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Assessment of a simplified experimental procedure to evaluate impact sound reduction of floor coverings
Applied Acoustics 79 (2014) 92–103
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Applied Acoustics journal homepage: www.elsevier.com/locate/apacoust
Assessment of a simplified experimental procedure to evaluate impact sound reduction of floor coverings Andreia Pereira a,⇑, Luís Godinho a, Diogo Mateus a, Jaime Ramis b, Fernando G. Branco c a
CICC, University de Coimbra, Department of Civil Engineering, Pólo 2, Rua Luís Reis Santos, 3030-788 Coimbra, Portugal University of Alicante, Department of Physics, System Engineering and Signal Theory, Mail Box 99, 03080 Alicante, Spain c INESC, University de Coimbra, Department of Civil Engineering, Pólo 2, Rua Luís Reis Santos, 3030-788 Coimbra, Portugal b
a r t i c l e
i n f o
Article history: Received 24 April 2013 Received in revised form 27 September 2013 Accepted 13 December 2013
Keywords: Impact noise Floor coverings Small sized setup Experimental
a b s t r a c t The impact noise reduction provided by floor coverings is usually obtained in laboratory, using the methodology described in the standard EN ISO 140-8, which requires the use of standard acoustic chambers. The construction of such chambers, following the requirements described in the EN ISO 140-1, implies a significant investment, and therefore only a limited number exists in each country. Alternatives to these standard methodologies, that allow a sufficiently accurate evaluation and require lower resources, have been interesting many researchers and manufacturers. In this paper, one such strategy is discussed, where a reduced sized slab is used to determine the noise reduction provided by floor coverings, following the procedure described in the ISO/CD 16251-1 technical document. Several resilient coverings, floating floors and floating slabs are tested and the results are compared with those obtained using the procedures described in the standards EN ISO 140-8 and EN ISO 717-2. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction One of the causes of annoyance inside buildings is impact sound produced on floors by falling objects or people walking. In each country the acoustic codes define acoustic demands in order to minimize the annoyance produced by these noises, which in general require the evaluation of impact noise levels in the receiving room and the calculation of the corresponding weighted index. These requirements have been motivating industry to develop materials and solutions that allow for the reduction of impact sound pressure levels, which, in general, are composed of flexible floor coverings, floating floors and floating slabs. Indeed, works such as those of Schiavi et al. [1], Asdrubali and D’Alessandro [2], Asdrubali et al. [3] or Rushforth et al. [4] clearly demonstrate the present motivation of researchers in studying and developing new solutions to help mitigating impact noise. The acoustic behaviour provided by these solutions may be evaluated experimentally through the use of the procedure defined in the ISO 140-8 [5]. This standard describes the normalized test to obtain the impact sound reduction provided by floor coverings and requires the use of two acoustic chambers, one placed above the other, with about 50 m3, separated by a concrete slab with approximately 10 m2, where the specimen is applied. The system ⇑ Corresponding author. E-mail addresses: [email protected] (A. Pereira), [email protected] (L. Godinho), [email protected] (D. Mateus), [email protected] (J. Ramis), [email protected] (F.G. Branco). 0003-682X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apacoust.2013.12.014
is excited by a normalized tapping machine and the sound pressure level in the receiving room is recorded. The impact sound reduction is then calculated by performing the difference between the sound pressure levels obtained with and without the tested solution. With these curves it is also possible to obtain the weighted impact sound reduction by following the procedure defined in the EN ISO 717-2 [6] standard. Such method involves the use of infra-structures with high construction and maintenance costs, and therefore only a limited number exists in each country. In the initial stage of product development it may be more interesting to use other methodologies, with lower costs, to predict impact sound reduction of floor coverings. Several researchers have been using non-standard strategies to obtain the acoustic behaviour of floor coverings, such as Godinho et al. [7], which used a reduced size acoustic chamber to evaluate the impact sound reduction of locally reactive floor coverings. Recently a new methodology for the evaluation of impact sound reduction is being investigated to evaluate impact sound reduction of floor coverings based on the use of small sized specimens. The procedure, described in the ISO/CD 16251-1 [8] technical document, requires a setup composed of a simply supported bare concrete slab of small dimensions, and vibration measurements are carried out when a normalized tapping machine excites the system without and with the floor covering. The impact sound reduction is then calculated by evaluating the difference between these two levels. This procedure may be applied to evaluate the impact sound reduction of locally reacting floor coverings, such as carpets, vinyl
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or linoleums, applied on heavyweight floors. In the works of Bjor [9] and Foret et al. [10] preliminary results on the behaviour of this system are described, indicating that the it may provide promising results, approaching those obtained when using the normalized method of ISO 140-8 for resilient coverings and floating floors. Besides these publications, as far as the authors know there are no other references regarding this method; therefore, this work aims to go further into the understanding of the acoustic behaviour of this system. Thus, the setup was built in the acoustic lab of the Department of Civil Engineering of the University of Coimbra. Initial measurements were carried out to describe the dynamic behaviour of the system, trying to identify the vibration modes and the influence of the measurement positions, as well as of the source positions. Several floor coverings were tested such as resilient floor coverings and floating floors with different resilient underlays. The results are compared with normalized ones (following the procedure defined in the ISO 140-8). Small sized floating slabs of different dimensions, ranging between 0.25 m2 and 0.96 m2, were also built and tested in the setup. Since these slabs have small dimension when compared with those usually adopted in full-scale tests, it is expected that the behaviour and obtained results will be somehow different. It is our aim to understand the dynamic behaviour of small dimension slabs to assess the possibility of using this system to study these solutions in a preliminary stage of research. Other authors such as Miškinis et al. [11], addressed the issue of the floating slab dimensions, by performing in situ tests, following the procedure defined in the ISO 140-7 [12], where floating slabs, 5 cm thick, and dimensions varying between 0.5 m2 and 13.4 m2 were tested. In that work, even though the number of tested specimens was small, the authors verified that the impact sound reduction depends on the size of the floating slab and that small sized floating slabs provide higher values of the weighted sound reduction index than the normalized result for a floating slab of 10 m2. In fact even in laboratories with standard facilities, it may be more practical to perform tests using smaller sized floating slabs. It is therefore important to fully understand the advantages and limitations of using such elements in the acoustic tests. This paper is organized as follows: in the next section a description of the setup is presented; there follows the analysis of the dynamic behaviour of the system; the measurement results obtained from testing floor coverings, floating floors and floating slabs are then displayed and comparison with standard results is carried out.
