Façade Elements for Natural Ventilation and Sound Insulation. Bianca C. D. de Araújo, Sylvio R. Bistafa. Reprinted from JOURNAL OF BUILDING ACOUSTICS

Transcription

1 Façade Elements for Natural Ventilation and Sound Insulation by Bianca C. D. de Araújo, Sylvio R. Bistafa Reprinted from JOURNAL OF BUILDING ACOUSTICS Volume 19 Number MULTI-SCIENCE PUBLISHING CO. LTD. 5 Wates Way, Brentwood, Essex CM15 9TB, United Kingdom

2 BUILDING ACOUSTICS Volume 19 Number Pages Façade Elements for Natural Ventilation and Sound Insulation Bianca C. D. de Araújo 1, Sylvio R. Bistafa 2,a 1 School of Architecture - Federal University of Rio Grande do Norte, R.G. do Norte, Brazil 2 Polytechnic School - University of São Paulo, São Paulo, Brazil a Corresponding Author: sbistafa@usp.br (Received 4 December 2010 and accepted 23 September 2011) ABSTRACT The indoor environmental comfort requires nowadays the pursuit of architectural alternatives with less energy demand. In hot and humid climates, natural ventilation is one of the design strategies that can be applied to reduce energy consumption in buildings. According to this vision, elements with apertures (cobogó or combogó, as they are popularly known in northeastern Brazil) are architectural elements that provide permanent natural ventilation, sun protection and natural lighting. However, there are acoustic problems associated with the sound insulation that they provide. The present study had the goal of developing elements with apertures to mitigate the conflict between the thermal-acoustic comfort demands. The sound insulation performance of the elements was evaluated in situ according to the ISO standard. The natural ventilation characteristics of the elements were investigated by means of computer simulations, using methods of computational fluid dynamics - CFD. Four types of elements had been proposed, which were acoustically evaluated in different mounts on a wall-like facade. The results showed how difficult is to control the transmission of sound at low frequencies in facades with apertures, however, satisfactory sound insulation results were obtained with one type of mounting, which gave an Weighted Standardized Level Difference D nt, w value of 27 db. The CFD simulations show that because of its geometrical characteristics, this mounting also provides the best flow conditions for internal natural ventilation, despite the fact that its % of open area of 12% is not the highest amongst the proposed elements. Calculated values of measures of comfort ventilation show that the mountings with the best sound insulation characteristics are also capable of providing adequate values of ventilation ratios and air changes per hour in the test suite that was used for measuring sound insulation. Keywords: building acoustics, architectural acoustics, facade elements with apertures, sound insulation of facades, natural ventilation in facades, acoustic and thermal comfort in buildings, comfort ventilation.

3 26 Facade Elements for Natural Ventilation and Sound Insulation 1. INTRODUCTION In the preliminary studies of a new architectural design, passive alternatives should be considered regarding the demands of the energy systems. These alternatives are based on technical concepts of environmental comfort that have justification in the bioclimatic architecture. With that goal in mind, it is necessary that the building elements and components make use of the potentiality of the existing climate to fulfill the thermal requirements of the built-in environment. In the Brazilian northeastern tropical regions near sea-shore, the climate is hot and humid, with small fluctuations of daily and seasonal temperatures, being the relative humidity often high. In these regions, the buildings should be shaded to avoid external heat gain. Additionally, natural ventilation, wherever possible, should be used to cool the building, heated not only by solar radiation but by internal heat gains as well. Natural ventilation may be desirable, even when the outdoor air temperature is higher than the indoor air temperature. [...] the simplest strategy for improving comfort when the indoor temperature, under still air conditions, is felt as too warm is to open the window and to enhance comfort by ventilation: providing comfort through higher indoor air speeds. Introducing outdoor air with a given speed into a building may provide a direct physiological cooling effect even when the indoor air temperature is actually elevated. This is particularly the case when the humidity is high, as the higher speed increases the rate of sweat evaporation from the skin, thus minimizing the discomfort people feel when their skin is wet. Such comfort ventilation may be desirable, from the physiological viewpoint, even when the outdoor air temperature is higher than the indoor air temperature, because the upper temperature limit of comfort is shifted upward with a higher air speed. Therefore, even if the indoor air temperature is actually elevated by ventilation with a warmer outdoor air, the effect on the comfort of the occupants, up to a given temperature limit, might be beneficial. This is specially the case in hot, humid regions[...]. (Givoni, 1994, pp. 38). However, the use of natural ventilation, to control the built-in environmental comfort in places with hot and humid climate, requires constructive solutions and passive elements at costs that encourage their adoption in the popular tropical regions. There is a passive element that meets these requirements, popularly known in the northeastern of Brazil as cobogó or combogó, with potential to creative solutions in the architectural design. Besides ventilation, these elements provide sun protection and can filter the intense natural light of low latitudes. All these characteristics embedded in a single component of easy manufacturing and at low cost. Perhaps the characteristic that hinder the most the use of cobogós in the natural ventilation strategy is its intrinsic poor sound insulation. Despite the fact that elements with apertures have been in use in Brazil for a long time, there is no known study that had considered the combined acoustic and thermal effect of cobogós on the comfort of the built-in environment. The question of how to solve the conflicting requirements of acoustic and ventilation is a recurring aspect of elements with apertures that needs to be evaluated.

