Building Acoustics

298 Building acoustics

the curve in the higher frequency range is probably due to flanking transmission, which
even with laboratory facilities will show up at extreme level differences.
Finally, all structural connections between the two elements will necessarily
diminish the sound insulation. As indicated in Figure 8.18 b) the binders used are 3.5 mm
thick steel bolts. There are distributed over the whole wall area with a centre-to-centre
distance of 500 mm.

Leca, 150 mm

Plaster, 10 mm

Leca, 100 mm

Mineral wool

50 mm

a)

Binder,

3.5 mm )

Gap

b)

Figure 8.18 Lightweight concrete double wall. a) Mounted on a common frame; b) Mounted on separate
frames.

8.3 Sandwich elements

The concept of a sandwich applies to a large group of multilayer elements with
applications in building constructions, in transport systems and not least, in plane and
ship constructions (high speed boats). The literature in this field is rather extensive,
which stems from the fact that modelling the different layers may be quite different
depending on the given application. In our context of building acoustics we shall define a
sandwich element as a three-layer structure having two thin plates (face sheets) bounded
by a lightweight core material. As opposed to conventional double leaf constructions,
where the cavity has an infill of porous material, we now have a core as a continuous and
solid coupling element between the outer sheets (see Figure 8.19).
The face sheets may not have the same material properties or have the same
thickness but that is the normal case. In addition, we have characterized them as being
“thin”, which in practice implies that they are metal sheets, fibreboard, chipboard,
plasterboard etc. As for the core material we find plastic foam as well as mineral wool,
the latter having densities in the range of 100–150 kg/m^3 and also cut to make the
direction of the fibres normal to the sheets. A much used type of core is the honeycomb,
a beehive plate where the material may be metal or plastic. A very cheap variant, found
in door leaves, is a core made up of cardboard rings, i.e. short cylinders of cardboard.
Elements with honeycomb cores offer lightweight and high stiffness elements.
They become nearly incompressible in the crosswise direction, which implies that we
may characterize the core by its shear stiffness only. Such a description cannot be used in
a number of other core materials, e.g. plastic foams as polyurethane, polystyrene etc. In