AN IMPOSSIBLE OBJECT On the Analysis of the National Phenomenon
2. La nación: significante vacío u objeto imposible
The so-called impact noises caused by objects falling on the floor or the foot-steps of people are noises that are transmitted essentially through structures and involve the structure of the floor- slab. The acoustic requirement that characterises the behaviour of these building components concerning impact noises is the level of foot-traffic noise.
The law (D.P.C.M. 5/12/1997) establishes that the levels of foot-traffic noise be measured in-situ when construction has been completed;
this means that the results obtained do not depend solely on the materials employed but also on the quality of the installation.
The achievement of acoustic requirements cannot depend solely on the acoustic insulation but is also the result of the correct functioning of the whole system defined as “mass-spring-mass”. It is also clear that not all insulating materials are suitable for such a function and must therefore be chosen based on several specific characteristics, such as dynamic rigidity and the viscous creep under compression (see details ahead). Considering that the level of acoustic insulation of floors must be measured in-situ, it is possible to predict it with sufficient precision by using the European Standard EN 12354-2 that proposes two calculation models, the “detailed model” and the “simplified model”.
The “simplified model” in the second part of standard EN 12354-2 allows an estimate of the evaluation index of normalised foot-traffic noise on the basis of the evaluation index of the building elements determined in compliance with the standard EN ISO 717-2:1996.
The “simplified model” is limited to overlying rooms of a conventional size for dwellings and it is applicable only to homogeneous constructions (brick walls and/or concrete) with floating floors.
Figure 3.3.4 Sound transmission between overlying rooms.
Foot-traffic noise
Direct transmission
Flanking transmission
© 2008 Valsir S.p.A.
The calculation model does not consider the quality of laying or workmanship and any possible defects since these cannot be quantified. It is presumed that the installation has been carried out in compliance with regulations, without acoustic bridges and with a floating floor free of constraints. Accuracy of the “simplified model”. The European Standard EN 12354-2 highlights that 60% of the predictions present an error of ±2 dB as compared with the measured values, whereas 100% of the predictions present an error of ±4 dB as compared with measured values.
The evaluation index of normalised foot-traffic noise is calculated by using the following equation:
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The evaluation index of normalised foot-traffic noise is calculated by using the following equation
Ln,w,eq is the evaluation index of normalised foot-traffic noise of a homogenous floor without acoustic insulation (“nude” floor) depending on the mass per unit area m’ expressed in kg/m2. Homogenous floors are floors in full concrete poured when laying, in full cellular concrete, made with perforated bricks, made with joists, slabs in concrete or with joists in concrete. This index is calculated with the following equation valid for floors with a mass per unit area from 100 kg/m2 to 600 kg/m2:
Ln,w,eq = 164 - 35 log(m’) [3.3.2]
The values of are indicated in the following table.
Table 3.3.1 Determination of the evaluation index of normalised foot-traffic noise.
Relation between mass of unit area of “nude” floor and evaluation index of normalised foot-traffic noise m’
[kg/m2] 100 150 200 250 300 350 400 450 500 550 600
Ln,w,eq
[dB] 94.0 87.8 83.5 80.1 77.3 75.0 72.9 71.1 69.5 68.1 66.8
Correction factor for the flanking transmission of foot-traffic noise
K is the correction factor for the flanking transmission of foot-traffic noise that is added to the direct transmissions. This correction factor depends on the mass for unit area of the “nude” floor and the mean mass per unit area of the walls of the disturbed room without any type of acoustic insulation; the latter is calculated as the average of the superficial masses estimated according to the dimensions of the single walls. The K values are indicated in the following table.
Table 3.3.2 Determination of the correction factor K.
