This article throws light upon the top twelve acoustic enclosures for noise control. The acoustic enclosures are: 1. Noise Barriers 2. Acoustic Curtains 3. Semi-Enclosures 4. Close-Fitting Enclosure 5. Basic Panel SRI 6. Enclosure Construction 7. Panel Joints 8. Acoustic Doors 9. Ventilation 10. Windows 11. Internal Absorption 12. Isolation of Vibrations.
Noise Control: Acoustic Enclosure # 1. Noise Barriers:
Any solid medium placed substantially perpendicular to the ground will act as a barrier to sound, i.e., it will provide a measure of noise reduction in the area “shadowed” by the barrier. Depending on the nature of the medium (of this barrier), it may reflect the sound (in the case of hard, dense surfaces), or absorb it (in the case of soft, porous media).
Usually, both of these processes are present in most cases; but the nature of the medium makes one of them predominant. In addition, the degree of noise reduction provided by a barrier also depends on the physical form of the barrier (specifically its effective height and length).
Buildings, stored materials and other solid obstructions can all act as noise barrier on an open site. Earth embankments can also act as sound barriers. On the other hand, unless such noise barriers are suitably sited, they may merely transfer a noise problem from one area to another.
In order to be most effective, a noise barrier must be placed as close as possible either to the source of noise or the position of its receiver. Moreover, there should be no gaps or joints in the barrier, otherwise sound would leak through these. Ideally, the length of the barrier should be at least ten times its height. If this is not possible, then the barrier should curve around the source of noise.
To act as an effective insulator of sound, the material of noise barrier should have a surface density (also known as “superficial weight”) of at least 7 kg/m2. A number of common building construction materials meet this requirement, as shown in Table 1.
Building debris, sand bags, mounds of earth, or even barriers built from old tyres and other discarded materials are other possibilities for permanent barriers. Top soil can often be used to “seal” such barriers. If noise barriers are to be constructed on purpose, wood wool slabs fixed to timber posts can form a very effective, durable and low-cost barrier.
Sound absorbent materials, on the other hand, are normally poor insulators of sound (in the sense that such materials readily transmit sound through them. The usefulness of absorbent materials lies in their use as linings to reduce the reflections of sound incident on particular surfaces.
They are especially useful for the lining of screens or semi-enclosures where the purpose is to:
(a) Reduce the sound generated by a source of noise (such as a noisy machine or tool); and
(b) Reduce, at the same time, the overall noise level (in the vicinity of the noise source), which would otherwise be reflected by the unlined sound barrier, and thus add to the discomfort of the machine operator.
The average values of the absorption coefficient of some typical sound-absorbing materials (used for lining of covers and enclosures) are listed in Table 2. The absorption coefficient, when multiplied by 100, represents the percentage of sound absorbed by the material.
A simple sound barrier can give, in general, an anticipated noise reduction of up to 15 db. In the majority of practical cases, though, a noise reduction of about 10 dB is regarded as more typical.
Acoustic treatment of noise in a particular area involves basically a choice between modular acoustic panels (forming a rigid structure) or non-rigid acoustical curtains of lead-loaded vinyl (or other similar materials). Rigid steel barriers (acoustical steel panels) offer good attenuation of sound. Such barriers are capable of providing a noise reduction of 15 dB or more.
The main disadvantage of such steel barriers is that they are fairly heavy and relatively costly. In addition, they are also difficult to erect and dismantle. From this point of view, thick plywood is a relatively lighter material; but plywood does not offer the same noise reduction performance as steel.
Noise Control: Acoustic Enclosure # 2. Acoustic Curtains:
In contrast to rigid noise barriers, acoustic curtains provide easy maintenance and operational flexibility; but they are quite limited from the point of view of actual noise reduction. This is especially true if such curtains are designed as barriers (to contain sound), and not as absorbers of sound.
However, the acoustic performance of curtains can be enhanced by an addition of absorptive foam to their interior surface. In this case, the level of noise absorption achieved the curtains depends on the thickness of the foam layer and the area covered by it.
This, however, presents certain practical problems:
(a) Thinner layers of foam are poor absorbers of noise (but they allow for complete coverage of the curtain).