2. Setup description The test facility used in this work was built according to the procedure described in the Standard ISO/CD 16251-1. This setup is composed of a concrete slab with dimensions 1.2(m) 0.8(m) 0.2(m), simply supported on four elastic supports, as displayed in Fig. 1. A resilient material made of agglomerated rubber granulate, with a total thickness of 10 mm, was interposed between the slab and the supports. The same material was also placed between the room floor and the support so as to minimize vibration transmission from the hosting room. The effectiveness of this elastic material was evaluated by performing prior vibration measurements to estimate the vibration level difference between the room floor and the concrete slab. These measurements were carried out using a normalized tapping machine placed on the room floor and a set of accelerometer positions defined in the room floor and on upper surface of the slab. The results are displayed in Fig. 2, using a 1/3 frequency band scale, ranging between 12.5 Hz and 4000 Hz. The analysis of this plot allows identifying, in the lower 40 Hz frequency band, that the sound
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Fig. 1. Picture of the setup used to obtain impact sound reduction of floor coverings.
level in the floor and in the slab are very similar indicating that at this frequency no attenuation is provided. This frequency corresponds to the first resonance frequency of the spring-mass system. Above this value the elastic material provides an increased attenuation of the noise generated in the floor room. In order to understand the behaviour of this system, a set of preliminary measurements were performed, where the vibration level on the upper surface of the slab, excited by an impact point load (a wood hammer was used) was recorded. The results are displayed in Fig. 3a, in a linear scale, between 10 Hz and 1000 Hz. This result evidences the presence of two peaks. The first one was identified as the mass-spring-mass resonance frequency of the system. The first resonance frequency can be computed theoretically pffiffiffiffiffiffiffiffiffiffi as f0 ¼ 1=ð2pÞ k=m, where k = EA/L is the dynamic stiffness of the elastic support, E is the Young modulus, A and L are the surface area and thickness of the elastic material, respectively. For this purpose, the dynamic stiffness of the elastic material was evaluated experimentally following the procedure described in ISO 9052-1 [13] standard. From the result of that test, the theoretical resonance frequency calculated using the above equation is f0 ¼ 38 Hz, which matches well the plot in Fig. 3a. A finite element simulation was additionally performed in order to compute the first flexural mode of the slab, and to further confirm the resonance frequency of the mass-spring system. The mode shapes computed using the finite element software LISA are depicted in Fig. 3b and c, for which eigenfrequencies around 40 Hz and 550 Hz were obtained. Indeed, these figures allow a better understanding of the dynamic behaviour of the system, clearly revealing that a rigid body movement occurs for the lower frequency, and that a flexural movement of the slab is registered at the higher frequency.
3. Test procedure The measurement procedure consists of acquiring vibration levels in the lower surface of the slab, in 1/3 octave frequency bands, between 100 Hz and 3150 Hz. The ISO/CD 16251-1 technical document also prescribes that at least four accelerometer positions, randomly chosen, shall be used, distanced of at least 10 cm from the edges. The source used is a normalized tapping machine placed in at least two distinct different positions (with a minimum distance
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Fig. 2. Vibration measurements performed to evaluate the effectiveness of the elastic material placed between the room floor and the supports of the setup, so as to minimize vibration transmission from the hosting room.
of 30 cm from each other, and no hammer shall be closer to the edges of the plate than 10 cm). The measurement procedure is similar to that defined in the EN ISO 140-8, although here acceleration measurements are used instead. Firstly, the average vibration level is obtained from measurements recorded in the concrete slab without the floor covering specimen (La,0). Then the floor covering is applied on the concrete slab and the corresponding average vibration level provided by the tapping machine acting on the covering is achieved (La,1). The impact sound reduction is then obtained by performing the difference between these levels according to the following expression:
DLa ¼ La;0 La;1 ;
Fig. 3. Dynamic behaviour of the slab system when subject to a point impact load: (a) frequency response in terms of acceleration; (b) mode shape for the mass-spring vibration and (c) first flexural mode of the slab.
1,2
1,2
0,15
0,15 A5
A1
A2
A6
A2
A6
A3
A7
A3
A7
(a)
A8
A4
A5
0,1
A4
0,8
A1
0,1
P1
where La;0 ¼ 20 logða0 =aref Þ is the average vibration level obtained in the bare concrete slab (without the specimen); La;1 ¼ 20 logða1 =aref Þ refers to the vibration level obtained when the floor covering is applied; and aref ¼ 106 m=s2 , is the reference acceleration. Measurements of background vibrations should also be performed in order to evaluate whether those vibrations may influence the results. If the difference between the measured vibration levels with the tapping machine acting and the background vibration levels is less than 10 dB, then it may be necessary to perform corrections. The weighted sound reduction index of the floor covering is obtained by applying the procedure described in the ISO 717-2 as follows: the vibration level measured with the floor covering on the concrete slab is transposed to a reference concrete floor, 12 cm
0,8
P3 P2
ð1Þ
A8
(b)
Fig. 4. Positions of the normalized tapping machine and of the accelerometers: (a) upper surface of the concrete slab and (b) lower surface of the concrete slab.
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Fig. 5. Average impact sound reduction and standard deviation for the three source positions regarding the concrete slab with: (a) carpet; (b) floating floor using a polyethylene foam and (c) floating slab using a rubber resilient layer.
thick, with a weighted impact sound pressure level of 78 dB; then the weighted impact level provided by the floor covering applied on this floor, Lw,a,r, is obtained by adjusting the reference curve described in the ISO 717-2. The weighted sound reduction index is then given by:
DLa;w ¼ 78 Lw;a;r :
ð2Þ
4. Dynamic behaviour of the setup After building the above described setup, measurements were carried out in order to understand its dynamic behaviour and to define a measurement procedure. With this aim a grid of eight
accelerometer positions placed in the upper and lower surfaces of the slab, as displayed in Fig. 4, and a set of three source positions were defined (P1, P2, P3 as displayed in Fig. 4). The measurements were performed using an accelerometer B&K 4370 which allows measurements between 0.2 Hz and 3500 Hz, and a signal acquisition system Symphonie (01 dB) with the software dBBati 32 for signal treatment. The present section displays the average sound reduction curves and standard deviation using different accelerometer positions and source positions. Analysis of background noise is also performed to assess if any correction is required. The three types of impact sound treatment solutions are here addressed: resilient coatings, floating floors and floating slabs.
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Fig. 6. Average impact sound reduction when the tapping machine is acting on position P1 and standard deviation regarding the concrete slab with: (a) carpet; (b) floating floor using a polyethylene foam and (c) floating slab using a rubber resilient layer.
4.1. Influence of the tapping machine positions The influence of the tapping machine positions in the sound reduction curves was investigated by performing measurements over the eight accelerometer positions defined in the upper surface of the slab for the three defined source positions, identified in Fig. 4. Fig. 5 displays the average impact sound reduction curves and standard deviations with relation to source position, for the three types of impact sound reduction solutions here addressed. As expected, the average sound reduction curves provided by the carpet and the floating floor increase smoothly with frequency, behaving similarly as the ones obtained using a normalized procedure. In the case of the floating slab the response although with amplitudes increasing with frequency, displays oscillations. The standard deviation provided by the three source positions varies
between: 0.21 dB and 1.94 dB for the carpet; 0.25 dB and 2.87 dB for the floating floor and 0.24 dB and 3.04 dB for the floating slab. The larger values of the standard deviation are recorded in the 1/3 octave 2500 Hz and 3150 Hz frequency bands. In all other frequencies the deviations are never greater than 2 dB. 4.2. Influence of the accelerometer’s position The influence of the accelerometer positions was addressed at eight positions defined in the upper surface of the slab. Fig. 6 displays the average impact sound reduction obtained from measurements in these positions when the source is at position P1 and the corresponding standard deviation. Again the three types of impact sound attenuation solutions were tested. The standard deviation lies between: 0.33 dB and 5.36 dB for the carpet; 0.76 dB and
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Fig. 7. Average impact sound reduction for acceleration positions defined on the lower surface of the slab and on the upper surface of the slab: (a) concrete slab with carpet; (b) concrete slab with floating floor using a polyethylene foam and (c) concrete slab with floating slab using a rubber resilient layer.
