In addition to lead and lead-loaded products, there are many other materials used for sound attenuation and vibration control. We shall examine these other materials and some of their applications.
Materials with high sound absorption qualities generally have soft porous surfaces. When sound waves come in contact with these absorptive surfaces, air travels in and out of the pores in the material because of pressure changes produced by the sound. Frictional forces that result from this action convert the sound energy into heat, even though the initial amount of energy is quite small.
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Every time sound waves travel through the material at each reflection, energy is dissipated and there is a resulting reduction in the reverberant sound level of the particular enclosure. The use of acoustical material can cut down sound levels in an entire plant as well as limited areas.
1. Acoustical Foams:
Many different types of foams are used widely by industry for sound and vibration isolation. Most industrial foams are available in pore size from 10 ppi to 100 ppi (pores per linear inch). Textures are generally coarse and abrasive in the 10 ppi grades to soft and downy in the 100 ppi grades. Acoustical foams that have excellent high and low temperature features can withstand temperature as high as 250°F, so that they can be sterilised with boiling water or steam.
One of the most important factors determining the behaviour of a foam sound absorber is its resistance to air flow. In flexible urethane foams, this concept is called ‘permeability’. Permeability measurements are made by first measuring air flow resistance of the foam and then inverting. For example – completely closed cell foam would totally resist air flow and have a zero permeability. As a more open cell or reticulated foam is achieved, the measurement of permeability rises to 30 per cent, 50 per cent, 70 per cent and so on.
A common misconception concerning flexible urethane foams is the belief that two foams that look alike (have the same thickness, cell size and density) will provide equal sound absorption. In reality, these two foams may have permeabilities differing by several orders of magnitude and have widely different acoustical properties. To understand this one must understand how sound or noise becomes absorbed.
In principle, noise contacts the foam structure in the form of sound pressure waves. The pressure wave within the foam structure is converted to heat energy and is dissipated. The ability of the foam to absorb sound is determined primarily by its permeability and its thickness. Thickness is easily seen and measured, but permeability is neither easily measured nor visually compared.
A very practical, on-the-spot-method of determining whether a foam has permeability is to put it against your lips and blow on it. If it totally resists the pressure you put on it, the foam is not permeable. If you can blow through it, it has a degree of permeability.
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To understand the reason for the enormous variations of permeability encountered in flexible urethane foams, it is necessary to know something about foam geometry. Urethane foams are formed by mixing reactants that simultaneously polymerise to yield a polymer and generate a gas, which causes the reacting mass to expand into a foam.
Initially these gas nuclei are very small (the number of gas nuclei generated per unit volume determines cell size). However, as the reaction proceeds the gas bubbles grow larger until, at the point where the porosity of the expanding mass has reached a level of about 72 per cent (density of about 20 PCF), the gas bubbles become distorted into polyhedrons and the contact points develop into planes.
At the instant of completion of expansion the fully expanded low density foam consists of nested polyhedrons (ideally these are dodecahedrons having 12 facing planes). Each common face between the polyhedrons contains a thin polymer film or membrane. At this point, the result is a closed-cell foam, completely pneumatic and having zero permeability.
To produce an open-cell foam it is necessary to design the formulation and processing conditions so that at the exact point the polymer mass is expanded and set into a foam of the desired density, some of the films or membranes separating the cells collapse and establish communication between the cells. Assuming the cells are dodecahedrons, it is necessary for one-sixth of the membranes to open to achieve an open cell foam.
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Thus, urethane foams have a strand and membrane configuration, the strands defining the edges of the nested polyhedrons and the membranes the planes or faces (often called pores) of the polyhedrons. The permeability of a foam is proportional to the number of ruptured membranes.
The membranes comprise only a very small portion of the polymer mass, about 1-2 per cent. In the foam industry, the term ‘level of reticulation’ is often used to indicate the percentage of the membranes that are missing. Foam containing no membranes is said to be 100 per cent reticulated.