(b) Thicker foams have good noise absorption properties; but they generally need to be applied in strips or segments. This reduced their overall effectiveness.
(c) Exposed foam surfaces are subject to abrasion and other damages. (This limitation may be overcome by applying a protective outer covering.)
(d) Foams may present the hazards of fire and toxic fumes.
In order to overcome the above-mentioned limitations inherent with the foam-lined acoustic curtains, a number of appropriate materials have been developed. One such material incorporates a lead septum as its central element (to act as a noise barrier), with layers of glass-fibre on one or both sides (to act as an absorber of sound).
The outer surfaces are protected by layers of aluminized glass-fibre cloth or glass-fibre scrim. Both of these covering materials provide structural integrity to the glass- fibre layers. In addition, the aluminized cloth also provides environmental protection.
Such specially-designed materials may be finished with a quilted surface in order to produce an attractive outer surface. In addition, quilting also has the desirable property of providing acoustic decoupling of the noise impacting on and penetrating the curtain.
However, a disadvantage of quilting is that, due to the presence of stitch holes, the substrata are not fully protected against harsh environments. This implies that plain surface finishes should normally be used under harsh environmental conditions.
Noise Control: Acoustic Enclosure # 3. Semi-Enclosures:
These are also known as partial enclosures or acoustic sheds to distinguish them from complete enclosures (used in the case of noisy machines). Partial enclosures are structures erected around a source of noise, but not fully enclosing the source; thus leaving sufficient space for the operator to operate the noisy machine or tool.
In partial enclosures, the main structure may or may not be of sound insulating material; but it will have an inner lining of sound-absorbent material in order to reduce the reflections of sound inside the acoustic shed.
In the case of portable acoustic sheds, fairly lightweight construction (e.g., plywood on timber framing) is used. It is important that there should be no gaps at the joints or comers of an acoustic shed. On the other hand, block work is to be preferred for permanent acoustic sheds.
Even in this case, however, all joints should be properly made. Any gaps between the sides and ground can be sealed with a flap of suitable heavy- grade, flexible material. The recommended thickness for the sound absorbent lining is 5.0 cm (or 2.5 cm, if mounted on battens). Mineral wool or glass-fibre linings normally require a wire mesh or perforated screen covering to ensure that they stay in place.
In partial enclosures, sound will be radiated directly through the open end of the shed. This, however, can be stopped with the help of an acoustic screen. The screen used for this purpose may or may not be lined with a sound absorbent material on the side of the operator.
Semi-enclosures for machines have openings (either for access or ventilation) The main structure, in this case, consists of a sound insulating material, with a suitably high surface density (10 kg/m2 or more), and a sound-absorbent lining at least 2.8 cm thick.
It is possible to reduce the thickness of the lining, though if any high-frequency noise is present. The favoured materials for the sound-absorbent linings are glass-fibre or rock-wool (behind a wire mesh or a perforated screen) or acoustic tiles.
Typical sound reduction obtained by various types of partial enclosures is shown in Table 3.
Noise Control: Acoustic Enclosure # 4. Close-Fitting Enclosure:
Now we turn our attention to complete enclosures. We shall consider close-fitting enclosures in the present section. For the specific case of the source and recipient of noise in the same room, the approximate reduction in reverberant sound pressure level at a point in the room, due to enclosure of the source, is given by the relation
((SPL)1 -(SPL)2= SRI-(10log10Se-10log10 Ae), (1)
where (SPL)1 = reverberant sound pressure level (in dB) in the room before enclosure- (SPL)2=the same after the enclosure; SRI = sound reduction index (in dB) of the enclosure wall; Se= surface area of the enclosure (in m2 ) radiating sound into the room; and Ae=total absorption (in Sabines) inside the enclosure. The last parameter Ae may be calculated from the relation
Ae = αe Se…. (2)
where αe = average absorption coefficient inside the enclosure.
Among the parameters which can be used to maximise the reduction of noise in an enclosure, the overall surface area Se can be more or less discounted. The reason is that, whether the enclosure is a large “walk-in” type or a small one the difference in overall surface area likely to occur in practice has only a small effect on the overall performance.
