Building Acoustics

316 Building acoustics

should be obvious that such stiff connections along the boundaries have a destructive
effect on the working of a floating floor but the requirement as to the stiffness of such
connections is not evident. Experience shows that such connections may have a stiffness
that is a lot stiffer than the elastic layer before a reduction in the effect of the floating
floor is detected.

63 125 250 500 1000 2000 4000

Frequency (Hz)

0

10

20

30

40

50

60

Reduction in impact sound level

'

L

(dB)n

Length (m)

L = 0

L = 0.1

L = 0.5

L = 2.6

Solid "bridge"

Floating floor

Concrete floor

Brick wall

Figure 8.33 The effect of solid line contacts between a concrete floating floor and the adjoining wall. The
length of the line bridges is the parameter on the curves. Measurements by Gösele (1964).

Extensive experiments (e.g. Holmås and Bjørklund (1979); Austnes and Hveem
(1983)) on lightweight floating floors using floorboards, combinations of chipboard and
plasterboard, parquet etc. show that stiff boundary connections have substantially less
effect than in the case of heavy floating floors. Holmås and Bjørklund (1979) used a
laboratory set-up with a wood joist primary floor combined with various types of
lightweight floating floor. Fixed connections were established between the floating floor
and a boundary wall in contact with the primary floor, the effect being an increase in the
transmitted impact sound level at frequencies above approximately 300 Hz, this when
placing the tapping machine at positions near to the boundary wall (< 1 metre) as
compared to other tapping positions. This clearly indicates the local reaction of such
lightweight floating floors, an effect making them less critical with regards to non-
intentional structural connections.

8.4.4 Properties of elastic layers

Assuming that the mass m per unit area of the floating floor is substantially less than the
mass of the primary floor, Equation (8.5) may be simplified to