6.30 dB for the floating floor and from 0.38 dB to 9.39 dB for the floating slab. The lower values are registered in the lower frequencies and they progressively increase for higher frequencies. In the measurement procedure, the eight accelerometer positions were always used. With respect to the acceleration position, although it is easier to perform records in the upper surface of the slab, it may be useful to register vibration levels in the lower surface when the specimen size covers a significant area of the slab surface. In order to assess differences in the records provided by these different positions, measurements were also performed using a similar grid of positions defined in the lower surface and in the upper surface of the slabs for the three tapping machine positions (see Fig. 4b). The average sound reduction curves obtained for these two grids of receivers (displayed in Fig. 7) reveal similar behaviours, although the curve obtained using the upper surface of the slab reveals a
smoother trend. In spite of the differences registered throughout the frequency range and of the smoother behaviour of the results from the top surface, it must be noted that positioning the accelerometers at the lower surface of the slab can be quite useful whenever the specimen covers a large part of the slab, hindering the use of the upper surface to perform measurements.
4.3. Influence of background noise Background noise was also evaluated using the carpet solution, since it is the best performer among the tested materials, and thus leading to the lowest vibration levels. The vibrations levels obtained with and without the tapping machine are plotted in Fig. 8. From the analysis of this figure one verifies that the average background level is always 40 dB bellow the vibration level
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Fig. 8. Average vibration level measured on the slab with the carpet with and without the tapping machine.
Fig. 9. Impact sound reduction provided by floor coverings obtained using the standard and simplified methodologies: (a) carpet; (b) vinyl with flexible underlayer; (c) standard vinyl and (d) linoleum.
measured with the tapping machine, and therefore no correction is required.
and sound reduction indexes obtained using both the ISO/CD 16251-1 and the ISO 140-8 procedures. 5.1. Floor coverings
5. Comparisons of results provided by the simplified and standard method The above described setup was used to evaluate the acoustic behaviour provided by the three types of solutions used to control impact sound transmission: floor coverings; floating slabs and floating floors. This section discusses the sound reduction curves
Several floor coverings were tested such as carpets, standard (rigid) vinyls, vinyls with flexible underlayers and linoleums (a total of 11 different specimens were tested). Among these coverings four were chosen to illustrate the behaviour of the system: a carpet; a standard vinyl; a vinyl with a flexible underlayer and linoleum. Fig. 9 illustrates the impact sound reduction curves for
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Fig. 10. Weighted impact sound reduction indexes obtained from the normalized vs. standard procedure for several floor coverings.
Fig. 11. Impact sound reduction curves provided by floating floors obtained using the standard and simplified methodologies: (a) floating floor with glued cork and (b) floating floor with a rubber 4.5 mm underlayer.
these coverings obtained with the simplified method and also with the normalized procedure (as described in the ISO 140-8). From the analysis of these figures one can observe that the sound reduction curves provided by the simplified method
approach those obtained using the ISO 140-8 standard procedure. In fact only in the higher frequencies it is possible to notice more significant differences, which are more pronounced for the coverings displaying a poorer acoustic performance (see Fig. 9c–e).
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Fig. 12. Weighted impact sound reduction indexes obtained from the normalized vs. standard procedure for several floating floors.
Fig. 13. Responses in 1/12 octave frequency bands provided by floating slabs with 0.5(m) 0.5(m) placed over an uniform rubber layer, 4.5 mm thick: (a) slab 4 cm thick and (b) slab 8 cm thick.
The comparison of the overall weighted sound reduction index between the ISO 140-8 and the ISO/CD 16251-1 (see Fig. 10) shows that for coverings displaying higher acoustic performances (between 15 dB and 19 dB), both indexes are very similar and a maximum deviation of 1 dB was obtained. For the materials with lower acoustic performance (bellow 10 dB), the deviations found were greater but always less than 2 dB from the standard results. These small differences are related to the procedure used to obtained the index, with the unfavourable deviations from the reference curve being located in the low and medium frequencies, where the curves provided by both methodologies approach. 5.2. Floating floors In this subsection we display the results obtained for the several floating floors tested (a total of 6 different floating floors were tested with resilient underlayers of cork, rubber, flexible polyurethane and polyethylene). Fig. 11 displays the sound reduction curves obtained using the simplified and standard procedures, for two of the floating floors tested, the first consists of a floor where the resilient cork underlayer is glued to its lower surface, whereas the second case corresponds to a floating floor resting on a rubber underlayer, 4.5 mm thick. For the floating floors the corresponding impact sound reduction curves provided by the simplified and standard methods display a similar behaviour, although in the medium and higher
frequencies greater differences are found. When comparing the weighted sound reduction indexes provided by the two procedures (see Fig. 12), we find that the larger deviations assume values of only 2 dB; again, this good agreement is mostly justified by the fact that the unfavourable deviations in relation to the reference curve (defined in the ISO 717-2) occur mainly in the low and medium frequency range, where both results approach.
5.3. Floating slabs In this subsection the results obtained when testing floating slab systems using the procedure of the ISO/CD 16251-1 document are discussed. So as to evaluate the influence of the floating slab dimensions, three slabs 0.04 m thick were built with the following dimensions: 0.5(m) 0.5(m); 0.8(m) 0.8(m) and 1.2(m) 0.8(m), exhibiting areas of 0.25 m2, 0.64 m2 and 0.96 m2 respectively. It is important to bear in mind that manufacturers display results for different floating slab thickness which may hamper the comparison of different solutions. Therefore an analysis of thickness influence was also performed for floating slabs with 0.5(m) 0.5(m) and varying thicknesses of 4 cm, 8 cm and 12 cm. The tests were carried out for two resilient underlayers: a uniform 4.5 mm rubber underlayer, whose sound reduction index given by the manufacturer is 20 dB and a non-constant thickness rubber underlayer 15/7 mm thick, with maximum and minimum
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thickness of 15 mm and 7 mm respectively, with a sound reduction index, given by the manufacturer of 24 dB. Fig. 13a displays the impact sound reduction obtained using a slab with 0.5(m) 0.5(m) and thickness of 4 cm placed over a resilient rubber layer with uniform thickness of 4.5 mm. This result is displayed in a narrow frequency scale of 1/12 octave bands to provide insight on the behaviour of this solution. As expected the impact sound reduction increases with frequency, however the curve displays a set of oscillations, which are related with the dynamic behaviour of the solution. In order to better understand these oscillations a test was carried out using a hammer as a source to excite the floating slab and acceleration measurements were recorded on the surface of this small sized floating floor. Fig. 13a also displays the corresponding average vibration level. This result was shifted downwards of 80 dB to provide comparison between the two curves. Comparing these two results we find that they display several oscillations and in most frequencies they coincide denoting that, in fact, to an increase in the vibration level of the floating slab corresponds a drop in the impact sound reduction provided by the solution. The oscillations in 1/12 octaves of 777 Hz and 1959 Hz are particularly sharp and will be noted in the following responses calculated in 1/3 octave frequency bands. We may also note from the experimental analysis that the mass-spring-mass resonance frequency is situated on the low frequency range at about 80 Hz. A similar test was performed using a floating slab of 8 cm thick and the results, displayed in Fig. 13b allows similar
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conclusions. In this case the resonance frequency of the system is situated, as expected, at a lower frequency in relation to the 4 cm floating slab, at about 50 Hz. Fig. 14a1 and b1 display the impact sound reduction obtained for the tested floating slabs, 4 cm thick, with different dimensions. In these figures the standard result (using a floating slab with 10 m2) was also included for reference. From the analysis of Fig. 14a1, where the resilient layer is composed of a rubber material with 4.5 mm, it is possible to verify that the curves provided by the different slabs, although showing a tendency to approach the normalized curve, display several oscillations throughout the frequency range, that were previously discussed, and clearly influence the 1/3 octave response. For smaller dimensions these oscillations are situated in the higher frequencies, and as the slab dimensions increase they move to lower frequencies, although still within the frequency range of interest. From the three slab dimensions analysed, the curve that best approaches the normalized result is that provided by the larger slab, measuring 1.2(m) 0.80(m). The results shown in Fig. 14b1, concerning tests performed using a resilient layer with a variable thickness 15/7, also exhibit a set of oscillations regarding the normal modes of the slabs, as explained previously. In this case, the impact sound reduction provided by the smaller slab, with 0.50(m) 0.50(m), displays increased amplitudes in relation to the normalized result. As the size of the slab increases, the resulting curve tends to approach the normalized result.