The area term, therefore, can be regarded as more or less constant for the particular machine being enclosed. The design parameter offering the most scope for achieving a given target reduction in the noise level is the sound reduction index R of the enclosure wall It follows, therefore, that in the design of enclosures most attention should be given to the ways and means of achieving a sufficiently high sound reduction index.
We note here, however, that the above relation for the reduction in sound pressure level (Eq. (1)) is valid only for the transmission of airborne noise from the inside of the enclosure to outside.
It may so happen that the mechanical excitation of the enclosure structure by the enclosed machine and the subsequent transformation of structure-borne vibrational energy into the airborne acoustic energy outside the enclosure, may well be the dominant component of the total energy received by the recipients of noise.
When this is the case, Eq. (1) does not apply, and there is no simple way to calculate the amount of mechanically transmitted energy. In order to minimise the effect of this uncertainty, the design of an enclosure should aim to keep mechanical transmission of energy to an absolute minimum.
There are three basic rules for maximising the performance of close-fitting acoustic enclosures.
These can be summarised as follows:
(a) The overall sound reduction index of the structure of enclosure must be as high as possible.
(b) Maximum absorption of sound at all frequencies of interest must be provided inside the enclosure.
(c) The mechanical isolation between the structure of enclosure and the machine must be as complete as possible.
We discuss the practical implementation of these basic rules in the following sections. We start with the first basic rule, viz., the maximisation of sound reduction index (SRI). In this connection, there are two basic assessments to be made.
The first of these is to fix the sound reduction index (SRI) to be provided by the basic panel structure. The second is concerned with the amount of reduction in the potential SRI (that provided by the basic panel structure) due to the effect of various “weak” areas that are inevitably present in any practical structure.
Noise Control: Acoustic Enclosure # 5. Basic Panel SRI:
The most significant parameter determining the sound reduction index of a given partition is its superficial weight (also known as “surface mass” or “surface density”). In fact, the amount of acoustic energy transmitted by a partition, and hence its sound transmission coefficient and sound reduction index, is calculable from the well-known “mass law”.
In the form most widely used in acoustic design and related fields, the mass law for the sound reduction index SRI may be written as:
SRI = 20 log10 (Mf) – 43 dB2 … (3)
where M = superficial weight of the panel (in kg/m2) and f = the frequency under consideration (in Hz). This relation, as written above, applies to the particular case where the sound energy is impinging on the partition from a direction normal to its surface. This condition is probably true for much of the energy generated inside a close-fitting enclosure.
In a large plant room/however, the sound energy would tend to reach the enclosure walls from many directions, though it may not be the case of true random incidence presented by the fully reverberant test laboratory.
This type of incidence (the one lying somewhere between the normal incidence and true random incidence) is known as “field” incidence. Random or field incidence modifies the mass law to some extent.
It has been found that the sound reduction index is maximum for the normal incidence, minimum for random incidence and somewhere in between for the field incidence, for any given frequency. Moreover, the SRI increases with frequency for all the three cases.
For the specific case of a close-fitting enclosure, it would be more conservative (and on the safer side) to assume field incidence (rather than normal incidence) for design purposes.
In order to select the panels required for the enclosure of a particular machine, the first step is to determine the SRI required at a particular frequency. When this has been done, the next step is to determine the required superficial weight of the panel with the help of Eq. (3), using the value of SRI determined earlier. This procedure should be repeated for all frequencies of interest.
The third step is to decide as to the form of the panel to be constructed to achieve the required superficial weight. In the case of close-fitting enclosures, for example, it is unlikely that the normal masonry materials will be suitable.
Enclosures over individual machines will almost certainly be of temporary nature (since the machine may have to be moved at some time). At the very least, the machine may need to be completely demounted for major repairs, etc.
This requirement implies that the enclosure should take some form of modular panel construction. The basic mass skin of the panel could be timber (plywood or chipboard), plasterboard, or asbestos cement.
All of these materials have their own special advantages, provided the surface density required does not result in an inordinate panel thickness. This possible drawback may sometimes be overcome by making use of a sandwich construction with the required surface density provided by the lead sheet.
