Here is a list of enclosures and barriers used for reducing noise in industries.
1. Enclosures:
Although strides of progress have been made in source noise reduction, some types of industrial and manufacturing equipment such as hammer mills, power presses, plastic grinders, diesel engines, etc., are still quite noisy. In addition there are many equipment installations where the cost to develop noise reduction measures for old vintage equipment is prohibitive.
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In these situations, the acoustical engineer must consider enclosing the equipment in either a partial or total enclosure. The thought of an enclosure is initially repulsive to the plant engineers, the maintenance staff and the operator because of the anticipated nuisance associated with the loss of visibility, accessibility and added maintenance. These penalties are not inherent and a well-designed enclosure can provide bonus features such as mist and smoke control.
It must be emphasised that noise reduction at the source always enjoys top priority, but with a systematic approach and careful attention to design detail, enclosures can be one of the most powerful noise reduction measures available to the acoustical engineer.
In industry today, a practical and efficient method of reducing noise in a system is to enclose it, thus cutting off some of the sound waves. Enclosures can be constructed from most common building materials. However, the degree of noise reduction is dependent upon the surface weight, as well as the internal damping and stiffness of the material.
All too often the engineer designing an enclosure begins by considering what materials or combinations or materials will be selected for the enclosure. To be sure, this decision must be made, but several steps should be taken first which will simplify the selection process and assure a balanced design.
These recommended steps are outlined below:
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Step 1:
As in any good design discipline, the initial step should be to establish design criteria (design goals) and to determine the corresponding acoustical performance required of the enclosure to meet the criteria. The design goals may be hearing damage risk criteria and/or annoyance criteria in noise-sensitive areas.
Regardless, most criteria can be expressed in terms of octave band sound levels and for most cases, these will suffice. Therefore, the first step is to establish a realistic octave band criterion at some location or perhaps several locations from the noise source.
For industrial equipment, machines, motors, etc., the operator station and at ear level 3 ft from major machine surfaces are preferred locations. These locations are also consistent with the many recommended measurement standards or procedures such as developed by industrial associations, such as ASHRAE, NMTBA and GAGI.
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Step 2:
The second step is to determine or predict the octave band noise levels of the equipment at the locations selected in step 1. The octave band noise level data can be measured either as installed or from data as supplied by the equipment manufacturer. However, actual measurements at the locations selected in step 1 are obviously preferable. With the criteria established and measurements obtained, the required acoustical performance of the enclosure can be calculated.
To illustrate this method, consider the following example-
Example:
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It is desirable to enclosure three plastic grinders in separate enclosures. The resultant noise levels must not exceed 90 dBA at 3 ft from the complex. What is the required acoustical performance (noise reduction) of the enclosures? The measured noise levels at 3 ft for one grinder are shown in line 2 of Table 28.1.
Solution:
In this example, the criterion or design goal is 90 dBA; hence an octave band equivalent of 90 dBA is selected, as shown in line 1 of the computation table. It must be emphasised that this criterion is not unique but provides uniform levels for conservative design purposes. Hence, if all resultant levels are below these criteria levels, the combined overall level will also be below 90 dBA.
Next the measured or manufacturer-supplied noise levels at the desired location are listed, as shown in line 2. Since there are three grinders, one must account for the sound accumulation. Using the chart in Fig. 28.6, we can see that three equal sources increase the level by 5 dB, which is included in line 3. The net noise level for the three grinders at 3 ft would then be the sum of line 2 and line 3, which is shown in line 4.
The required spectral noise reduction of each enclosure is shown in line 5 and is just the difference between the criteria given in line 1 and the net noise levels of line 4. With the required acoustical performance (noise reduction) determined (line 5), the next step is to select the type of enclosure and materials which will meet these design goals.
There are two basic types of enclosures- total and partial. With a well-designed total enclosure, there is practically no lower limit to the resultant noise levels. Partial enclosures, on the other hand, have very serious acoustical performance limitations and must be dealt with accordingly.
The measured noise reduction does not necessarily provide direct quantitative information regarding the acoustical performance of the enclosure. To see this, note that if the outside measurement location is some distance away from the enclosure, there will be considerable attenuation due to divergence. As such the difference in levels will include factors which are not related directly to the acoustical performance of the enclosure.
