Building Acoustics

318 Building acoustics

(8.50). In cases where r < 10 kPa·s/m^2 , moreover when we cannot assume that sskeleton >>
sair , the method is not suitable for determining s.
Some typical data of the dynamic elasticity modulus are given in Table 8.3, valid
for a static load of 2 kPa. The dynamic stiffness and dynamic E-modulus are dependent
on the static load and this dependency will not be the same for different materials. As an
example we find that the E-modulus of rock wool, having a density in the range 150–175
kg/m^3 , is approximately 0.3 MPa at a load of 2 kPa. Decreasing the load to 1 kPa the E-
modulus will decrease by some 20%, whereas it will increase by some 30% with a load
of 4 kPa.

Table 8.3 Dynamic modulus of elasticity.

Material Density

(kg/m^3 )

Dynamic E-modulus (MPa)

(static load ≈ 2 kPa)

Glass wool approx. 125 0.11–0.13

Rock wool 150–175 0.27–0.33

Rock wool 110–135 0.25–0.30

Polystyrene foam 10–20 0.30–3.0

Polyurethane foam 33–72 7–19

Cork 120–250 10–30

Examples As pointed out above, the stiffness of the enclosed air may contribute
substantially using thin elastic layers. Using a layer of thickness 10 mm only, assuming a
porosity V ≈ 1.0, we get sair ≈ 10 MPa/m, which is in the same order of magnitude as
sskeleton of a very elastic material.
A floating floor of 50 mm concrete on an elastic layer of 25 mm glass wool will
have a resonance frequency of approximately 40 Hz. Using the same elastic layer
together with a floating floor of type as shown in Figure 8.31, i.e. a combination of 22
mm chipboard and 13 mm plasterboard, the resonance frequency will now be
approximately 90 Hz.

8.4.5 Floor coverings

Floor coverings such as carpets, vinyl, vinyl combined with felt and linoleum etc. are, as
opposed to floating floors, purely a means of reducing the impact sound transmission. A
soft covering changes the shape of the force impact from the tapping machine, thereby
influencing the mechanical power transmitted to the floor below. The reduction in the
impact sound power should therefore, in principle, be predicted from the difference in
transmitted force with and without the covering. It has, however, been difficult to set up
a good prediction model due to problems of characterizing the properties and behaviour
of such coverings subjected to this kind of impact.
The main feature is that the speed of the hammer will decrease from its initial value
v 0 when hitting the covering layer, to zero at the maximum compression of the layer.
Thereafter the hammer will return. The time for this process is certainly dependent on the
effective stiffness of the layer related to the area of the hammer(s) Sh, the latter being 7
cm^2. We may express this stiffness as