This article throws light upon the top eight materials used for acoustic treatment of noise. The materials are: 1. Acoustic Foams 2. Asbestos Products 3. Balsa Wood 4. Building Materials 5. Glass Products 6. Mineral Products 7. Lead Products.
Acoustic Treatment of Noise: Material # 1. Acoustic Foams:
Acoustic foams are sponge-like materials. Such materials possess the ability to absorb sound energy by changing it into heat. The capacity of acoustic foams to absorb acoustic energy depends primarily on their permeability and thickness of the cellular structure and, to a lesser extent, on the pore size and surface treatment. On the other hand, the rigidity of the foam has very little or no effect on its acoustic performance.
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As a general rule, the higher the permeability the greater the sound absorption capacity of a foam. However, there is an optimum permeability for a foam of particular thickness. For optimum acoustic performance, the thinner the foam, the lower the permeability has to be.
This requirement has a significant effect on the acoustic performance, particularly at lower frequencies. Other parameters being the same, the smaller the pore size (i.e., the larger the number of pores per unit area), the greater the capability for absorption of sound. Controlling all these parameters mentioned above, foams can be “tailored” (during manufacture) to provide optimum acoustic properties.
Acoustic foams are especially suitable for the acoustic treatment for middle and high frequencies. For low-frequency applications, on the other hand, acoustic performance of foams can be improved by spacing them away from hard surfaces. Acoustic foams can be made from any “expandable” material. However, the main type of foam used for acoustic applications is the flexible polyurethane foam of open pore structure.
Polyurethane foams offer several advantages:
(a) These foams are inert;
(b) They are fire-retardant (except that they may fume); and Finally
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(c) They can be dyed of painted (for decoration) without appreciably affecting their sound absorbent properties.
Polyurethane foams are also produced with a polymer film surface (to resist abrasion), bonded to open-weave fabrics, sheet vinyl, sheet lead, and other similar materials.
Acoustic Treatment of Noise: Material # 2. Asbestos Products:
As is well known, the primary use of asbestos is for heat-and- fire applications and these utilize non-combustible asbestos/cement moldings, insulation boards and other similar products. Such materials, however, may also have good and desirable acoustic properties.
In addition to some of the asbestos products named above, there are several others for specialised applications. For example, asbestos films may be applied (by spray) to structural steel work and other building surfaces for fire insulation.
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Foamed asbestos is produced from asbestos fibre and kaolin in sheet form (usually with aluminium foil backing) as a thermal insulation. It can also be used for acoustic insulation. Typical sound absorption properties of low-density foamed asbestos are given in Table 1.
Acoustic Treatment of Noise: Material # 3. Balsa Wood:
Balsa wood has a particular industrial application as a core material for rigid, lightweight laminates with aluminium sheet, and also as a thermal insulant lining. In the applications as a core material, balsa is generally used in end-grain configuration, bonded to two outer skins.
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Typical sound transmission loss through solid balsa wood is shown in Table 2. Balsa wood gives a mean transmission loss of about 3.8 dB per cm thickness over the frequency range 100 – 3,000 Hz. On the other hand, mean transmission loss over the frequency range 125-400 Hz can be taken as 4.2 dB per cm thickness. We note, however, that both of these figures apply to low-density (112-128 kg/m3) balsa wood.
The approximate value of the sound transmission loss through a balsa- wood panel may also be obtained from the empirical relation
L=14 +
… (1)
where L = mean sound transmission loss (in dB), d = density of balsa-wood panel (in kg/m3), and t = thickness of the panel (in cm).
Balsa can also be classified as a moderately sound-absorbent material, since it is a light weight material with a porous structure. If balsa wood is used for this purpose (i.e., absorption of sound) in the form of tiles or cladding, sound absorption performance is considerably improved with end-grain configuration.
However, no specific figures are available in literature for the sound-absorbent properties of balsa panels in end-grain configuration.
Acoustic Treatment of Noise: Material # 4. Building Materials:
Brickwork provides good insulation against sound, particularly if plastered on one or both sides as shown in Table 3. Clinker block is a light-weight aggregate concrete block using furnace residue, and containing no more than 10-20% un-burnt carbonaceous material.
Clinker block is a standard building material for cavity walls and partitions. Its sound reduction performance, when plastered, is almost directly comparable with a similar thickness of brickwork (see Table 3). 
Comparative sound transmission loss of various building materials is shown in Table 4.
Other materials in the same category (generally classified as light-weight aggregate) concerned with different types of light-weight concrete are as follows:
(a) Foamed slag (used for heavier concrete and foundation work);
(b) Expanded clay;
(c) Expanded pulverized fuel ash;
(d) Expanded slate;
(e) Pumice;
(f) Kieselguhr (for production of diatomaceous concrete) and
(g) Vermucilite (yielding light-weight cavities; density range 300-900 kg/m3; can be cut, sawn, nailed and screwed).
In passing, we may also mention cork under this section. Insulation corkboard is generally produced in the density range 100-320 kg/m . The lower densities (80-100 kg/m3) have good sound absorption properties in the middle frequency range. Slightly higher densities are used for thermal insulation.
Densities of about 175 kg/m3 upwards are useful for the anti- vibration treatment, and also for insulation against impact cm- percussive noise.