The number of membranes present in a foam is, of course, the major factor in determining its permeability. Expressing permeability as its inverse, flow resistance, fully reticulated foams have a flow resistivity ranging from about 20 Rayls per inch for a very fine cell foam to about 1 Rayl per inch for a very large cell foam. However, a foam of any cell size which has most of its membranes intact, will exhibit a very high flow resistivity, often greater than 1000 Rayls per inch.
The conventional bun stock process for urethane foam manufacture presents the foamer with some formidable obstacles. The bun or loaf of foam, perhaps 36 inches thick, emerging from the machine and destined to be cut into sheets has an inherent variation in permeability from top to bottom. Maintaining average permeability involves cutting and testing sections, so rapid feedback for process control is difficult, if not impossible.
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The optimum permeability for an absorber depends upon its thickness. As a general rule the thinner the sheet, the higher its flow resistivity to achieve optimum performance. Again, the thin-sheet casting process is advantageous since the permeability of the single sheet being produced can be optimised for its thickness.
However, the bun stock manufacturer is faced with the choice of compromising on an average permeability or producing buns having different permeabilities for each sheet thickness. The second choice leads the bun stock manufacturer to the additional serious problem of controlling inventory.
The ability to control permeability in foam casting and thus achieve predictable acoustical results has many advantages for an engineer concerned with noise control. Principally, he can now select a foam absorber having good performance in any desired frequency range. These factors can be written into specifications with the assurance that the supplier can deliver a consistently uniform foam absorber for each application.
2. Foam with Polymer Film Surface:
The foam consists of an acoustic foam for absorbing sound at peak efficiencies combined with a polymer film surface. The polymer film surface resists scuffing and abrasion, is self-repairing against punctures and has no tendency to tear. The material is specifically designed for areas demanding a high degree of sound absorption in a visible foam surface.
3. Acoustical Panel:
An acoustical panel is a composite of a 5 mil white polyester film chemically bonded to a foam structure that utilises controlled permeability for optimum acoustical performance. The product also utilises a pressure-sensitive adhesive backing for ease of application.
The film facing, combined with the proper air flow resistance of foam, yields excellent low frequency absorption performance. The facing, while being attractive, is durable and well suited for use in environment in which it would be exposed to rough handling, grease, moisture, solvents and other contaminants. The acoustical panel has good low-frequency performance.
4. Flexible Polyurethane Foam:
The flexible polyurethane foam with a random coefficient of absorption is equal to 2 lb/cu ft of fibrous glass, the most efficient noise absorber known over a broad frequency spectrum. Available in standard thicknesses of ½ and 1″ and with pressure-sensitive self-adhesive backing for easier installation, it is nontoxic and nonirritating to handle. Requiring little mechanical support because of high tear and shear strengths, it can be free-hung on wires or attached to vertical surfaces with adhesives.
This foam is a sound absorption material used for middle and high frequency applications in which an attractive appearance is desired, such as hard walls and vehicle headliners. It is also effective as absorbing baffles. Because it can be formed to compound curves with a minium wrinkling, it is used for vehicle headliners.
It can also be used as absorbing liners for appliances, truck cabs, golf carts, snowmobiles and office equipment. For low-frequency applications, the efficiency of this product can be improved by spacing it away from a hard surface. It has an expected service life of 10 years under normal, in-plant or vehicle conditions. It will meet random incident absorption coefficient, ageing, durability and flammability requirements.
5. Various Uses for Polyurethane Foam:
Flexible polyurethane foam is a highly versatile material that can solve a wide variety of acoustical problems. The number of newly acclaimed uses is constantly growing, based on different specific application requirements and unusual foam structure designs that successfully meet them. In the increasingly important area of noise reduction, polyurethane foam has proven to be practical. There is no secret ingredient involved in the sound-absorption successes of polyurethane foam.
6. Important Variables in Selection:
The effect of many variables on the ability of flexible polyurethane foam to absorb unwanted sound has been investigated. These variables included permeability, pore size, thickness, density, stiffness and surface treatment.