In large total enclosures, the most important design consideration is the overall transmission loss of the walls. The transmission loss, depends to the first order on surface density and hence there is a wide assortment available, depending on whether thick masonry, modular panels or relatively light acoustical curtains are selected. For example, in large jet engine test cells, heavy concrete 12 to 18 inch thick is common, with TL values exceeding 70 dB.
We shall not spend much time in this area other than to emphasise that when heavy-duty noise control enclosures are required, such as test facilities for aircraft engines, large gas turbines, diesel engines, etc., masonry wall construction should be the first consideration. With masonry, only performance penalties associated with airflow and penetrations such as windows, doors, etc., need further detail design effort.
Often, large total machine enclosures are constructed from commercially available modular panels. Shown in Fig. 28.2 is an illustration of the basic construction elements in these panels. Typically, these panels are available in combinations of lengths of 4, 5, 6, 8 and 10 ft and widths of 2, 3 and 4 ft. With respect to thickness, 2 and 4 inch are most common, with higher transmission loss for the thicker panels as illustrated in Fig. 28.3.
The outside sheet metal skin is typically 16-gauge galvanised steel and the inside surfaces is typically 22-gauge perforated steel, 10 per cent to 30 per cent open area. The interior sound-absorbing fill is usually fibrous glass or mineral wool (4 to 6 lb/ft3), often protected from contaminants by a thin 1 or 2-mil polyethylene film (bag).
For added versatility, these panels are available with windows installed, usually double-glazed (two panes) and supported in rubber isolator gaskets. Panels with high-performance acoustical doors and seals are also available to the engineer and designer, to complete the list of features.
Because of the design flexibility of these panels, their applications are limitless. In summary, total and partial enclosures constructed of these modular panels provide a high level of acoustical isolation along with ease of assembly and flexibility.
The factor limiting the acoustical performance of enclosures is often the number and size of acoustical leaks. Shown in Fig. 28.4 is a chart for calculating the effect of an acoustical leak. Note that an enclosure with a transmission loss potential of 45 dB is reduced to 20 dB if an opening of 1.0 per cent is present.
Further, the performance reduces to a mere 10 dB if the opening is 10 per cent. From this example, it is easy to see that great care must be given to design and construction detail in order to minimise acoustical leaks. This is not always easy, for air, stock, materials, products, scrap, etc., usually must be moved in and out of the enclosure at high speeds.
Finally, for machine enclosures, quick operator accessibility and continuous visibility are almost always required. It is meeting these requirements while preserving the acoustical performance that challenges the design engineer. Examples of large industrial equipment that lends itself to total enclosures include cold headers, swagers, bar machines, transfer presses, tumbling equipment, sandblasting nozzles, large fans, compressors, etc.
For those areas such as boiler rooms, generator rooms, pump rooms, in-plant offices, etc., where machine enclosures are not feasible, a personnel room enclosure for the watch engineers or operators will significantly reduce daily noise exposure. These personnel rooms are often installed at control panels or located centrally such that critical gauges or meters can be observed and recorded. In this way, only short periodic excursions into the noisy areas are required for monitoring remote gauges, meters, etc.
In summary, the disadvantages of total enclosures are obvious; costly additional floor space is generally required and some production penalties associated with machine operation and maintenance must be anticipated. In short, there are usually some cumbersome access and visibility problems to be solved.
An often-overlooked positive feature of the total enclosure is the ease with which air contaminants can be collected. Here noxious or toxic fumes, dust, oil mist, etc., common in industry, are locally contained in the enclosures and exhausted or vented to a central collection system. The enclosure approach is usually much more effective than collection by hoods or with ceiling fans after dispersion throughout the production areas.
When modular panel enclosures are not practical, a custom panel enclosure design approach can be undertaken. Here the enclosures panels are often attached to machine surfaces and may only enclose the major sources of noise.
Examples of successful application of custom enclosures include small punch presses (eyelet machines), cold headers, cutoff saws, pumps, plastic moulding equipment, grinders, hydraulic units, compressors, etc. It should be emphasised that because of the design detail involved in adapting these enclosures to complex surfaces and equipment configuration, the engineer must anticipate a rather extensive design and development programme.
