Building Acoustics

Multilayer elements 315

8.4.3 The influence of structural connections (sound bridges)

Structural connections, which we may refer to as sound bridges, may be of vital
importance in the performance of a floating floor. This is certainly a case completely
analogous to the effect of such structural connections in other types of multilayer
constructions, e.g. when adding a lining to a basic construction. The conditions are
however a little different for impact sound compared with airborne sound, as the
influence of a sound bridge on the impact sound depends on whether the top floor is
locally reacting or not, where there is no simple criterion to use. There is obviously a
gradual transition between these extreme cases depending on material properties,
thickness etc. making us dependent on experiments or experience to decide on a proper
design. All investigations do, however, show that heavy floating floors such as concrete
are extremely vulnerable to sound bridges, being in the form of point connections to the
primary floor or as connections to the adjoining walls.
This was shown early on by Gösele (1964) in a laboratory experiment introducing
point contacts to the primary floor in the form of cylinders of gypsum, 30 mm in
diameter. The results are shown in Figure 8.32, where the parameter for the curves is the
number of such bridges. It should be noted that just one of these bridges reduces the
improvement to nearly half its value.

63 125 250 500 1000 2000 4000

Frequency (Hz)

0

10

20

30

40

50

60

Reduction in impact sound level

'

L

(dB)n No. of bridges

n = 0

n = 1

n = 10

Figure 8.32 The influence of sound bridges, cylinders of gypsum of diameter 30 mm, on the measured impact
sound improvement of a concrete floating floor. Data from Gösele (1964).

Correspondingly, Gösele (1964) performed a similar experiment where he made
solid contact between the concrete top slab and one of the adjoining walls. This made in
effect a direct structural connection between the floating floor and the primary floor. The
result is shown in Figure 8.33 together with a sketch of the situation. The parameter on
the curves is the length of the solid connection between the floating slab and the wall. It