Building Acoustics

284 Building acoustics

treating the case of a double leaf lightweight partition with periodically placed studs. We
shall postpone the treatment of such mechanical connections to the next section.

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Non-resonant coupling Non-resonant coupling

Winput

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2

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Non-resonant coupling Non-resonant coupling

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Figure 8.6 SEA model for the set-up shown in Figure 8.5. Subsystems 1 and 5 represent the sending and
receiving room, respectively. Subsystem 3 is the cavity between the leaves.

Referring back to the discussion in Chapter 7, we should expect that predictions
using SEA would be accurate when the cavity is lightly damped. A cavity completely
filled with a porous material may hardly be characterized as a resonant subsystem.
Brekke (1979) used a set-up as shown in Figure 8.5, a double leaf construction of 12 mm
chipboard with an absorber lining that was placed in a measuring opening of dimensions
2.25 x 1.25 metres. The system was modeled using SEA according to the scheme shown
in Figure 8.6. The system of equations is easy to formulate, estimating the loss factors is
harder. By determining these factors mainly from independent measurements, the fit
between measured and calculated results were reasonably good (see Figure 8.7).
Crocker et al. used a double leaf construction, which they called a double panel,
aluminum panels of 3.2 mm thickness placed in an opening of 1.55 x 1.97 meters. The
cavity, having a depth of 71 mm, was empty. The fit between measured and calculated
results was good but based on estimated data for the internal material losses (see Figure
8.7).

8.2.2 Double walls with structural connections

An accurate prediction of sound insulation indexes of double wall constructions
with different types of structural connection between the leaves (see Figure 8.1) has been
and still is a challenge. In the cases cited above, which were using SEA modelling, the
inclusion of structural connections has been an obvious extension. This includes
coupling along a line, i.e. ribs or studs in lightweight partitions as well as discrete point
connections, the latter being binders in heavy walls as brick or concrete. These
connections may be modelled as separate modal subsystems or purely as coupling
elements.
Sharp (1978) introduced an extension to the simple set of calculations in Equations
(8.7) covering point and line connections for lightweight walls, assuming that these
connections were infinitely stiff. An extension of the work of Sharp, taking the stiffness
of the connections into account, has been suggested by Davy (1991). Later developments
have been on models, partly of the type “smeared” model by representing the studs by