In short, for these enclosures to meet both the acoustical and functional design goals, the following steps, as a minimum, are strongly recommended:
Phase I – Preliminary Design:
1. Complete preliminary design drawings.
2. Review the preliminary design with cognizant production engineering, maintenance and operating personnel.
3. Finalise the design.
Phase II – Prototype Installation and Evaluation:
1. Fabricate and install the prototype enclosure.
2. Evaluate the prototype enclosure in terms of acoustical performance and production impact.
3. Review the evaluation and finalise the design.
With respect to the design and construction of these custom enclosures, only guidelines can be given. The steps to establishing design parameters, goals or criteria naturally apply here and will not be repeated.
With respect to panel construction, the most popular configuration is a combination of sheet metal and a commercially available composite of absorbing and barrier materials. Shown in Fig. 28.5 is an illustration of this laminate composite bonded to 16-gauge sheet metal, along with typical acoustical performance.
Now, clearly, the performance can be improved by increasing either the surface density of the outer sheet metal layer or the inner septum or both. The inner 1″ layer of foam provides absorption to reduce reverberant buildup and the 1/4″ layer decouples the barrier layers.
To avoid wicking of water, oil or industrial solvents, etc., the exposed surface of the foam is usually treated to close the pores or is covered with a thin plastic or vinyl film. In those cases where heavy wear can be expected, the foam can be protected with an additional layer of perforated or expanded sheet metal, 20 per cent open area (minimum).
Additional barrier performance can be obtained by using the constrained-layer sheet metal laminates or applying a thin layer of damping material to the sheet metal layer. With more extensive design and development, the outer layer of these enclosure panels can be fabricated from moulded fibrous glass, plastic, etc., which usually adds some esthetic value.
Because of the individual aspects of custom enclosures, little more can be said regarding design or material selection. As in the case of modular panel enclosures, the overall performance of the enclosure will probably be determined by the unavoidable acoustical leaks associated with conduit or stock penetrations, windows, doors, access areas, etc. Let us now consider each of these areas and those design guidelines which minimise acoustical leaks.
Rare is the enclosure installation where numerous penetrations into the enclosure are not required. These penetrations include, for example, electrical conduits, plumbing, stock feed and discharge openings for exchanging large volumes of air. Conduit or plumbing penetrations are rather easy to seal by working sheet lead (1/64 or 1/32″) or dense vinyl around the pipe or conduit. Where the penetration openings are well oversized, the treatment illustrated should be applied to both the inner and outer sides of the panel.
With respect to moving air in and out of enclosures, there are two basic approaches. The first and often the easiest, is to utilise either commercially available parallel baffle or circular flow-through silencers as a sound trap in the inlet and/or exit ducts. These types of silencers are mentioned specifically since they have very low pressure losses, which is often a major design requirement.
Illustrated in Fig. 28.6 is a typical silencer installation for the intake of a fan enclosure. At a glance, one might think that the silencer is just a large hole, but as we shall see, the noise reduction of the silencer is equivalent to the enclosure walls. This approach is especially applicable in enclosing combustion engines, forced draft fans, compressors, ventilation fans, rotary positive displacement blowers, burners, etc.
The second basic approach, which is especially applicable to small enclosures, is to include a series of lined bends or labyrinth sound traps. In most installations, this approach will compromise the acoustical performance of the enclosure to some degree. However, if the lined bends are carefully constructed along the guidelines net overall attenuation of 15 to 20 dB can be readily achieved for dominantly high-frequency noise sources.
Illustrated in Fig. 28.8 is a total enclosure designed for a plastic scrap granuliser which includes a labyrinth sound trap for ventilation. This enclosure reduced noise levels by more than 17 dBA.
In summary, the use of commercially available silencers or carefully designed labyrinth sound traps will allow large volumes of air to be brought into an enclosure with little or no loss of acoustical isolation performance.
Access panels and doors are often the culprits that sharply reduce the noise reduction capability of well-designed enclosures. A few design guidelines for access doors will now be presented. A simple and very effective access panel door design is illustrated in Fig. 28.9. By far the most common error is to select a lightweight door material such as aluminium. As such the noise passes easily through the lightweight door, reducing the noise reduction capability of the enclosure.
In addition, the door must obviously fit well and if possible, be airtight. To assure an airtight fit, include labyrinth rubber seals and a positive pressure-type latch to compress the seals and resist vibration. Other equally good or better access door designs exist, but this configuration emphasises the basic construction guidelines.