The panel materials mentioned above, however, suffer in varying degrees from their susceptibility to damage, lack of structural strength and the difficulty of sealing That is the reason why the majority of modular panels used now a days for close-fitting acoustic enclosures utilize plain sheet steel as the fundamental mass barrier.
Steel may not appear, at first sight, to be the most suitable material for a sound reduction panel. Although steel has the advantage of its high density and structural strength, it has very little internal damping. The result is that the sound reduction index of steel shows many departures from the mass law (arising from a number of resonance and coincidence effects).
This disadvantage of steel, however, is very effectively offset by the second requirement for enclosure panels, viz., that they should provide as much internal absorption of acoustic energy as possible.
Thus the addition of any of the commonly used absorptive materials to a steel sheet exerts a considerable amount of damping on the panel. The result is that the resonance and coincidence effects mentioned above become far less pronounced, and the panel obeys the mass law much more closely.
The damping of sound is achieved whether the absorptive medium is bonded to the panel, or merely supported against the inside surface of the panel by means of a second steel sheet, which thus forms a double-skin panel. Among these, the second alternative is more widely used in practice.
The inner skin is usually formed of some “open” material (such as perforated sheet steel, welded or woven wire mesh, or expanded metal), which exposes the absorptive face of the lining material to the internal sound field in the enclosure.
Noise Control: Acoustic Enclosure # 6. Enclosure Construction:
The basic construction of a pre-fabricated acoustic panel is an outer cover of plain sheet steel, a layer of absorptive material of sufficient thickness, and an inner “open sheet” retaining skin. Though having a double-skin construction the panel thus formed will probably behave, at low frequencies, more like a homogeneous, damped panel of the same total weight.
In the middle to high frequency range, however the behaviour of the panel will probably be better than that predicted by the mass law (i.e., an extra sound reduction of about 5 dB at 500 Hz, and even more at higher frequencies. This means that the total superficial weight of the panel can be less than that predicted by the mass law.
As with all noise-control products, it is best to use the performance figures supplied by manufacturers, and based upon laboratory tests, for specific designs of enclosures. On the other hand, one can utilize the guidelines mentioned above to obtain a design estimate of the likely superficial weight of the double skin panel.
For example, if frequencies up to about 250 Hz are the deciding factor, one should use the prediction of the mass law corresponding to field incidence. On the other hand, if the noise in the middle and high frequency range (500 Hz and above) is the problem, a superficial weight of about 75% of that predicted by the mass law will probably be sufficient.
Obtaining the required superficial weight of the panel is simply a matter of selecting appropriate steel sheet thickness and absorbent material density, so that they add up to the total required.
If the mass barrier is to be provided by the outer skin alone, and the weight required demands excessive thickness, it is a common practice to increase the overall depth of the panel (up to as much as 10 cm), and make up part of the extra depth with a mass in fill (such as plaster board or lead sheet) to give the required overall weight.
In that case, the rest of the space between the outer and inner skins is filled with the absorptive material.
Effect of “Weak” Areas:
If the close-fitting enclosure could be constructed as a completely uniform one-piece box, then the overall sound reduction index as used in the noise reduction formula, given in Eq. (1), would be virtually the same as that of the basic panel from which the box is constructed. This is indeed the case for very small machines coverable by a “lift-on” box, and where no ventilation or access is required.
On the other hand, real enclosures almost always suffer from a number of potentially “weak” areas. Unless very carefully designed, such weak areas will have the effect of reducing the value of the overall sound reduction obtainable from the basic panel construction. The reason for this is easily seen if the transmission of sound through a non-homogeneous panel is considered on an energy-sharing basis.
The sound reduction index (SRI) is given by the relation
SRI = 10 log10 (1/T), …(4)
where T is the sound transmission coefficient, defined as the ratio of the acoustic energy transmitted through the partition to the total acoustic energy incident upon it.
