Building Acoustics

306 Building acoustics

measurements was a little smaller than required for a standard laboratory measurement,
which is 10 m^2. The predicted results are averaged values using data calculated for
approximately 10 single frequencies inside each one-third-octave band.
Summing up sandwich elements: A critical factor in designing for good sound
insulation is the core shear stiffness. By reducing the core thickness without changing
any material parameters one may draw two important conclusions. The stiffness of the
core will increase, moving the lowest dilatational resonance up in frequency. At the same
time the bending stiffness of the element will decrease, which results in a reduced phase
velocity for the anti-symmetric wave motion in the middle frequency range, thereby
obtaining a higher reduction index.

8.4 Impact sound insulation improvements

Adding a lining to the ceiling is one way of improving the airborne and/or the impact
sound insulation of a floor, using a floating floor is another. The added floor “floats” on
an elastic medium placed upon the primary floor construction. The elastic component is
normally a continuous layer of mineral wool, plastic foam etc. but may be discrete load-
bearing mounts, e.g. rubber mounts or rubber strips. A floating floor may then be
considered as a generalised form of vibration isolation. Principal solutions for the case of
a concrete floating floor slab are shown in Figure 8.25.

Floating floor

Elastic layer

Primary floor

Elastic bearing

Porous material

(non-bearing)

a)

b)

Figure 8.25 Principal types of concrete floating floors. a) On a continuous elastic layer, b) on elastic load-
bearing unit mounts.

As far as the airborne sound insulation is concerned, such a construction is nothing
less than a double wall construction without stiff structural connections. As for the
improvements in the impact sound insulation, however, not only the properties of
floating slab and the elastic layer are important, the properties of the ISO tapping
machine may also influence the results. The latter applies in particular to cases where the
floating floor is not a concrete slab but a lightweight construction, e.g. floorboards,
parquet etc. We shall therefore mostly be concerned with the impact sound properties of
such floating floors.