Where high acoustical performance doors are required for large room or test facility enclosures, commercially available units are strongly recommended. These doors are available as complete assemblies that is, with jambs, sills, seals and so forth.
Windows always present a problem in enclosure design.
However, the major problems can be simplified if two basic design features are followed:
1. The thicker the pane (glazing), the higher the transmission loss. Panes typically selected are safety glass or transparent plastic ¼” to 1″ thick. Two panes (double glazing) spaced 2″ or more apart should be considered for extremely high noise levels such as gas turbine or jet engine control room windows.
In addition, they should be installed nonparallel, i.e., canted slightly relative to each other. Further, in two-pane installations, a desiccant of some kind or heating elements must be placed between the panes to absorb moisture and prevent condensation or fogging.
2. Panes should be gasketed or mounted in rubber or dense polyurethane foam. Here again, where high acoustical performance is required, commercially available window units are recommended.
With respect to isolation performance, single-pane windows generally follow the mass law up to the coincidence region, where pronounced performance losses or dips of 10 dB or more can be anticipated. With commercially available window assemblies, performance equivalent to masonry block can be attained.
2. Use of Lead as a Noise Barrier:
A wide selection of products and materials are available for noise reduction, one of which is lead. Lead is used to combat noise pollution in a variety of applications. The metal is applied alone and in combination with various other materials to meet OSHA regulations in numerous industrial, businesses, educational and residential complexes.
Table 28.2 lists types of lead-containing materials that are used for industrial noise control. Lead can be specified by sheet weight (1 square foot of 1/64″ sheet lead weighs 1 pound). The type of lead alloy does not have to be specified because the alloy content does not significantly affect the weight or the efficiency of lead as a noise barrier.
Acoustical properties of lead for controlling noise are very good. Qualities such as limpness, mass and internal damping is essential for decreasing and controlling sound and lead possesses all three. Lead can be incorporated into other noise-reduction materials and due to its mass, it can easily be pounded into thin sheets for application-an important design characteristic when maximum space is to be utilised. Mass and limpness should be considered when installing lead or lead-loaded fabric enclosures because these two qualities are most important in sound reduction.
Basic considerations and procedures that should be followed for proper installation of enclosures are listed here:
1. When installed, sheet lead should not be fastened rigidly to stiffer surface skins because of their tendency to degenerate the limp qualities of the lead.
2. When lead is laminated to another material, its weight should be kept in a composite skin nearly equal to or in excess of, the weight of the other material.
3. Viscoelastic adhesives or intermittent fasteners should be used when laminating lead panels, rather than continuous fastenings.
4. Leaded panel skins should be used for double-wall construction instead of one skin even when the total weight of both is the same.
5. All seams, doors and perimeter joints should be caulked or fitted with gaskets to eliminate entrance of any noise.
6. Whenever sound can circumvent a lead shield, all passages should be thoroughly covered. These passages include back-to-back panels, windows, cabinets, electrical outlets, ventilation ducts, the space above a suspended ceiling and any cracks in walls, however small. All of these passages should be completely blocked with leaded materials.
Sheet lead used for industrial noise control is specified in pounds per square foot. Manufacturers of such sheets make them available in weight from ¼ pound per square foot at 1/128″ thickness to 8 pounds at thickness. Sheet lead is easily altered to any dimensional need; sheets can be cut with scissors, moulded and contoured by hand and applied to most surfaces with adhesives, as well as laminated to substrates such as aluminium and steel.
Another material used for industrial sound attenuation is sheet lead and polyurethane sandwich material, which can be applied in such places as the inside of machine guards or shrouds. The lead-foam material is available from the manufacturer with ½-pound or 1-pound per square foot sheet lead laminated between various thicknesses of polyurethane foam.
Thickness of the foam can be obtained anywhere from ¼ to 2″ in increments of ¼”. Lead-foam material can be obtained with either two or three layers of sheet lead within the laminate. This material may be cut with ordinary scissors or steel rule dies when repetitive parts are required. The material can be anchored down with adhesives and can be shaped and moulded.