Correction factor K [dB]
Mass per average area of the flanking wals [kg/m2]
100 150 200 250 300 350 400 450 500
Mass per unit area of the “nude” slab [kg/m2]
150 2 1 1 1 1 0 0 0 0
200 2 1 1 1 1 0 0 0 0
250 3 2 2 1 1 1 1 1 1
300 3 2 2 1 1 1 1 1 1
350 3 2 2 2 1 1 1 1 1
400 3 3 2 2 2 1 1 1 1
450 3 3 2 2 2 2 1 1 1
500 3 3 2 2 2 2 1 1 1
550 4 3 3 3 2 2 2 2 2
600 4 3 3 3 2 2 2 2 2
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Evaluation index of the reduction of foot-traffic noise
is the evaluation index of the reduction of foot-traffic noise caused by the presence of a floating floor and it depends on the mass per unit area m’ of the floating floor (concrete screed created over the soundproof mat) and the dynamic rigidity s’ of the soundproof mat.
This relation is represented graphically in Figure 3.3.5.
∆Lw = 30·log 3.125 · + 3 m’
s’ [3.3.3]
Figure 3.3.5 Evaluation index of the reduction of foot-traffic noise.
15 20 25 30 35 40 45
4 6 8 10 20 30 40 50
Index of the reduction level of foot-traffic noise ∆Lw [dB]
Dynamic rigidity of the soundproof mat [MN/m3]
Mass of floating floor [kg/m2]
60 80 100120 140160 28
V-ACUSTIC Dynamic rigidity 21 MN/m3
© 2008 Valsir S.p.A.
The dynamic rigidity s’, expressed in MN/m3, is measured in compliance with EN 29052-1 and describes the capacity of the material, if correctly installed, to elastically deform itself under the action of the distributed load of screed by converting the acoustic energy into mechanical energy (movement).
V-ACUSTIC thanks to its excellent dynamic rigidity of s’=21 MN/m3 allows foot-traffic noise to be reduced by 28 dB.
There are practical correlations that are worth remembering between dynamic rigidity, the mat thickness and the acoustic characteristics:
the greater the thickness of the soundproof mat the lower the dynamic rigidity;
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the lower the dynamic rigidity the better the soundproof characteristics;
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elevated dynamic rigidity means the material has a reduced capacity to deform and therefore means low acoustic performance.
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An excessively low dynamic rigidity, however, can mean the soundproof mat deforms too much; if the thickness of the mat reduces excessively following installation or over time (up to 1-3 mm), the acoustic performance of the product will be considerably reduced.
When comparing soundproof products not only the dynamic rigidity values must be considered but also the compressibility of the product
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Considering, therefore, a compressibility during installation of 2 mm and a temporary squashing that reaches 0.25 mm, during the installation phase, a total reduction in thickness of the V-ACUSTIC soundproof mat of about 2.25 mm must be considered.
3.3.3.1 Calculation example
Consider a slab of concrete and masonry 20+4 that has a 1.5 cm layer of plaster underneath with trellis beams at a distance of 50 cm with a mass per unit area of 340 kg/m2. The walls of the shell have a mass per unit area of 150 kg/m2. The floating screed has a weight per unit area of 100 kg/m2.
Figure 3.3.6 Structure considered in the example.
Floating screed 100 kg/m2
Flanking wall 150 kg/m2
Flanking wall 150 kg/m2 Slab
340 kg/m2
Edging stripe V-BAND
Soundproof mat V-ACUSTIC
© 2008 Valsir S.p.A.
The evaluation index of the normalized foot-traffic noise of the nude slab is:
Ln,w,eq = 164 - 35 · log(m’) = 164 - 35 · log(340) = 75 dB,
the corrective factor for the lateral transmission of the foot-traffic noise taken from the table is:
K = 2 dB,
The evaluation index of the reduction of foot-traffic noise given by the V-ACUSTIC soundproof mat with a screed slab with a mass per unit area of 100 kg/ m2 is:
∆Lw = 28 dB.
Thereforem the evaluation index of he normalised foot-traffic noise of the insulated slab is:
L’n,w = Ln,w,eq + K - ∆Lw = 75 + 2 - 28 = 49 dB.
The value is below the 63 dB specified by the Italian Ministerial Decree D.P.C.M. 5/12/1997 for residential buildings. Take into account that the margin between the calculated value and the maximum value allowed by the Decree, that is quite high, is often reduced by defects in installation, acoustic bridges generated by the contact with the skirting board or cross-over of the pipes supplying the floor heating systems.
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TECHNICAL CHARACTERISTICS OF THE COMPONENTS