Floating slabs, 4 cm thick
Floating slabs with 0.50(m)x0.50(m)
(a1)
(a2)
(b1)
(b2)
Fig. 14. Impact sound reduction provided by the several tested floating slabs, with different dimensions: (a) floating slab over uniform rubber layer, 4.5 mm thick and (b) floating slab over rubber layer, 15/7 mm thick.
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Measurement according to the ISO 140-8 using a floating slab with 0.4x0.6x0.05
Measurement according to the simplified procedure using a floating slab with 0.5x0.5x0.04
Fig. 15. Impact sound reduction obtained for a concrete floating slab over several flexible polyurethane foams, with different densities and thicknesses, using two different measurement procedures.
The results for different thicknesses of the floating slab with 0.5(m) 0.5(m) are plotted in Fig. 14b1 and b2. From the analysis of these figures one verifies that when the thickness changes from 4 cm to 8 cm, the impact sound reduction increases significantly and in the higher frequencies strong oscillations appear in the curve related to the greater thickness which are related to modal behaviour of the slab. For the floating slab, 12 cm thick, the resulting impact sound reduction curve displays, however, smaller amplitudes than the 8 cm thickness slab. This behaviour may be related to the greater weight of this slab that is compressing the resilient layer, promoting an increase in sound transmission to the support slab. When the resilient layer is a rubber mat, 15/7 mm thick, the results indicate, again an increase in the acoustic behaviour when the thickness increases from 4 to 8 cm. With the increase of thickness from 8 to 12 cm the corresponding results tend to approach. The last figure (Fig. 15) displays the results provided by a floating slab over a flexible polyurethane foam with densities of 120 kg/ m3 and 200 kg/m3 respectively and varying thicknesses of 5, 10 and 30 mm. These solutions were tested using two methods: (i) the first consists of performing the test following the procedure described in the NP EN ISO 140-8, but using a small floating slab 0.4(m) 0.6(m) 0.05(m) [14]; (ii) the second method follows the procedure given in the ISO/CD 16251-1 using the floating slab with 0.5(m) 0.5(m) 0.04(m). These tests were performed with the aim of assessing the possibility of perceiving similar conclusions.
From the analysis of the results provided by Fig. 15 we verify that the curves obtained from both methods are not smooth and display oscillations related with the vibration modes of the floating slab. Interestingly, we find that the tests provided by both methods allow the same conclusion: the polyurethane foam with 30 mm provides the best acoustic behaviour while the foam with thickness 10 mm displays the smaller amplitudes of impact sound reduction. From the analysis of the results achieved using the EN ISO 140-8 procedure we see that the increase in the foam thickness allows an increase in impact sound reduction mainly in the lower and medium frequencies, while for the results provided by the ISO/CD 16251-1 this behaviour is not noticed, and the sound reduction improvement occurs more uniformly throughout the frequency range. 6. Conclusions This paper reports the results obtained from the implementation of an experimental procedure described in the ISO/CD 16251-1 technical document that may be used to predict impact sound reduction using small sized specimens on heavy-weight floors. In this method, vibration levels are recorded on a concrete slab, of reduced dimensions, without and with the floor covering when a normalized tapping machine is acting. The impact sound reduction is then obtained from these vibration measurements by performing the difference between them. The setup was then used to test several specimens of floor coverings, from resilient coverings, floating floors and floating slabs.
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The impact sound reduction results obtained for resilient coverings were found to approach those obtained using the normalized procedure defined in the EN ISO 140-8. The weighted sound reduction provided by both methods was also calculated and the results provided by the method defined in the ISO/CD 16251-1 technical document provided maximum differences of 2 dB in relation to those obtained from the normalized method. Similar conclusions were drawn from the tests performed on the floating floors also displaying a local reactive behaviour. The tests performed on the floating slabs allowed the identification of differences in relation to the normalized curves. Although the curves display a trend to approach the standard curves, oscillations are identified due to the vibration modes of the reduced sized slab, indicating a plate like behaviour. These oscillations are positioned in the frequency range of interest also for the floating slab with the dimensions of the bar slab. Although not allowing a direct comparison of both these and the normalized results, the tests performed on this setup using floating slab may allow a comparison among different resilient materials and therefore may be used to perform tests in a preliminary stage of research. References [1] Schiavi A, Pavoni Belli A, Corallo M, Russo F. Acoustical performance characterization of resilient materials used under floating floors in dwellings. Acta Acustica united Acustica 2007;93:477–85. [2] Asdrubali F, D’Alessandro F. Impact sound insulation and viscoelastic properties of resilient materials made from recycled tyre granules. Int J Acoustic Vibration 2011;16(3):119–25.
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[3] F. Asdrubali, F. D’Alessandro, S. Schiavoni, G. Baldinelli, Lightweight screeds made of concrete and recycled polymers: acoustic, thermal, mechanical and chemical characterization, in: Proceedings of FORUM ACUSTICUM 2011, 27 June - 1 July 2011, Aalborg, Denmark (in CD-Rom). [4] Rushforth I, Horoshenkov K, Miraftab M, Swift M. Impact sound insulation and viscoelastic properties of underlay manufactured from recycled carpet waste. Appl Acoustics 2005;66:731–49. [5] Standard ISO 140-8: 1997(E). Acoustics-measurement of sound insulation in buildings and of building elements - Part 8: Laboratory measurements of the reduction of transmitted impact noise by floor coverings on a heavyweight standard floor. [6] Standard ISO 717-2: 1996. Acoustics - rating of sound insulation in buildings and of building elements - Part 2: impact sound insulation. [7] Godinho L, Masgalos R, Pereira A, Branco FG. On the use of a small-sized acoustic chamber for the analysis of impact sound reduction by floor coverings. Noise Control Eng J 2010;58(6):658–68. [8] ISO/CD 16251-1: Acoustics-Laboratory measurement of the reduction of transmitted impact noise by floor coverings on a small floor mock-up - Part 1: Heavyweight compact floor. [9] O-H. Bjor, Simplified measurement of the reduction of transmitted impact noise by floor coverings, Proc 2010, BNAM 2010 - Bergen. [10] R. Foret A, J. Chéné, C. Guigou-Carter, A comparison of the reduction of transmitted impact noise by floor coverings measured using ISO 140-8 and ISO/CD 16251-1, Forum Acusticum 2011, Aalborg, Denmark, 28 Junho a 1 Julho de 2011, pp. 1371–76. [11] Miškinis K, Dikavicˇius V, Ramanauskas J, Norvaišiene˙ R. Dependence between reduction of weighted impact sound pressure level and specimen size of floating floor construction. Mater Sci 2012;18(1):93–7. [12] Standard ISO 140-7: 1998(E). Acoustics-measurement of sound insulation in buildings and of building elements - Part 8: Field measurements of impact sound insulation of floors. [13] Standard ISO 9052-1: 1989. Acoustics-determination of dynamic stiffness Part 1: Materials used under floating floors in dwellings. [14] N. Sousa, Assessment of impact sound insulation provided of solutions using polyurethane foam, MSc Thesis, FEUP, Portugal, 2008.