Another sound control material used widely by industry is leaded vinyl sheet, an example of which is two sheets of lead-loaded vinyl laminated to a core of glass fibre cloth to give added strength and durability. This material is specified by pounds per square foot and is available in 0.5, 0.75, 1.0, 1.5 and 3.0-pound weights.
It is commonly applied in the same way as sheet lead and can be used with acoustical wool. Lead-loaded epoxy is another type of sound-reducing material. Powdered lead mixed with epoxies makes an excellent damping compound to control structure-borne noise. This substance can be applied by trowelling it to surfaces to reduce noise and vibration.
3. Plenum Barriers:
The use of suspended ceilings is very popular in modern office construction. However, this type of ceiling installation allows a space or plenum between it and the previous ceiling from which it is hung. An acoustical problem may result, usually because the material employed in a suspended ceiling is lightweight and does not provide an adequate sound barrier.
A better system is to use plywood-lead sheets, with 2-lb lead, around a frame. These plywood-lead sheets can be covered with different types of wood panels. Office doors can also be made of the same materials. An office with lead-plywood ceiling panels and doors is more efficient in achieving sound attenuation; however, any sound made within the office will go through the ceiling and into the open space above.
The noise will then find its way through the ceilings of other offices. The flow of sound through the plenum can be stopped by hanging plenum barriers. Installing such systems does not require excessive materials and is more feasible than laying sheet lead over ceiling tiles.
Plenum barriers are also used for sound attenuation in piping and air duct systems. In such systems, holes are cut into the barrier, thus leaving an air leak around the pipe or duct that should be caulked or taped. In addition, the pipes and ducts may be lined with lead to lessen the radiated noise.
To develop such a noise attenuation system the pipes should first be insulated with paper-backed fibre-glass material or mineral wood insulation applied tight enough to support lead. Next, lead sheets are wrapped around pipe in sections with each piece overlapping the preceding one by 1½”.
The lead sheets are sealed with a standing seam with the overlap taped. For air ducts, a resin-bonded fibre-glass board is commonly employed. This material is normally available in 2 by 4-feet boards. The lead is then applied to the ducts in the same manner as for the pipes. Sheet lead plenum barriers are widely used for sound attenuation in office systems.
How to Install Plenum Barriers:
No special tools or skills are needed for installation of plenum barriers. The material is very malleable, 1/64″ thick and weighs 1 pound per square foot. Lead will stick with adhesives and can also be taped or cemented into place.
Three methods of installing sheet lead plenum barriers are discussed here:
i. For method number one, first cut the sheet lead 4″ longer than the total height of the plenum. Then make notches in the top corners about 1½ × 2″ in size. Then trim the 2″ edge over a batten length of metal that is equivalent in length to the width of the sheet minus 3″. Lift the sheet with batten and nail on 6″ centres to the blocking or the soffit of the slab. Finally, hang the bottom end of the lead sheet approximately 2″ out over the ceiling construction. This method is illustrated in Fig. 28.12.
ii. In method number two, first cut the sheet lead 7″ longer than the total height of the plenum. Make notches at the top two corners 1 ½ × 5″ in area and turn the 5″ edge 1½ times around a wooden batten strip that is 3″ shorter than the width of the sheet. Now lift the sheet with batten to the slab and fasten it on 12″ centres with screws. Finally, hand the bottom 2″ of the sheet over the ceiling construction. This method is illustrated in Fig. 28.13.
iii. In method number three, cut the lead sheet 5″ longer than the height of the plenum. Then make 1½ × 3″ notches in each top corner and turn the 3″ edge over a 1½” black iron channel that is 3″ shorter than the width of the sheet. Lift the sheet with channel to the slab and fasten down on 24″ centres with screws. Then hand the bottom 2″ of the sheet over the ceiling construction. This method is illustrated in Fig. 28.14.
When the plenum barrier is installed, seams can be formed by bending a 1½” tab located at each vertical edge and sealing it with 3″ tape. This procedure is illustrated in Fig. 28.15. For the barrier to give maximum performance in sound attenuation, it must be completely airtight, which can be achieved by gasketing and caulking at all joints.
Installation of Barriers around Pipes and Wires:
When installing a plenum barrier, pipes and wires may often interfere. To circumvent such obstacles without sacrificing efficiency of the barrier, the following procedure may be employed. First, cut the lead sheet from the bottom to the point at which the pipe or wire will enter. Next, make slits in the sheet to provide for the pipe circumference.