The effect of permeability on the overall absorption capability of foam proved interesting. In general, completely closed-cell foam shows minimal capability to absorb sound energy, regardless of thickness. Some open-cell foams have excellent absorptiveness in 1-inch to 2-inch thicknesses. Fully reticulated open-pore foam in thicknesses of 2 inches and greater has outstanding broad-band absorption capabilities.
In testing the effect of pore size, foams ranging from 20 to 93 ppi (pores per lineal inch) were used. These tests showed that, with all other variables constant, the smaller the pore—or the greater the number of pores per lineal inch—the greater the absorption capability. Also, the thicker the piece of a particular foam, the greater the noise reduction.
For example – the sound energy absorbed was measured by various thicknesses of an 80 ppi, fully reticulated open-pore foam. Some of the results are: 6″ foam absorbed nearly 100 per cent of the noise from 250 to 6000 Hertz; 4″ foam absorbed 97 per cent noise from 1000 to 6000 Hz, 93 per cent at 500 Hz and 60 per cent at 250 Hz; 2″ foam absorbed 95 per cent at 4000 to 6000 Hz, 52 per cent at 500 Hz.
To determine the effects of density and stiffness, reticulated foams ranging from 1 to 6 pounds per cubic foot were first tested and then tested again after being rigidised by means of plastic coatings and metalising. Resultant variations in absorption capabilities were negligible in all cases.
In certain noise control applications it is necessary to protect the foam from its environment. This can be accomplished by laminating a film to the foam, making it more resistant to abrasion and impervious to liquid penetration. However, complete lamination of film surface to foam gives good absorptive characteristics in only a narrow frequency range. It has been found that a partial lamination considerably improves the sound absorption characteristics of the package.
An important property of industrial component materials today is the level of fire retardency. Many polyurethane foam products are formulated to provide a high level of fire retardency and such formulations have been successfully tested in accordance with accepted industry standards. It is also important to note that polyurethane foam can be dyed or painted to achieve specific colour effects, throughout a wide range, without appreciably reducing its ability to absorb sound energy.
In manufacturing sound-absorbing foams, the variables—including permeability, pore size, thickness and surface finish—are taken into careful consideration to produce products that precisely meet specific application requirements. Foams should be selected carefully by industrial designers who have prior knowledge of the frequency range involved, desired thickness, fire retardency, appearance/colour, cleanability abrasion resistance and of course, cost.
7. Uses for Foam in the Field of Noise Reduction:
If one word were required to describe the essence of this product’s contribution to the noise war effort, the word would be ‘innovation’. Flexible polyurethane foam has reduced successfully noise in such current industrial applications as manufacturing plant machinery, metal-removing machinery, and industrial vehicle interiors for various uses including agriculture and construction and large over-the-road trucks. In addition, a growing number of new uses are rapidly gaining acceptance.
Polyurethane foam is being used very successfully in reactive mufflers to further reduce noise from pneumatic control values. A 1″ thick foam disc is placed adjacent to the outlet of the muffler, as part of the unit. This application increases the effective noise reduction of the muffler by an additional 9 dBA.
Another unique and interesting muffler design is found in computer housings, where sound waves pass through access holes into a hollow pillow-like polyurethane foam structure. The sound hits two internal baffles and then is directed, right and left, out the ends. In all cases, noise levels have been reduced considerably.
Foam also is specified in air-flow acoustical mufflers, especially where space is restricted.
Air paths can be designed as eccentric curves within foam blocks, for two reasons:
(i) To give the noise a longer dissipation path to travel and
(ii) To cause the noise to impinge on a more absorptive foam area, due to the erratic route.
Units currently using this concept have reduced noise levels by 8 to 12 dBA.
In a large business office, the noise from a copy machine vacuum cleaner disturbed employees working nearby. The installation of a 1-inch thick ring of very fine pore reticulated polyurethane foam around the outlet of the vacuum pump reduced the noise level by 9 dBA.
Polyurethane foam was recently designed into several components of a complex plastic pelletising machine. This involved partial modification of the enclosures for the inlet and outlet chutes, the cutting mechanism and the feed mechanism. Noise output from the machine was reduced by 13.5 dBA.