4 BUILDING ACOUSTICS Volume 19 Number An alternative is to conceive cobogós with the acoustics requirements in mind, that allows for natural ventilation and with as much as possible blockage of external noise. In this context, the present study proposes some types of cobogós that have been evaluated acoustically in a wall-like facade, which have been developed to mitigate the conflict between ventilation and sound insulation requirements. Elements with apertures have been in use in Brazil since the 1930s, when the architects Amadeu Coimbra, Ernest and Anthony Boekman Góis used them in a building in the city of Olinda Pernambuco. The name cobogó was created from the first syllable of their last names. Figure 1 shows an image of the cobogó-lateral-facade of that building, which was recently renovated. Figure 1. Image of the first cobogó-lateral-facade in a building in city of Olinda Pernambuco/Brazil Source _b6a7f517cf.jpg.

5 28 Facade Elements for Natural Ventilation and Sound Insulation Over the years, the use of cobogós has spread throughout the national territory. Initially, they were molded in mortar, cement and sand, in molds of wood or clay, and dried in ovens. As shown in Figure 2, nowadays they are fabricated from different materials, including glass, ceramic, aluminum, wood and others. Figure 3 shows the cobogó-facade of Conjunto Residencial Parque Guinle in Rio de Janeiro. Along with trellises and louvers, the cobogós were adopted by the modernist Figure 2. Images of cobogós commercially available in Brazil. Figure 3. Image of the cobogó-facade of Conjunto Residencial Parque Guinle in Rio de Janeiro/Brazil (Source projetos_int.php?id_projetos=35).

6 BUILDING ACOUSTICS Volume 19 Number architecture in residential, educational and public buildings throughout Brazil, with aesthetic results rescuing the characteristics of the buildings from the colonial period. From the 1960s onwards, the use of elements with apertures began to decline due to the adoption by the architects of glazed facades, typically found in the northern hemisphere needless to say, highly inappropriate to the Brazilian climate and economic reality. The use of cobogós has been based on empiricism, sometimes with unsatisfactory results, which have undermined the potential use of these elements. The few available studies have focus on the ventilation characteristics of cobogós (e.g. Bittencourt, 1995). There is no study known to the authors that have explored the conflicting aspects of ventilation and sound insulation of cobogós. 2. FACADE ELEMENTS FOR NATURAL VENTILATION AND SOUND INSULATION Four types of blocks have been proposed to build a wall-facade with apertures for ventilation, capable of partially blocking the sound transmission: Small-Box (SB), Large- Box (LB), Small-Box with Slit (SBS), and Large-Box with Slit (LBS). Figure 4 shows the schematics of the proposed blocks. The geometry of the blocks is simple, for keeping the costs of building the mold (in wood or steel) as low as possible. Also, the idea is that they should resemble regular blocks that could be used in building construction as well, if needed. The blocks were made from cement and sand mixed in a 1:3 ratio, providing mm mm m mm mm mm mm mm mm mm mm mm mm Small-box (SB) Large-box (LB) mm mm mm mm m mm mm mm mm mm Figure mm Small-box with slit (SBS) Schematics of the proposed blocks mm mm Large-box with slit (LBS)

7 30 Facade Elements for Natural Ventilation and Sound Insulation adequate stacking resistance, impermeability, and so they could be painted. The manufacturing follows traditional procedures for making regular blocks for constructions. Openings, vents, etc are elements that interrupt the homogeneity of a partition, and can significantly degrade its sound insulation performance. An open area of only 1% can reduce the sound insulation of a homogeneous partition from 30 to around 20 db. To build a facade with apertures for natural ventilation, and at the same time to keep the sound transmission through the facade low, the blocks were mounted together in Cavity Cavity Aperture Cavity (a) Mounting scheme of the blocks SB and LB mounted sideways in the wall-facade, with indication of the location of the cavities (b) Mounting scheme of the blocks with slits SBS and LBS Figure 5. Images of the mounting schemes of the proposed blocks.