The amount of energy transmitted through an area Si of the partition, having a transmission coefficient Ti is equal to the product SiTi (i = 1,2,…, n and n = no. of elemental areas of the partition). Thus the average transmission coefficient T1V for the complete panel is given by the weighted mean
and, on the basis of Eq. (4), the average sound reduction index for the complete partition may be written as
(SRI)av = 10 log 10 (1/Tav). … (8)
It is this average value of sound reduction index which should be used in the basic formula given in Eq. (1).
In an enclosure, the “weak” areas (such as doors, windows, ventilation openings, etc.) will, more often than not, have a higher transmission coefficient than the basic panel.
Thus the average transmission coefficient of the whole enclosure will be higher than that of the basic panel, i.e., the average sound reduction will be lower. Obviously, the weakening effect of such areas of different construction will be much less if the individual sound reduction index of each of such “weak” areas in the following sections.
Noise Control: Acoustic Enclosure # 7. Panel Joints:
Almost without exception, practical acoustic enclosures are constructed from individual panels joined together at their edges to form the complete enclosure. The size of individual panels is fixed by the overall size of the surface of which it forms a part, the size of the opening or viewing area required, or simply the available size of the basic sheet material of which it is constructed.
The main requirement of joints between the panels is that they must allow no leakage of acoustic energy through them. From this point of view, a good practical guide to the design of “leak-proof panel joints is to regard them as having to be airtight. This will invariably require the use of some form of soft, flexible sealing strip of rubber.
There are three methods in common use in the current commercial designs of panel joints:
These methods are:
(a) Joining of edges by integral flange;
(b) Joining of edges by separate channel strip; and
(c) Complete panel support by a separate frame.
Noise Control: Acoustic Enclosure # 8. Acoustic Doors:
Access is probably the most important facility required in an enclosure. It is perfectly feasible to provide access to the enclosure by making one or more complete panels removable. It is more usual, however, for one or more panels to be attached by hinges to form a door.
This is especially true in the case of enclosures where frequent access for maintenance, work loading, or even direct operation of the machine is required.
Acoustic doors are of fundamental importance in the field of noise control. It is of fundamental importance to construct the door leaf to the same specification as the basic enclosure panel, wherever possible.
In addition, one should also ensure that the construction of the enclosure in the region of the door is sufficiently stiff to prevent the progressive sag or other distortion of the door frame, leading to the appearance of gaps around the edges when the door is closed.
Enclosure doors must also be provided with the same degree of sealing as is available to doors in the conventional masonry surroundings. Sliding doors can be used to good effect in pre-fabricated enclosures; but without positive seal compression on closure, their effectiveness is limited to a noise reduction of about 15-20 db.
Noise Control: Acoustic Enclosure # 9. Ventilation:
In a vast majority of cases, noisy machines requiring an enclosure are either consuming or generating energy. In either case, one by-product will be heat radiation. If the machine has to be enclosed, then the resulting heat must be removed; otherwise the temperature inside the enclosure would rise to an unacceptable level.
The simplest way to remove the surplus heat is through ventilation, which is essential also if machine operators have to spend any appreciable amount of time inside the enclosure.
The basic design of the ventilating system for an acoustic enclosure is essentially the same as that for any room; but more careful attention has to be paid to the acoustics of the system.
The ventilating system will certainly have fan noise; but it will almost always be much less than the noise produced by the offending machine. Of more concern, in this case, is the weakening of the acoustic structure of the enclosure by the holes required in it to pass the air in and out.
The solution to this problem, though, is quite straight forward. It requires merely the fitting of a duct-type silencer to both intake and discharge openings. With the enclosure of machines other than internal combustion engines, for which the volume of air required is much less, the air-flow may be ducted to and from the enclosure, with the fan mounted outside the enclosure.
This arrangement is perfectly acceptable under the following circumstances:
(a) Silencers on both ducts are located immediately next to the wall or roof of the enclosure.
(b) Any connecting ducting pieces, between the outside of the enclosure and the duct silencer, are made of suitably heavy or lagged material to prevent breakout of un-attenuated noise through the connector wall.
(c) The effects of fan noise on the surrounding areas are examined and treated as required.