Acoustical wall treatment is a new and rapidly growing architectural application for foam. Materials consist of a fine-pore reticulated foam with a fiberglass woven fabric laminated to it. The treatment is applied like thick wallpaper.
Flexible polyurethane foams have found application as components in sound transmission barriers. For this use the foam is laminated to a lead-loaded vinyl backing and a perforated textile facing so that it resembles a blanket, in what is called ‘flexible paneling’. The product can be stretched between frames or installed on a wall for modernistic architectural applications or simply draped, tent like, over a piece of noisy machinery in an industrial plant.
At 250 Hz, using a 0.4 pound-per-square-foot vinyl, this blanket application reduced noise by 16 dBA. At 250 Hz, using a 0.766 pound-per-square-foot vinyl, it reduced noise 21 dBA. At 1000 Hz, the 0.4 pound-per-square-foot vinyl blanket reduced noise 52 dBA and the 0.766 pound-per-square foot vinyl model reduced noise by 59 dBA. Not only is this blanket an effective transmission barrier, but the perforated facing makes it an excellent sound absorber as well.
Open-pore reticulated polyurethane foam has many basic composite forms. Its varied capabilities as a highly efficient sound-absorptive material are well documented and its versatility in application is limited only by the imagination and creativity of the design engineer. In the responsible analysis of noise problems and their solutions and in virtually any engineering application, open-pore polyurethane foam deserves prime consideration.
8. Aircraft Applications:
A polyurethane foam material, specially compressed into sound-absorbing layers, has substantially reduced the noise levels in the two-man crew compartment of the supersonic F-111, the multipurpose military aircraft. The foam serves as an acoustical liner for air inlet ducts to the crew compartment and muffles the noise associated with the plane’s air-conditioning system, which supplies the crew with temperature- controlled and pressure-regulated air.
9. Uses of Nylon in Noise Reduction:
Nylon Gears:
The noisiest part of any machine is usually gearing simply because of the inherent function of transmitting energy from the power source to the production end of the machine. Metal-to-metal gears clang, whine and otherwise generate noise that is harmful to a worker’s ears. Originally selected as a long-wearing alternative to bronze, steel and phenolic gearing, the noise-reduction properties of a nylon gear have only recently become an important factor in its selection to replace some other material.
In the past, the paper industry has used more nylon gears than any other industry. They can be found on such machinery as dryers, winders, filters and cutters.
A large west coast mill of a major American paper company was experiencing excessive noise from steel gearing on its high-speed continuous rewinder machines. These high-speed machines, used in the manufacture of bathroom tissue and roll towels, generated gear noise that severely limited the amount of time the operator could stay at his station without the use of ear protectors.
In searching for a method of increasing operator productivity without resorting to ear protectors, which are at best a stop gap measure that limits workers effectiveness, the paper mill engineers decided to use nylon for every other gear on the drive train of the rewinding machines. Result: an immediate measurable reduction in the noise level.
10. Noise Reduction with Glass:
Noise and its effects on man have received considerable attention in recent years and now rank among the most important problems in environmental control. Architects, home owners and industrial manufactures are all becoming more aware of the acoustical comfort and privacy problems associated with building construction. Additionally, studies have shown that although the normal ear can hear an amazing range of sound levels, prolonged exposure to high sound levels may lead to ear damage in the form of reduced hearing sensitivity.
Standard laboratory methods for testing the acoustical properties of materials have long been available. However, standard field methods for testing specific construction materials and their practical applications have been developed only recently.
Glass has been found to be an excellent sound reduction material. Inch for inch, it is better than most brick, tile or plaster and is about equal to medium-density concrete. Studies have also shown that as the thickness of the glass increases, the sound reduction or sound transmission class (STC) rating also increases.
However, increasing the interlayer thickness in laminated glass units does not increase STC ratings. Further experiments have demonstrated that higher STC ratings can be achieved by the proper construction of double-glazed units. In addition, these units are superior to single or laminated glass in insulating against heat or cold.
