vibrating surface to which they are applied.
3. Sandwich materials (laminates) made up of metal sheets bonded to mastic (sheet metal viscoelastic composites) are much more effective vibration dampers than single-layer materials; the thickness of the sheet-metal constraining layer and the viscoelastic layer should each be about one-third the thickness of the vibrating surface to which they are applied. Ducts and panels can be purchased already fabricated as laminates.
Since a vibrating body or surface radiates noise, the application of any material that reduces or restrains the vibrational motion of that body will decrease its noise output.
Three basic types of redress vibration damping materials are available:
1. Liquid mastics, which are applied with a spray gun and harden into relatively solid materials, the most common being automobile ‘undercoating’.
2. Pads of rubber, felt, plastic foam, leaded vinyls, adhesive tapes or fibrous blankets, which are glued to the vibrating surface.
3. Sheet metal viscoelastic laminates or composites, which are bonded to the vibrating surface.
Admittedly, machine design is a complex business and fundamental changes in design cannot be made at the dictate of a particular customer, however desirable from the acoustical point of view. What is urgently required, however, is for acoustic factors to be taken into consideration from now onwards whenever a new machine or piece of equipment is being designed. The noise aspect of design should be given just as much attention at the design stage as the machine’s efficiency, ease of maintenance and all the other factors which are rightly given pride of place in the designer’s plans.
Many other mechanical parts which come into contact with each other or with the main structure of the machine should also be studied afresh. These include such components as push rods, cams and driving quadrants, which modern techniques often permit of being bushed with rubber or other compliant material where they come into contact. Many such components can also now be made of nylon.
Once manufacturers of machines and equipment include quiet-running as one of the essential qualities to be built into their products, the acoustic consultant will be able to recommend the exact machine required from the performance point of view, in the knowledge that its acoustic characteristics will be no less satisfactory. As a corollary of this, the manufacturer should be able to furnish data concerning the acoustic performance of the equipment he supplies.