The machine being enclosed, in many cases, may be the one involved directly in production (such as automatic pressing from strip feed), as opposed to the so-called “service” machinery (e.g., an air compressor). In addition to any other features, the design of enclosures for the machines directly involved in production requires openings for feeding in and taking out work.
As with any other opening, the overriding requirement in this case is to preserve the acoustic integrity of the panel (i.e., to provide an “attenuated” opening). For machines which require long or continuous feeds, the routine solution is to provide an acoustically lined tunnel through which the work has to pass.
Other machines may require a supply of material only periodically, the continuous feed coming from their own hopper, as in the case of granulators for recycling scrap plastic.
It may often be sufficient in such cases to provide a flap in the enclosure the flap being hinged rather than sliding – so that a good seal can be ensured when the flap is closed. If the flap is horizontal (or even inclined), its weight may be sufficient to seal it adequately; otherwise a quick-release wedge catch will be required.
Noise Control: Acoustic Enclosure # 10. Windows:
Machine enclosure often require some means of inspecting their interior without necessarily opening the access doors. Inspection is necessary from the point of view of both safety and production operations.
For example, it is much cheaper to leave gauges and meters (indicating machine performance) attached to the machine itself than to extend them outside. Moreover, operators feeding an enclosed machinery from the outside also require visual access to the machine. This is achieved through the use of windows.
The design of windows for high noise reduction is of universal importance in buildings. In this respect, acoustic windows are not very different from acoustic doors. Design of windows for acoustic enclosures is similar to that for doors and windows in a building where high noise reduction is required.
There is, however, the obvious limitation in design flexibility due to the restricted airspace that can be accommodated between panes of double glazing. The loss of sound attenuation resulting from this restriction can be offset to some extent by using heavy panes.
Even then, the eventual sound reduction index of the window itself will very likely be less than that of the basic panel construction. It follows, therefore, that if the average sound reduction index for the panel as a whole if not to be seriously reduced, areas of inspection windows should be as low as possible.
Entries for Services:
The services required to be supplied to or from the enclosed machines may include one or more of the following:
(a) Electrical power;
(b) Compressed air;
(f) Hydraulic fluid;
(g) Fuel; and
(h) Engine exhaust gases.
As in the case of work feed, the objective here is to bring the various pipes and conduits through the enclosure wall without weakening it acoustically. When a new installation for a noisy machine is designed, the services can be brought in under the wall via a trough set in the foundation.
If the trough is covered and absorbent material laid in under the cover to fill the gaps around pipes, etc., attenuation of sound through the trough will be adequate. The services which can be transported in flexible pipes may be passed through the airways of ventilation air silencers
More often, however, services have to be brought through the actual enclosure wall. When this is the case, one may provide a very heavy plate panel which carries connectors on both sides, so that the required service is brought up and connected to the outside, and a separate section joins the inside connector to the machine.
Alternatively, one may provide a clearance hole in the wall panel and pass the service through, after making provision for adequate sealing of the hole.
Of these two alternatives, the method of making a specific panel for the services is preferable, especially if more than one service has to be introduced The reason is that if these services has to be introduced. The reason is that if these services are taken through the wall, each has to be sealed separately.
Noise Control: Acoustic Enclosure # 11. Internal Absorption:
The object of providing absorptive treatment inside the enclosure is the minimise the build-up of reverberant energy there. In the absence of an enclosure, the acoustic energy generated by the machine is free to radiate in all directions.
On the other hand, if the machine is enclosed in a box with non-absorbent internal surfaces, then the sound energy originally free to travel in all directions is reflected back, and the sound pressure level inside the enclosure increases due to multiple reflections of sound.
Since the sound reduction index of the panels of an enclosure remains constant (being a property only of the enclosure construction), it follows that the sound pressure level outside the enclosure construction), it follows that the sound pressure level outside the enclosure will vary directly as the sound pressure level inside it.
Thus it is evident that if the sound level inside the enclosure can be reduced, a corresponding noise reduction will be obtained outside. Such a reduction is achieved by providing the maximum possible amount of noise absorption inside the enclosure.
Almost invariably, the absorption required inside the enclosure will be broad band in nature. Thus the acoustic treatment most often used in close-fitting enclosures is the use of blanket-type porous absorber. This is true even in the case of enclosures for machines which produce strong discrete-frequency noise.