Noise reduction can be achieved in three ways:
1. By reducing the sound level of the noise source.
2. By dissipating the sound near the receiver or the listener.
3. By blocking the transmission path between the source and the receiver.
The first solution is the most effective, but also the most difficult to achieve in practice. The second solution is also most impractical. The third solution—blocking the transmission path—involves the placement of a relatively impervious partition somewhere between the noise source and the receiver.
Glass offers the potential for controlling noise and also establishing visual communication. The performance of a glass structure is primarily dependent upon the response characteristics it has to a spectrum of energy.
To understand the mechanism of sound transmission through partitions of any structure, one must study the single glass wall because it forms the basic element of most structures. A modest average insulation of 35 dB can be obtained by using a single-glass wall weighing about 6 pounds per square foot. However, if an average insulation of 40 dB or more is required, the single-glass wall construction becomes decreasingly efficient and the double-glass type of construction, which also provides good thermal insulation, should be used.
A double glass partition is defined as a pardon in which points on opposite sides of the structure do not necessarily move the same way at the same time. It is commonly constructed by placing two single glass panes in series separated by an air gap.
In the case of double glass units, the effect of the coincidence frequency is the same as that for single unit glass units. The air space has little effect on coincidence frequency. Experiments show that the coincidence dip for double glass units occur at the same frequency as that for single glass units of the same frequency as that for the single glass units of the same glass thickness.
For double glass units, the sound transmission loss depends on several factors including frequency, weight, and method of edge restraint, damping and stiffness. The most important variable of all is edge restraint. It has been proven experimentally that two different edge restraints with the same glass combination give different sound transmission class ratings.
The following three parameters must be considered when designing double glass units:
1. Air space,
2. Glass thickness and
3. Damping.
Generally, increasing the air space between two glass panels results in increased sound transmission loss. However, studies show that maximum sound insulation is reached with the optimum spacing of four inches. An air space wider than four inches acts as a convective space for vibrating waves, thereby reducing performance.
A combination of glass thicknesses can improve the performance of the unit by overcoming the resonance and coincidence effects. The ¼” and ⅜” glass combination, along with restrictions on the air space, gives the best performing unit.
3. Damping:
Damping is extremely important to the performance of the unit. The experimental results show that a ⅜” and ¼” glass panel, separated by a 2″ air space with proper damping in the edge attachment, has an STC rating of 45. With little or no damping, the sample has an STC rating of 42.
Four experimental tests were performed using two pieces of ¼” glass separated by various air spaces (1″, 2″, 3″ and 4″). These studies were conducted to determine the relationship between the size of the air space and the sound transmission loss rating of the window.
The studies showed that the sound transmission loss increased as the air space increased. For example, the sound transmission class (STC) rating rose from 39 for the ¼” glass, 1″ air space, ¼” glass configuration. Fig. 29.1 graphically displays the experimental results and they are also compared in Table 29.1.
Three additional studies were performed in an effort to determine the effects of glass thickness on the sound transmission loss rating. In each case, there was an air space of two inches and the thicknesses of the glass panels were varied. Three configurations were used: ¼” glass. 2″ air space, ⅜” glass; and ¼” glass, 2″ air space, ½” glass.
Experimental results are shown graphically in Fig. 29.2. In general the ¼” glass, 2″ air space, ⅜” glass configuration was the best performing unit. They are also compared in Table 29.2.
Experiments were conducted using double glass units with different edge attachment in an effort to determine whether or not the type of mounting had any effect on the sound transmission loss. Results using a ¼” glass, 2″ air space, ⅜” glass configuration demonstrated that the sound transmission loss was greater with the resilient edge attachment than with the stiff edge attachment.
Windows with more than two glass panes are usually restricted to special situations, such as observation windows in studios. Theoretical studies have shown that a multiple unit readily transmits energy below a limiting frequency dependent on the mass of each panel and the distance between the glass panels. Above this frequency, the slope of sound transmission loss as of function of frequency becomes greater as the number of glass panels increases.