Multilayer elements 311
At very low frequencies one may see frequency dependence near to the Cremer model
whereas at higher frequencies one may experience frequency dependence nearer to 6 dB
than 9 dB per octave. This is probably caused by wave motion in the elastic layer, i.e.
characterizing the layer by its compressional stiffness only is not appropriate. As
opposed to this effect, measured results on floating floors on discrete mounts give a good
fit to predicted data using Equation (8.46).
8.4.2 Lightweight floating floors
A couple of principal solutions when it comes to lightweight floating floors as
floorboards, parquet etc. are sketched in Figure 8.28. In both cases there is a continuous
elastic layer but in the first case the top floor is directly coupled to the elastic layer. We
shall refer to this as a “surface mounted” case. In the second case, referred to as “line
mounted”, the top floor is mounted on beams or slats, the latter constituting the couplings
to the elastic layer. This type of mounting may have the advantage that the dynamic
stiffness of the enclosed air layer is diminished, at the same time also increasing the static
stiffness of the top floor. In this line mounting case one may also replace the continuous
elastic layer by unit mounts fastened below the beams, i.e. a mounting resembling the
one shown in Figure 8.25 b).
a)
b)
Floorboard
Elastic layer
Primary floorSlatsFigure 8.28 Lightweight floating floor on a heavy basic floor. a) “Surface” mounted, b) “Line” mounted.The improvements gained by using such lightweight floors are distinctly different
from the ones of concrete slab form. As mentioned above, this is partly due to a more
local reaction by the lightweight floors. At the same time, however, the mass impedance
of the tapping hammers may no longer be neglected in comparison with the input
impedance of the floor. In this case, we shall use the complete Equation (8.40) which
may be written
2
11
nz
0z
4
40 lg 10 lg 1 , where.ff mB
Lf
ff π⎛⎞ ⎡⎤⎛⎞
'=⋅ +⋅ +⎜⎟ ⎢⎥⎜⎟ =
⎝⎠ ⎢⎥⎣⎦⎝⎠ mh