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Applied Acoustics journal homepage: www.elsevier.com/locate/apacoust
Assessment of a simplified experimental procedure to evaluate impact sound reduction of floor coverings Andreia Pereira a,⇑, Luís Godinho a, Diogo Mateus a, Jaime Ramis b, Fernando G. Branco c a
CICC, University de Coimbra, Department of Civil Engineering, Pólo 2, Rua Luís Reis Santos, 3030-788 Coimbra, Portugal University of Alicante, Department of Physics, System Engineering and Signal Theory, Mail Box 99, 03080 Alicante, Spain c INESC, University de Coimbra, Department of Civil Engineering, Pólo 2, Rua Luís Reis Santos, 3030-788 Coimbra, Portugal b
a r t i c l e
i n f o
Article history: Received 24 April 2013 Received in revised form 27 September 2013 Accepted 13 December 2013
Keywords: Impact noise Floor coverings Small sized setup Experimental
a b s t r a c t The impact noise reduction provided by floor coverings is usually obtained in laboratory, using the methodology described in the standard EN ISO 140-8, which requires the use of standard acoustic chambers. The construction of such chambers, following the requirements described in the EN ISO 140-1, implies a significant investment, and therefore only a limited number exists in each country. Alternatives to these standard methodologies, that allow a sufficiently accurate evaluation and require lower resources, have been interesting many researchers and manufacturers. In this paper, one such strategy is discussed, where a reduced sized slab is used to determine the noise reduction provided by floor coverings, following the procedure described in the ISO/CD 16251-1 technical document. Several resilient coverings, floating floors and floating slabs are tested and the results are compared with those obtained using the procedures described in the standards EN ISO 140-8 and EN ISO 717-2. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction One of the causes of annoyance inside buildings is impact sound produced on floors by falling objects or people walking. In each country the acoustic codes define acoustic demands in order to minimize the annoyance produced by these noises, which in general require the evaluation of impact noise levels in the receiving room and the calculation of the corresponding weighted index. These requirements have been motivating industry to develop materials and solutions that allow for the reduction of impact sound pressure levels, which, in general, are composed of flexible floor coverings, floating floors and floating slabs. Indeed, works such as those of Schiavi et al. [1], Asdrubali and D’Alessandro [2], Asdrubali et al. [3] or Rushforth et al. [4] clearly demonstrate the present motivation of researchers in studying and developing new solutions to help mitigating impact noise. The acoustic behaviour provided by these solutions may be evaluated experimentally through the use of the procedure defined in the ISO 140-8 [5]. This standard describes the normalized test to obtain the impact sound reduction provided by floor coverings and requires the use of two acoustic chambers, one placed above the other, with about 50 m3, separated by a concrete slab with approximately 10 m2, where the specimen is applied. The system ⇑ Corresponding author. E-mail addresses: [email protected] (A. Pereira), [email protected] (L. Godinho), [email protected] (D. Mateus), [email protected] (J. Ramis), [email protected] (F.G. Branco). 0003-682X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apacoust.2013.12.014
is excited by a normalized tapping machine and the sound pressure level in the receiving room is recorded. The impact sound reduction is then calculated by performing the difference between the sound pressure levels obtained with and without the tested solution. With these curves it is also possible to obtain the weighted impact sound reduction by following the procedure defined in the EN ISO 717-2 [6] standard. Such method involves the use of infra-structures with high construction and maintenance costs, and therefore only a limited number exists in each country. In the initial stage of product development it may be more interesting to use other methodologies, with lower costs, to predict impact sound reduction of floor coverings. Several researchers have been using non-standard strategies to obtain the acoustic behaviour of floor coverings, such as Godinho et al. [7], which used a reduced size acoustic chamber to evaluate the impact sound reduction of locally reactive floor coverings. Recently a new methodology for the evaluation of impact sound reduction is being investigated to evaluate impact sound reduction of floor coverings based on the use of small sized specimens. The procedure, described in the ISO/CD 16251-1 [8] technical document, requires a setup composed of a simply supported bare concrete slab of small dimensions, and vibration measurements are carried out when a normalized tapping machine excites the system without and with the floor covering. The impact sound reduction is then calculated by evaluating the difference between these two levels. This procedure may be applied to evaluate the impact sound reduction of locally reacting floor coverings, such as carpets, vinyl
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or linoleums, applied on heavyweight floors. In the works of Bjor [9] and Foret et al. [10] preliminary results on the behaviour of this system are described, indicating that the it may provide promising results, approaching those obtained when using the normalized method of ISO 140-8 for resilient coverings and floating floors. Besides these publications, as far as the authors know there are no other references regarding this method; therefore, this work aims to go further into the understanding of the acoustic behaviour of this system. Thus, the setup was built in the acoustic lab of the Department of Civil Engineering of the University of Coimbra. Initial measurements were carried out to describe the dynamic behaviour of the system, trying to identify the vibration modes and the influence of the measurement positions, as well as of the source positions. Several floor coverings were tested such as resilient floor coverings and floating floors with different resilient underlays. The results are compared with normalized ones (following the procedure defined in the ISO 140-8). Small sized floating slabs of different dimensions, ranging between 0.25 m2 and 0.96 m2, were also built and tested in the setup. Since these slabs have small dimension when compared with those usually adopted in full-scale tests, it is expected that the behaviour and obtained results will be somehow different. It is our aim to understand the dynamic behaviour of small dimension slabs to assess the possibility of using this system to study these solutions in a preliminary stage of research. Other authors such as Miškinis et al. [11], addressed the issue of the floating slab dimensions, by performing in situ tests, following the procedure defined in the ISO 140-7 [12], where floating slabs, 5 cm thick, and dimensions varying between 0.5 m2 and 13.4 m2 were tested. In that work, even though the number of tested specimens was small, the authors verified that the impact sound reduction depends on the size of the floating slab and that small sized floating slabs provide higher values of the weighted sound reduction index than the normalized result for a floating slab of 10 m2. In fact even in laboratories with standard facilities, it may be more practical to perform tests using smaller sized floating slabs. It is therefore important to fully understand the advantages and limitations of using such elements in the acoustic tests. This paper is organized as follows: in the next section a description of the setup is presented; there follows the analysis of the dynamic behaviour of the system; the measurement results obtained from testing floor coverings, floating floors and floating slabs are then displayed and comparison with standard results is carried out.
2. Setup description The test facility used in this work was built according to the procedure described in the Standard ISO/CD 16251-1. This setup is composed of a concrete slab with dimensions 1.2(m) 0.8(m) 0.2(m), simply supported on four elastic supports, as displayed in Fig. 1. A resilient material made of agglomerated rubber granulate, with a total thickness of 10 mm, was interposed between the slab and the supports. The same material was also placed between the room floor and the support so as to minimize vibration transmission from the hosting room. The effectiveness of this elastic material was evaluated by performing prior vibration measurements to estimate the vibration level difference between the room floor and the concrete slab. These measurements were carried out using a normalized tapping machine placed on the room floor and a set of accelerometer positions defined in the room floor and on upper surface of the slab. The results are displayed in Fig. 2, using a 1/3 frequency band scale, ranging between 12.5 Hz and 4000 Hz. The analysis of this plot allows identifying, in the lower 40 Hz frequency band, that the sound
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Fig. 1. Picture of the setup used to obtain impact sound reduction of floor coverings.