8 BUILDING ACOUSTICS Volume 19 Number different ways as shown in Figure 5. As can be seen in Figure 5a, Blocks SB and LB were mounted sideways in the wall, with a cavity on each side of the aperture. Since these mountings provide unobstructed passages for the sound through the apertures, the mechanism for the attenuation of sound in its way through the apertures, would supposedly be by means of the dissipation of the acoustical energy in the reverberant field inside the cavities formed by the blocks. To explore this supposition further, in certain types of mountings, the cavities were partially filled with fibrous soundabsorbing material (glass-fiber), for increased sound absorption (Figure 5c). Although this material cannot be considered low-cost, and if it would have contributed to the sound insulation of the facade, the idea is to use, in real applications, fibers available locally such as coconut and palm tree fiber. The blocks with slits (SBS and LBS), when mounted as shown in Figure 5b, gives a longer and tortuous path for the sound, which would result in increased sound attenuation when compared to the straight paths of the previous type of mountings. Here, sound is also dissipated by absorption in the reverberant field inside the cavities formed by the blocks with slits, which can also be filled with sound-absorbing material. As shown in Figures 6 and 7, the blocks were mounted in a wall-like facade of a rectangular room (test suite) with a plan area of 15 m 2. The total area of the wall-facade was around 7.50 m 2, and the area occupied by the blocks was around 4.20 m 2 (1.40 m high by 3.00 m wide). Table 1 shows the types of mountings that were tested, the width of each aperture and the percentage of the open area of each type of mounting. In order to derive a baseline for sound insulation (maximum sound insulation of the wall-facade without apertures), all the blocks were tested in the facade without apertures, by mounting blocks SB and LB side-by-side and with the cavities facing the inside of the room (the slits of blocks SBS and LBS were also facing the inside of the room). Figure 6. Outside view of the test suite with the wall like facade partially mounted with one type of block (block LB).

9 32 Facade Elements for Natural Ventilation and Sound Insulation West facade Source South facade 5.00 Internal area P4D P1D 1.50 Test champer A = 15 m 2 height = 2.50 m P3D Test area 4.20 m 2 ( m) 1.50 P5D P2D North facade Outside P3F P2F P1F External area free field 3.00 East facade Roof projection Figure 7. Floor plan of the test suite used to measure sound insulation with indication of the microphone positions in red. 3. EXPERIMENTAL PROCEDURES The evaluation of sound insulation was experimentally accomplished with measurements in-situ according to the procedures established in the ISO Standard (1998). The rating of sound insulation was accomplished by applying the methods established in the ISO Standard (1996). All the equipment and measurement procedures followed the recommendations of the ISO Standard (1998). The loudspeaker was positioned outside the room, on the ground, at a distance of 3.5 m from the facade, with its axis making an angle of 45 with the normal to the wall-facade. Sound pressure levels in 1/3-octave frequencybands were measured at 3 points outside, and at 5 points inside the receiving room, which were then position-averaged. The background noise was always 10 db below the noise level generated by the loudspeaker. Reverberation times inside the receiving room were obtained from decay curves in 1/3-octave frequency-bands. By applying the Sabine formula, the equivalent absorption area of the room was estimated in 1/3-octave frequency-bands from the measured reverberation times. The main equipment used during the measurements were the following: sound level meter type 2 SOLO SLM with 1/3-octave band filters (12,5 Hz 20 khz) and the