The reason is that the use of such materials is dictated by the practical engineering limitations on the design of a practical modular panel for enclosures.
Thickness of the lining is the only significant performance parameter which the designer of acoustic panels has at his disposal. In general, the design rule for this purpose is – the lower the frequency, the thicker the absorptive material required for lining. Traditionally, the panel thickness for perforated modular enclosures is either 5 cm (for noise reduction of up to 35 dB) or 10 cm (to give a noise reduction of up to 45 dB).
Once the thickness of absorbent material has been selected, there is not much to choose between one type of material and another. The main design parameter for the choice of absorptive materials is their density, and this should be in the range from 30 to 100 kg/m2. Much lighter material may not have the required acoustic absorption properties. On the other hand, a much denser material may be wasteful.
When it comes to securing the absorptive lining to the inside of the enclosure panel, open-celled polyurethane foam slabs offer a definite advantage, since they require no support other than a suitable contact adhesive applied locally (even on the underside of ceiling panels).
On the other hand, although modern foams can have a very high fire resistance rating, foam slabs used for the acoustic lining of enclosure panels may prove to be a safety hazard, particularly if the machine to be enclosed is likely to splash oil or grease.
Foam slabs, therefore, are not the best choice for a lining material for enclosure panel’s The more usually employed material is some type of semi-rigid, resin bonded mineral or glass-fibre slabs which are readily available. Since these are laminated materials, they do require protection against mechanical damage, and physical support against the walls (especially under ceiling panels).
To take care of these requirements, the panels are provided with sheets of perforated steel, welded mesh, or expanded metal and supported off the wall by folded channel sections or battens. Of course, the total weight of lining and supporting skin must be included in the panel weight when estimating the likely sound reduction.
One should keep in mind, however, the fact that even mineral or glass- fibre slabs are not fully safe. Continuous splash of oil, for example, can be just as much of a safety hazard of progressively absorbed by mineral wool slabs as by the slabs of polyurethane foam slabs.
One method of combating this hazard is to wrap the slabs of absorptive material (before retaining them behind the protective facing) with a very thin polyester or similarly impervious film. This may have some effect on the absorption of high frequency noise; but there is virtually no effect on the low frequency absorption (which may probably be determining the design parameters of the enclosures).
Noise Control: Acoustic Enclosure # 12. Isolation of Vibrations:
The main objective of providing vibration isolation is to ensure that the acoustic enclosure is required to deal with airborne noise only from the machine enclosed by it.
In order to ensure that any sound radiation resulting from the mechanical vibrations in the wall is small, the following three conditions should be satisfied:
(a) The machine itself must be adequately isolated from its support slab or framing.
(b) All pipes and ducts (for services) to and from the machine, which pass through the enclosure wall, must have a flexible section between the wall and machine.
(c) In situations requiring very high reduction of sound, air-borne noise inside the enclosure must be prevented from entering the floor slab and being transmitted under the enclosure as mechanical vibration.
There is hardly anything special about providing adequate vibration isolation for a machine just because it is inside an enclosure. Even the provision of vibration isolation for service connections should present no basic difficulty.
Some services (e.g.. electrical power, fuel, cooling air, etc.) will be carried by cables or pipes which can be easily made flexible. On the other hand, services involving high pressures (e.g., compressed air, hydraulic oil, and high-pressure water) may pose some problems due to the fact that high-pressure flexible pipes tend to stiffen with use.
However, flexible pipes can still provide excellent isolation (even in the case of “high-pressure” services) if the following precautions are observed:
(i) Flexible hose should be located as close to the machine as possible.
(ii) The axis of the hose should be aligned at 90° to the direction of vibration.
(iii) Adjacent rigid piping should not be supported by hanging it from the flexible hose.
(iv) Torque loads on hoses should be avoided.
Instead of using flexible hoses for services, one may also pass the rigidly connected pipe through an over-sized hole in the enclosure wall and pack around with sealed flexible material. The effective prevention of airborne noise entering the foundation slab inside the enclosure will also require a floating floor inside.