level in the floor and in the slab are very similar indicating that at this frequency no attenuation is provided. This frequency corresponds to the first resonance frequency of the spring-mass system. Above this value the elastic material provides an increased attenuation of the noise generated in the floor room. In order to understand the behaviour of this system, a set of preliminary measurements were performed, where the vibration level on the upper surface of the slab, excited by an impact point load (a wood hammer was used) was recorded. The results are displayed in Fig. 3a, in a linear scale, between 10 Hz and 1000 Hz. This result evidences the presence of two peaks. The first one was identified as the mass-spring-mass resonance frequency of the system. The first resonance frequency can be computed theoretically pffiffiffiffiffiffiffiffiffiffi as f0 ¼ 1=ð2pÞ k=m, where k = EA/L is the dynamic stiffness of the elastic support, E is the Young modulus, A and L are the surface area and thickness of the elastic material, respectively. For this purpose, the dynamic stiffness of the elastic material was evaluated experimentally following the procedure described in ISO 9052-1 [13] standard. From the result of that test, the theoretical resonance frequency calculated using the above equation is f0 ¼ 38 Hz, which matches well the plot in Fig. 3a. A finite element simulation was additionally performed in order to compute the first flexural mode of the slab, and to further confirm the resonance frequency of the mass-spring system. The mode shapes computed using the finite element software LISA are depicted in Fig. 3b and c, for which eigenfrequencies around 40 Hz and 550 Hz were obtained. Indeed, these figures allow a better understanding of the dynamic behaviour of the system, clearly revealing that a rigid body movement occurs for the lower frequency, and that a flexural movement of the slab is registered at the higher frequency.
3. Test procedure The measurement procedure consists of acquiring vibration levels in the lower surface of the slab, in 1/3 octave frequency bands, between 100 Hz and 3150 Hz. The ISO/CD 16251-1 technical document also prescribes that at least four accelerometer positions, randomly chosen, shall be used, distanced of at least 10 cm from the edges. The source used is a normalized tapping machine placed in at least two distinct different positions (with a minimum distance
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Fig. 2. Vibration measurements performed to evaluate the effectiveness of the elastic material placed between the room floor and the supports of the setup, so as to minimize vibration transmission from the hosting room.
of 30 cm from each other, and no hammer shall be closer to the edges of the plate than 10 cm). The measurement procedure is similar to that defined in the EN ISO 140-8, although here acceleration measurements are used instead. Firstly, the average vibration level is obtained from measurements recorded in the concrete slab without the floor covering specimen (La,0). Then the floor covering is applied on the concrete slab and the corresponding average vibration level provided by the tapping machine acting on the covering is achieved (La,1). The impact sound reduction is then obtained by performing the difference between these levels according to the following expression:
DLa ¼ La;0 La;1 ;
Fig. 3. Dynamic behaviour of the slab system when subject to a point impact load: (a) frequency response in terms of acceleration; (b) mode shape for the mass-spring vibration and (c) first flexural mode of the slab.
1,2
1,2
0,15
0,15 A5
A1
A2
A6
A2
A6
A3
A7
A3
A7
(a)
A8
A4
A5
0,1
A4
0,8
A1
0,1
P1
where La;0 ¼ 20 logða0 =aref Þ is the average vibration level obtained in the bare concrete slab (without the specimen); La;1 ¼ 20 logða1 =aref Þ refers to the vibration level obtained when the floor covering is applied; and aref ¼ 106 m=s2 , is the reference acceleration. Measurements of background vibrations should also be performed in order to evaluate whether those vibrations may influence the results. If the difference between the measured vibration levels with the tapping machine acting and the background vibration levels is less than 10 dB, then it may be necessary to perform corrections. The weighted sound reduction index of the floor covering is obtained by applying the procedure described in the ISO 717-2 as follows: the vibration level measured with the floor covering on the concrete slab is transposed to a reference concrete floor, 12 cm
0,8
P3 P2
ð1Þ
A8
(b)
Fig. 4. Positions of the normalized tapping machine and of the accelerometers: (a) upper surface of the concrete slab and (b) lower surface of the concrete slab.
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Fig. 5. Average impact sound reduction and standard deviation for the three source positions regarding the concrete slab with: (a) carpet; (b) floating floor using a polyethylene foam and (c) floating slab using a rubber resilient layer.
thick, with a weighted impact sound pressure level of 78 dB; then the weighted impact level provided by the floor covering applied on this floor, Lw,a,r, is obtained by adjusting the reference curve described in the ISO 717-2. The weighted sound reduction index is then given by:
DLa;w ¼ 78 Lw;a;r :
ð2Þ
4. Dynamic behaviour of the setup After building the above described setup, measurements were carried out in order to understand its dynamic behaviour and to define a measurement procedure. With this aim a grid of eight
accelerometer positions placed in the upper and lower surfaces of the slab, as displayed in Fig. 4, and a set of three source positions were defined (P1, P2, P3 as displayed in Fig. 4). The measurements were performed using an accelerometer B&K 4370 which allows measurements between 0.2 Hz and 3500 Hz, and a signal acquisition system Symphonie (01 dB) with the software dBBati 32 for signal treatment. The present section displays the average sound reduction curves and standard deviation using different accelerometer positions and source positions. Analysis of background noise is also performed to assess if any correction is required. The three types of impact sound treatment solutions are here addressed: resilient coatings, floating floors and floating slabs.
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Fig. 6. Average impact sound reduction when the tapping machine is acting on position P1 and standard deviation regarding the concrete slab with: (a) carpet; (b) floating floor using a polyethylene foam and (c) floating slab using a rubber resilient layer.
4.1. Influence of the tapping machine positions The influence of the tapping machine positions in the sound reduction curves was investigated by performing measurements over the eight accelerometer positions defined in the upper surface of the slab for the three defined source positions, identified in Fig. 4. Fig. 5 displays the average impact sound reduction curves and standard deviations with relation to source position, for the three types of impact sound reduction solutions here addressed. As expected, the average sound reduction curves provided by the carpet and the floating floor increase smoothly with frequency, behaving similarly as the ones obtained using a normalized procedure. In the case of the floating slab the response although with amplitudes increasing with frequency, displays oscillations. The standard deviation provided by the three source positions varies
between: 0.21 dB and 1.94 dB for the carpet; 0.25 dB and 2.87 dB for the floating floor and 0.24 dB and 3.04 dB for the floating slab. The larger values of the standard deviation are recorded in the 1/3 octave 2500 Hz and 3150 Hz frequency bands. In all other frequencies the deviations are never greater than 2 dB. 4.2. Influence of the accelerometer’s position The influence of the accelerometer positions was addressed at eight positions defined in the upper surface of the slab. Fig. 6 displays the average impact sound reduction obtained from measurements in these positions when the source is at position P1 and the corresponding standard deviation. Again the three types of impact sound attenuation solutions were tested. The standard deviation lies between: 0.33 dB and 5.36 dB for the carpet; 0.76 dB and
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Fig. 7. Average impact sound reduction for acceleration positions defined on the lower surface of the slab and on the upper surface of the slab: (a) concrete slab with carpet; (b) concrete slab with floating floor using a polyethylene foam and (c) concrete slab with floating slab using a rubber resilient layer.
