Civil Engineering Reference
In-Depth Information
The top layer or floating floor may either be characterized as locally or resonantly
reacting, which here means that the force from the tapping machine is either transmitted
from the top layer to the primary floor just around the neighbourhood of the tapping
point or a reverberating bending wave field is generated. For a very stiff top layer, a local
reaction implies that the internal losses must be large; the wave field must be heavily
attenuated before it arrives at the boundaries and furthermore; the free reflected waves
created at the boundaries must also decay swiftly. This condition is not possible to realize
when it comes to concrete slabs, except may be in combination with thick layers of
asphalt etc.
A lightweight top layer of floorboards, parquet etc. may normally be characterized
as be locally reacting, primarily due to their lower stiffness in combination with higher
inner energy losses as compared with concrete slabs. The bending wavelength of
lightweight top floors is also substantially less than for concrete slabs as the latter
normally have a thickness 40-50 mm. These facts have, as will be shown below,
implications for the design of the connections between the top floor and the primary floor
at the boundaries.
Soft floor coverings, carpets etc. are, as opposed to floating floors, purely an agent
for improving the impact sound insulation. Such elastic layers change the shape of the
force impulse impacted by the tapping machine, thereby affecting the mechanical power
transmitted to the primary floor.
The achieved improvement, whatever the top layer used, is certainly not
independent of the type of primary floor. This is completely analogous to the
assumptions we were allowed to use when dealing with the improvement of linings as
opposed to the general case of lightweight double leaf partitions. In the former case we
could assume that the primary construction was unaffected by the presence of the lining.
In the same way, we shall start with prediction models for the improvement offered
by floating floors on heavy floor constructions. It should be mentioned that the basic
concrete floor slab specified in ISO 140 Part 8, which deals with laboratory
measurements of the improvement or reduction in transmitted sound by soft floor
coverings, shall have a thickness in the range 100-160 mm, preferably 140 mm. It is,
however, certainly of interest to know the reduction offered when placed on e.g. a
lightweight wood joist floor and we shall give examples of this case when dealing with
lightweight top layers. Lately, a laboratory standard for determining the reduction in
impact sound by floor coverings on a lightweight basic floor has been issued. Altogether,
three different lightweight floors have been specified (see ISO 140 Part 11).
8.4.1
Floating floors. Predicting improvements in impact sound insulation
Predicting the impact sound insulation improvement offered by a floating floor is not an
easy task. Analogous to the prediction of airborne sound insulation we shall have to take
account of both forced and resonant transmission, the latter being dependent on the
boundary conditions for the floating as well as for the primary floor. The boundary
conditions for these are not necessarily identical. Modelling the floating layer is also an
important task. May we consider the layer to act like an ideal spring or are we forced to
model it as medium supporting wave motion? We certainly cannot extensively deal with
all these factors; we shall limit our treatment to a “classic” model for forced transmission
in addition to an SEA model dealing with the resonant transmission.
The most well-known work, dealing with floating floor constructions, was
performed by Cremer in 1952, to be found in Cremer et al. (1988). In Cremer's model
the basic floor and the top floor are assumed to be two infinitely large and homogeneous
