Acoustics Material Study - Architectural Acoustics - Architecture - NIT Trichy

1. ARCHITECTURAL
ACOUSTICS
AR406
ASSIGNMENT - 2
1 0 1 1 1 4 0 1 2
K SABARATHINAM
MATERIAL STUDY
ACOUSTICS

2. MATERIAL STUDY
ACOUSTICS
INTRODUCTION
TYPE OF MATERIALS
SOUND ABSORBERS
ACOUSTIC FOAM
Acoustic materials play an important role in noise control solutions such as machinery housings or airborne sound
silencers.
The study of acoustics revolves around the generation, propagation and reception of mechanical waves and
vibrations.
The materials used for acoustic treatment of noise are -
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.
Polyurethane foams offer several advantages:
These foams are inert;
They are fire-retardant (except that they may fume); and Finally
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.
SOUND
ABSORBERS
SOUND
DIFFUSERS
NOISE
BARRIERS
SOUND
REFLECTORS

3. 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.
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.nsmission 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.
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.
ASBESTOS PRODUCTS
BALSA WOOD

4. 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). Typical Sound
Insulation PropertiesTypical Sound Insulation Properties
Comparative sound transmission loss of various building materials is shown in Table 4.
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.
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.m for floating floors, etc.
BULDING MATERIALS
GLASS PRODUCTS

5. 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.
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.
MINERAL PRODUCTS

6. 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:
Lead/foam sandwich;
Lead-loaded plastic sheet; and
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:
“Damping” compounds (for application by trowelling);
Casting compounds (for patching or filling voids); and
Mould tiles (damping tiles).
K SABARATHINAM
101114012