6.30 dB for the floating floor and from 0.38 dB to 9.39 dB for the floating slab. The lower values are registered in the lower frequencies and they progressively increase for higher frequencies. In the measurement procedure, the eight accelerometer positions were always used. With respect to the acceleration position, although it is easier to perform records in the upper surface of the slab, it may be useful to register vibration levels in the lower surface when the specimen size covers a significant area of the slab surface. In order to assess differences in the records provided by these different positions, measurements were also performed using a similar grid of positions defined in the lower surface and in the upper surface of the slabs for the three tapping machine positions (see Fig. 4b). The average sound reduction curves obtained for these two grids of receivers (displayed in Fig. 7) reveal similar behaviours, although the curve obtained using the upper surface of the slab reveals a
smoother trend. In spite of the differences registered throughout the frequency range and of the smoother behaviour of the results from the top surface, it must be noted that positioning the accelerometers at the lower surface of the slab can be quite useful whenever the specimen covers a large part of the slab, hindering the use of the upper surface to perform measurements.
4.3. Influence of background noise Background noise was also evaluated using the carpet solution, since it is the best performer among the tested materials, and thus leading to the lowest vibration levels. The vibrations levels obtained with and without the tapping machine are plotted in Fig. 8. From the analysis of this figure one verifies that the average background level is always 40 dB bellow the vibration level
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Fig. 8. Average vibration level measured on the slab with the carpet with and without the tapping machine.
Fig. 9. Impact sound reduction provided by floor coverings obtained using the standard and simplified methodologies: (a) carpet; (b) vinyl with flexible underlayer; (c) standard vinyl and (d) linoleum.
measured with the tapping machine, and therefore no correction is required.
and sound reduction indexes obtained using both the ISO/CD 16251-1 and the ISO 140-8 procedures. 5.1. Floor coverings
5. Comparisons of results provided by the simplified and standard method The above described setup was used to evaluate the acoustic behaviour provided by the three types of solutions used to control impact sound transmission: floor coverings; floating slabs and floating floors. This section discusses the sound reduction curves
Several floor coverings were tested such as carpets, standard (rigid) vinyls, vinyls with flexible underlayers and linoleums (a total of 11 different specimens were tested). Among these coverings four were chosen to illustrate the behaviour of the system: a carpet; a standard vinyl; a vinyl with a flexible underlayer and linoleum. Fig. 9 illustrates the impact sound reduction curves for
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Fig. 10. Weighted impact sound reduction indexes obtained from the normalized vs. standard procedure for several floor coverings.
Fig. 11. Impact sound reduction curves provided by floating floors obtained using the standard and simplified methodologies: (a) floating floor with glued cork and (b) floating floor with a rubber 4.5 mm underlayer.
these coverings obtained with the simplified method and also with the normalized procedure (as described in the ISO 140-8). From the analysis of these figures one can observe that the sound reduction curves provided by the simplified method
approach those obtained using the ISO 140-8 standard procedure. In fact only in the higher frequencies it is possible to notice more significant differences, which are more pronounced for the coverings displaying a poorer acoustic performance (see Fig. 9c–e).
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Fig. 12. Weighted impact sound reduction indexes obtained from the normalized vs. standard procedure for several floating floors.
Fig. 13. Responses in 1/12 octave frequency bands provided by floating slabs with 0.5(m) 0.5(m) placed over an uniform rubber layer, 4.5 mm thick: (a) slab 4 cm thick and (b) slab 8 cm thick.
The comparison of the overall weighted sound reduction index between the ISO 140-8 and the ISO/CD 16251-1 (see Fig. 10) shows that for coverings displaying higher acoustic performances (between 15 dB and 19 dB), both indexes are very similar and a maximum deviation of 1 dB was obtained. For the materials with lower acoustic performance (bellow 10 dB), the deviations found were greater but always less than 2 dB from the standard results. These small differences are related to the procedure used to obtained the index, with the unfavourable deviations from the reference curve being located in the low and medium frequencies, where the curves provided by both methodologies approach. 5.2. Floating floors In this subsection we display the results obtained for the several floating floors tested (a total of 6 different floating floors were tested with resilient underlayers of cork, rubber, flexible polyurethane and polyethylene). Fig. 11 displays the sound reduction curves obtained using the simplified and standard procedures, for two of the floating floors tested, the first consists of a floor where the resilient cork underlayer is glued to its lower surface, whereas the second case corresponds to a floating floor resting on a rubber underlayer, 4.5 mm thick. For the floating floors the corresponding impact sound reduction curves provided by the simplified and standard methods display a similar behaviour, although in the medium and higher
frequencies greater differences are found. When comparing the weighted sound reduction indexes provided by the two procedures (see Fig. 12), we find that the larger deviations assume values of only 2 dB; again, this good agreement is mostly justified by the fact that the unfavourable deviations in relation to the reference curve (defined in the ISO 717-2) occur mainly in the low and medium frequency range, where both results approach.
5.3. Floating slabs In this subsection the results obtained when testing floating slab systems using the procedure of the ISO/CD 16251-1 document are discussed. So as to evaluate the influence of the floating slab dimensions, three slabs 0.04 m thick were built with the following dimensions: 0.5(m) 0.5(m); 0.8(m) 0.8(m) and 1.2(m) 0.8(m), exhibiting areas of 0.25 m2, 0.64 m2 and 0.96 m2 respectively. It is important to bear in mind that manufacturers display results for different floating slab thickness which may hamper the comparison of different solutions. Therefore an analysis of thickness influence was also performed for floating slabs with 0.5(m) 0.5(m) and varying thicknesses of 4 cm, 8 cm and 12 cm. The tests were carried out for two resilient underlayers: a uniform 4.5 mm rubber underlayer, whose sound reduction index given by the manufacturer is 20 dB and a non-constant thickness rubber underlayer 15/7 mm thick, with maximum and minimum
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thickness of 15 mm and 7 mm respectively, with a sound reduction index, given by the manufacturer of 24 dB. Fig. 13a displays the impact sound reduction obtained using a slab with 0.5(m) 0.5(m) and thickness of 4 cm placed over a resilient rubber layer with uniform thickness of 4.5 mm. This result is displayed in a narrow frequency scale of 1/12 octave bands to provide insight on the behaviour of this solution. As expected the impact sound reduction increases with frequency, however the curve displays a set of oscillations, which are related with the dynamic behaviour of the solution. In order to better understand these oscillations a test was carried out using a hammer as a source to excite the floating slab and acceleration measurements were recorded on the surface of this small sized floating floor. Fig. 13a also displays the corresponding average vibration level. This result was shifted downwards of 80 dB to provide comparison between the two curves. Comparing these two results we find that they display several oscillations and in most frequencies they coincide denoting that, in fact, to an increase in the vibration level of the floating slab corresponds a drop in the impact sound reduction provided by the solution. The oscillations in 1/12 octaves of 777 Hz and 1959 Hz are particularly sharp and will be noted in the following responses calculated in 1/3 octave frequency bands. We may also note from the experimental analysis that the mass-spring-mass resonance frequency is situated on the low frequency range at about 80 Hz. A similar test was performed using a floating slab of 8 cm thick and the results, displayed in Fig. 13b allows similar
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conclusions. In this case the resonance frequency of the system is situated, as expected, at a lower frequency in relation to the 4 cm floating slab, at about 50 Hz. Fig. 14a1 and b1 display the impact sound reduction obtained for the tested floating slabs, 4 cm thick, with different dimensions. In these figures the standard result (using a floating slab with 10 m2) was also included for reference. From the analysis of Fig. 14a1, where the resilient layer is composed of a rubber material with 4.5 mm, it is possible to verify that the curves provided by the different slabs, although showing a tendency to approach the normalized curve, display several oscillations throughout the frequency range, that were previously discussed, and clearly influence the 1/3 octave response. For smaller dimensions these oscillations are situated in the higher frequencies, and as the slab dimensions increase they move to lower frequencies, although still within the frequency range of interest. From the three slab dimensions analysed, the curve that best approaches the normalized result is that provided by the larger slab, measuring 1.2(m) 0.80(m). The results shown in Fig. 14b1, concerning tests performed using a resilient layer with a variable thickness 15/7, also exhibit a set of oscillations regarding the normal modes of the slabs, as explained previously. In this case, the impact sound reduction provided by the smaller slab, with 0.50(m) 0.50(m), displays increased amplitudes in relation to the normalized result. As the size of the slab increases, the resulting curve tends to approach the normalized result.