Then, peel back the segments made by the slits and slide the wire or pipe through the hole. Push the segments back against the wire or pipe so that they are snug against it. Finally, tape around the segments and the slit to the bottom of the sheet. This method is shown in Fig. 28.16 through Fig. 28.19.
High Transmission Loss Ceilings:
High transmission loss ceilings can attenuate sound which is emanating from upper floors. Such a ceiling is illustrated in Fig. 28.20. Two-pound sheet lead is normally employed that can be fastened to board using staples or adhesives before installation. The sheets should be lapped at least 2 inches and carried up the perimeter walls at least 4 inches. The perimeter should be thoroughly caulked to obtain an airtight seal. Also, at each point of suspension isolation hangers should be placed. Last, the ceiling slab above should be covered with a fibrous absorptive material.
When a total enclosure is not feasible or practical, a partial enclosure should be considered.
Partial enclosures can be divided into two basic types:
1. Enclosures which totally enclose major noise sources but not the whole machine.
2. Enclosures that only partially enclose a machine or noise source.
With respect to the first type of a die area it enclosures for a punch press (automatic mode of operation). In this particular installation, the die area was acoustically isolated by enclosing the frontal and rear entry portals and the stock in-feed and scraps exit penetrations.
Specifically, here, the stock in-feed and front portal fixtures. Note the high level of visibility and numerous access doors. It should also be noted that panel and access door construction details follow the basic guidelines for custom designs.
With respect to the second type, is a partial barrier enclosure which isolates the operator from adjacent noisy machines?
A partial enclosure is quite effective and has wide application in food packaging areas. In a typical packaging line, empty bottles, jars or cans are taken from a case and placed on a belt conveyor to be filled. Along the way to the filler, there are areas where the bottles or cans accumulate and impact each other.
The result of these impacts is nearly continuous discrete noise at the natural frequencies or bell tones of the bottles or cans. An enclosure, placed over the conveyor at critical accumulation points has been shown to reduce noise levels up to 12 dBA.
With respect to partial enclosure design, the following guidelines should be followed:
1. Enclose as many sides of the noise sources as possible.
2. Treat the enclosure walls or panels heavily with absorbing materials.
From these examples and design guidelines, it is clear that enclosing the major source of noise on a machine can play an important role in controlling noise. Unfortunately, often considerable noise or vibratory energy escapes or flanks out of the partial enclosure through support brackets in a structure-borne manner.
The energy is subsequently radiated as sound and the anticipated noise reduction of the enclosure is compromised. As such this partial enclosure approach often has serious limitations when it is the only noise reduction measure applied.
The use of acoustical curtains as enclosures or partial enclosures has grown sharply. Their popularity stems from their acoustical effectiveness, versatility and ease of installation. Typically, the curtain materials are 0.5 or 1-lb/ft2 lead or barium-loaded (salt-loaded) vinyl.
The smooth vinyl materials are limp and highly resistant to the industrial environment. Here again the noise reduction is usually limited to the number of acoustical leaks and the amount of noise flanking over or under the curtain. As such, an overall noise level reduction of more than 10 dB is rarely achieved.
Another variation of the dense vinyl curtain is the transparent strip curtain. Here a canfilling machine is partially enclosed with transparent PVC strips (%” thick) with a 50 per cent overlap. Overall noise reduction in the range of 5 to 8 dBA was achieved, with the major portion of the sound energy in the high-frequency range (above 500 Hz). These strip curtains can be obtained commercially in a variety of thicknesses, length and overlaps. The acoustical performance of the strip curtains increase with both thickness and overlaps.
Often telephone conversation is severely limited in noisy industrial environment. An acoustically treated booth which greatly improves the intelligibility of telephone conversation. It should be noted that the interior of the booth is lined with absorbing material and covered with perforated sheet metal for protection. Environment where these treated booths offer excellent application are press rooms, weaving rooms, assembly areas, printing plants, packaging areas, cafeterias and so forth.
Thus, it must be emphasised that successful enclosure design and installation comprise one of the most challenging areas in acoustical engineering. This discipline requires not only a thorough knowledge of acoustical principles and materials but also extensive experience and cleverness in mechanical design.