Concrete rates as a good insulator of sound; but it will generally transmit impact sound unless it is combined with some other form of insulation or isolation treatment. Moreover, being hard, concrete surfaces also readily reflect the sound, i.e., concrete is a poor absorber of sound.
Lightweight concrete materials (known as aerated concrete or gas concrete) developed as thermal insulators have appreciably lower sound insulation than dense concrete. A typical figure in this connection is 42-45 dB reduction in sound level for a density of 640 kg/m3.
Gas concrete has also been used in the United Kingdom (and other countries) to make the inner leaves of cavity brick walls, and for the construction of roofing slabs.
Acoustic Treatment of Noise: Material # 5. Glass Products:
Glass is a very poor absorber of sound. Most of sound energy impinging on a glass surface is reflected. On the other hand, glass is an excellent insulator of sound. For a particular thickness, glass is about as good as medium-density concrete from the view-point of sound insulation. However, the performance of glass as an insulator of sound is limited by the practical consideration of its thickness.
The thickness of glass which can be employed in practice is limited due to economic as well as structural considerations. Thus single glass panels used in practical situations seldom offer a sound reduction better than about 35 db.
Foamed glass is an inert, impenetrable, low-density material produced as a thermal insulator. Foam glass is used as a pipe insulant and also for thermal insulation in flat roof constructions. Since the cells in foam glass are not interconnected, its sound absorption is probably low; but no specific figures are available.
Glass fibre wool, on the other hand, is both an excellent thermal insulant as well as sound absorber. Glass fibre wool is extensively used for lining ducts, silencers and acoustic hammers. In all the applications, glass wool is held in position by a perforated facing.
Glass wool is also widely used for facing in the sound absorbent treatment of hard surfaces, as an absorbent layer behind perforated tiles, and in felted form as a supporting medium for floating floors, etc.
Acoustic Treatment of Noise: Material # 6. Mineral Products:
Gypsum is a natural mineral. The hemihydrate and an hydrate types of gypsum are widely used for the manufacture of acoustic tiles, boards and panels. General building materials based on gypsum are insulating wall-boards, ceiling panels and impregnated wood-wool slabs.
Insulating wall-boards consist of a core of aerated gypsum plaster faced on both sides by mill board. One surface is normally covered with polished aluminium foil (to act as a reflective surface) facing the side of the cavity where the board is attached to timber studding.
Impregnated wood-wool slabs comprise compressed wood fibres impregnated (under pressure) with gypsum cement. These slabs are extensively used as a building material for roofing and/or light weight walling. Impregnated wood wool slabs provide both noise and thermal insulation.
Mineral wool (rockwool) is manufactured from a type of volcanic deposit, known as “diabase’, by melting the rock (together with limestone and coke) and converting it into fibres by a spinning process. The quality and size of the fibres is determined both by the selection of raw materials and the control of the manufacturing process (to produce a base material with consistent properties).
This base material is then further processed to produce a variety of mineral-wool products used for acoustic treatment. These products include slabs, pads, mats and quilts as acoustic panels of all types, and also rigid moulded sections. The acoustic properties and applications of various rock- wool products are given in Table 5.
Rockwool products are very extensively used in the building construction industry. These products provide thermal insulation, and possess fire-retardant properties. Certain rock wool products may combine both thermal as well as sound absorption properties. Other products are designed and manufactured specifically for acoustic or thermal insulation.
Acoustic Treatment of Noise: Material # 7. Lead Products:
Lead sheet is an exceptionally good sound-insulating material. This acoustic property of the lead sheet is the result of its high superficial weight (weight per unit area of the surface) as well as its natural limpness. This implies that a sound barrier incorporating lead can be lighter and thinner than other sound barriers offering a similar acoustic performance.
Until recently, the main factor limiting the use of lead sheets for noise insulation was the high cost of rolling the lead slab to the required thin sheets. This has now been overcome by the recent development of continuous cast thin-sheet lead.
Lead sheets are generally specified by their superficial weight (surface density). Thin lead sheets have the advantage that they can easily be cut (with scissors, for example), wrapped or draped around a structure, nailed, or stapled. In addition, thin lead sheets can readily be bonded to other surfaces.
In addition to lead sheets mentioned and discussed above, lead materials used for sound insulation also include the following:
(a) Lead/foam sandwich;
(b) Lead-loaded plastic sheet; and
(c) Leaded plastics.
Lead/foam sandwich consists of lead sheet laminated between the polyurethane foam. The lead in this case is usually a single layer (of about 0.4 mm thickness), with foam thickness ranging from 6 mm to 50 mm.
A particular advantage of lead/foam sandwich is that it can readily be cut, snapped by hand, and bonded to another surface. The polyurethane foam in the lead/foam sandwich provides isolation of the lead, maintaining its desirable limpness (for optimum acoustic performance).
Lead-loaded plastic sheets are usually vinyl or neoprene sheets loaded with powdered lead. This process yields a flexible curtain or blanket, which can be used as a sound barrier. Fabric reinforcement may also be incorporated in the lead-loaded plastic sheet. This type of sound-insulant material is free from the tendency to cold flow-which is the main limitation of sheet lead used as a draping material.
Leaded plastics usually consist of lead-loaded epoxy. Leaded plastics are prepared in several forms for various applications. Some of these are:
(a) “Damping” compounds (for application by trowelling);
(b) Casting compounds (for patching or filling voids); and
(c) Mould tiles (damping tiles).