Floating slabs, 4 cm thick
Floating slabs with 0.50(m)x0.50(m)
(a1)
(a2)
(b1)
(b2)
Fig. 14. Impact sound reduction provided by the several tested floating slabs, with different dimensions: (a) floating slab over uniform rubber layer, 4.5 mm thick and (b) floating slab over rubber layer, 15/7 mm thick.
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Measurement according to the ISO 140-8 using a floating slab with 0.4x0.6x0.05
Measurement according to the simplified procedure using a floating slab with 0.5x0.5x0.04
Fig. 15. Impact sound reduction obtained for a concrete floating slab over several flexible polyurethane foams, with different densities and thicknesses, using two different measurement procedures.
The results for different thicknesses of the floating slab with 0.5(m) 0.5(m) are plotted in Fig. 14b1 and b2. From the analysis of these figures one verifies that when the thickness changes from 4 cm to 8 cm, the impact sound reduction increases significantly and in the higher frequencies strong oscillations appear in the curve related to the greater thickness which are related to modal behaviour of the slab. For the floating slab, 12 cm thick, the resulting impact sound reduction curve displays, however, smaller amplitudes than the 8 cm thickness slab. This behaviour may be related to the greater weight of this slab that is compressing the resilient layer, promoting an increase in sound transmission to the support slab. When the resilient layer is a rubber mat, 15/7 mm thick, the results indicate, again an increase in the acoustic behaviour when the thickness increases from 4 to 8 cm. With the increase of thickness from 8 to 12 cm the corresponding results tend to approach. The last figure (Fig. 15) displays the results provided by a floating slab over a flexible polyurethane foam with densities of 120 kg/ m3 and 200 kg/m3 respectively and varying thicknesses of 5, 10 and 30 mm. These solutions were tested using two methods: (i) the first consists of performing the test following the procedure described in the NP EN ISO 140-8, but using a small floating slab 0.4(m) 0.6(m) 0.05(m) [14]; (ii) the second method follows the procedure given in the ISO/CD 16251-1 using the floating slab with 0.5(m) 0.5(m) 0.04(m). These tests were performed with the aim of assessing the possibility of perceiving similar conclusions.
From the analysis of the results provided by Fig. 15 we verify that the curves obtained from both methods are not smooth and display oscillations related with the vibration modes of the floating slab. Interestingly, we find that the tests provided by both methods allow the same conclusion: the polyurethane foam with 30 mm provides the best acoustic behaviour while the foam with thickness 10 mm displays the smaller amplitudes of impact sound reduction. From the analysis of the results achieved using the EN ISO 140-8 procedure we see that the increase in the foam thickness allows an increase in impact sound reduction mainly in the lower and medium frequencies, while for the results provided by the ISO/CD 16251-1 this behaviour is not noticed, and the sound reduction improvement occurs more uniformly throughout the frequency range. 6. Conclusions This paper reports the results obtained from the implementation of an experimental procedure described in the ISO/CD 16251-1 technical document that may be used to predict impact sound reduction using small sized specimens on heavy-weight floors. In this method, vibration levels are recorded on a concrete slab, of reduced dimensions, without and with the floor covering when a normalized tapping machine is acting. The impact sound reduction is then obtained from these vibration measurements by performing the difference between them. The setup was then used to test several specimens of floor coverings, from resilient coverings, floating floors and floating slabs.
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The impact sound reduction results obtained for resilient coverings were found to approach those obtained using the normalized procedure defined in the EN ISO 140-8. The weighted sound reduction provided by both methods was also calculated and the results provided by the method defined in the ISO/CD 16251-1 technical document provided maximum differences of 2 dB in relation to those obtained from the normalized method. Similar conclusions were drawn from the tests performed on the floating floors also displaying a local reactive behaviour. The tests performed on the floating slabs allowed the identification of differences in relation to the normalized curves. Although the curves display a trend to approach the standard curves, oscillations are identified due to the vibration modes of the reduced sized slab, indicating a plate like behaviour. These oscillations are positioned in the frequency range of interest also for the floating slab with the dimensions of the bar slab. Although not allowing a direct comparison of both these and the normalized results, the tests performed on this setup using floating slab may allow a comparison among different resilient materials and therefore may be used to perform tests in a preliminary stage of research. References [1] Schiavi A, Pavoni Belli A, Corallo M, Russo F. Acoustical performance characterization of resilient materials used under floating floors in dwellings. Acta Acustica united Acustica 2007;93:477–85. [2] Asdrubali F, D’Alessandro F. Impact sound insulation and viscoelastic properties of resilient materials made from recycled tyre granules. Int J Acoustic Vibration 2011;16(3):119–25.
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[3] F. Asdrubali, F. D’Alessandro, S. Schiavoni, G. Baldinelli, Lightweight screeds made of concrete and recycled polymers: acoustic, thermal, mechanical and chemical characterization, in: Proceedings of FORUM ACUSTICUM 2011, 27 June - 1 July 2011, Aalborg, Denmark (in CD-Rom). [4] Rushforth I, Horoshenkov K, Miraftab M, Swift M. Impact sound insulation and viscoelastic properties of underlay manufactured from recycled carpet waste. Appl Acoustics 2005;66:731–49. [5] Standard ISO 140-8: 1997(E). Acoustics-measurement of sound insulation in buildings and of building elements - Part 8: Laboratory measurements of the reduction of transmitted impact noise by floor coverings on a heavyweight standard floor. [6] Standard ISO 717-2: 1996. Acoustics - rating of sound insulation in buildings and of building elements - Part 2: impact sound insulation. [7] Godinho L, Masgalos R, Pereira A, Branco FG. On the use of a small-sized acoustic chamber for the analysis of impact sound reduction by floor coverings. Noise Control Eng J 2010;58(6):658–68. [8] ISO/CD 16251-1: Acoustics-Laboratory measurement of the reduction of transmitted impact noise by floor coverings on a small floor mock-up - Part 1: Heavyweight compact floor. [9] O-H. Bjor, Simplified measurement of the reduction of transmitted impact noise by floor coverings, Proc 2010, BNAM 2010 - Bergen. [10] R. Foret A, J. Chéné, C. Guigou-Carter, A comparison of the reduction of transmitted impact noise by floor coverings measured using ISO 140-8 and ISO/CD 16251-1, Forum Acusticum 2011, Aalborg, Denmark, 28 Junho a 1 Julho de 2011, pp. 1371–76. [11] Miškinis K, Dikavicˇius V, Ramanauskas J, Norvaišiene˙ R. Dependence between reduction of weighted impact sound pressure level and specimen size of floating floor construction. Mater Sci 2012;18(1):93–7. [12] Standard ISO 140-7: 1998(E). Acoustics-measurement of sound insulation in buildings and of building elements - Part 8: Field measurements of impact sound insulation of floors. [13] Standard ISO 9052-1: 1989. Acoustics-determination of dynamic stiffness Part 1: Materials used under floating floors in dwellings. [14] N. Sousa, Assessment of impact sound insulation provided of solutions using polyurethane foam, MSc Thesis, FEUP, Portugal, 2008.