10 BUILDING ACOUSTICS Volume 19 Number Table 1. Types of mounting tested with the proposed building-blocks. Width of Height of Type of each each % of block Condition of aperture aperture Mounting Open open used the cavities (mm) (mm) denomination Area (m 2 ) area* SB SBclosed 0 0 With glass-wool SB5gw Without glass-wool SB LB LBclosed 0 0 With glass-wool LB5gw Without glass-wool LB LB LBclosed 0 0 With glass-wool LB10gw Without glass-wool LB SBS SBSclosed 0 0 With glass-wool SBSgw ** Without glass-wool SBS ** LBS Without glass-wool LBS ** * The percentage of the open area is the ratio between the total area of the apertures and the total area of the facade occupied with the blocks (4.2 m 2 ). In the case of blocks SB and LB, the open area is given by the total area of the apertures. As can be seen in Figure 5a, these apertures are created right above and right below the region where two blocks are joined at the closed faces of two opposing cavities. **As can be seen in Figure 5b, for the blocks SBS and LBS, the open are cannot be considered as such, because there is no straight through passage for the air-flow. In these cases, the open area considered is a nominal area, given by the total area of the slits in one face of the wall. loudspeaker CSR 5500XA with a power of 200 W RMS and a frequency response in the range of 40 Hz to 20 khz. The reverberation times were measured by the interrupted white-noise method, by means of the preprogrammed setups of emission and reception of the SOLO SLM. 4. ANALYSIS AND DISCUSSION OF SOUND INSULATION RESULTS The Weighted Standardized Level Difference (D nt,w ), calculated according to the method established in the ISO Standard (1996), was used to compare the overall sound insulation performance of the mountings as a single-number (figure of merit). Values of D nt,w for each type of mounting tested are shown in Table 2. As expected, the closed mountings provide more sound insulation than the mountings with apertures, with D nt,w values of 35, 37 and 40 db, for the mountings SBclosed, SBSclosed and LBclosed, respectively. Mounting LBclosed provides the highest sound insulation among the closed mountings, because the blocks LB used in

11 34 Facade Elements for Natural Ventilation and Sound Insulation Table 2. Values of D nt,w for each type of mounting tested. Type of mounting D nt,w (db) % of open area SBclosed 35 0 SB5gw SB LBclosed 40 0 LB5gw 25 8 LB LBclosed 40 0 LB10gw LB SBSclosed 37 0 SBSgw SBS LBS 27 9 this type of mounting have thicker walls than blocks SB used in the mounting SBclosed (see the schematics of these blocks in Fig. 3). Although the small blocks with slits (SBS) are as thick as blocks LB, its D nt,w value of 37 db is lower because of the slits, which have the effect of reducing the total wall-mass. The mountings with apertures have D nt,w values considerable lower than the closed mountings, with D nt,w values ranging from 21 to 27 db. Among the mountings with apertures, mountings SB5gw and SB5 shows the lowest values for D nt,w because these mountings have the greater values of the % of open area of 19%. The blocks with slits show consistently better sound insulation than the other blocks. In fact, mountings SBSgw, SBS and LBS have the highest D nt,w values of 27 db, with % of open area of 12, 12 and 9%. Apparently, as can be seen in Table 2, there is no contribution of the glass-wool to the sound insulation when one compare mountings SB5/SB5gw and SBS/SBSgw. As a matter of fact, there is a counter-effect of reducing the D nt,w values of the mountings LB5/LB5gw and LB10/LB10gw by 1 and 2 db, respectively. Since this counter-effect cannot be justified on theoretical grounds, it can be attributed to the simplification of comparisons based on single-numbers. This in fact seems to be the case. The standardized level difference, D 2m,nT [ISO Standard (1998)] was used for comparisons of sound insulation in frequency-bands among the different mountings tested. Figure 8 shows the standardized level difference, D 2m,nT in octave 1/3-octaves frequency bands of mountings with and without glasswool in the cavities. For the mountings SB5 and SBS, there is an obvious increase in sound insulation, particularly at the higher frequency-bands above around 400 Hz, when glass-wool was inserted into the cavities of these mountings, despite the fact that their D nt,w values remained the same at 21 and 27 db, respectively. However, as can be seen in Figure 8, the insertion of glass-wool into the cavities of mountings LB5 and LB10 seem not to have been beneficial to the sound insulation in

12 BUILDING ACOUSTICS Volume 19 Number SB5-Dnt, w = 21 db SB5gw-Dnt, w = 21 db 30 db /3-octave band center frequency LB10-Dnt, w = 25 db LB10gw-Dnt, w = 23 db 30 db /3-octave band center frequency LB5-Dnt,w = 26 db LB5gw-Dnt, w = 25 db 30 db /3-octave band center frequency Continued

13 36 Facade Elements for Natural Ventilation and Sound Insulation SBS-Dnt,w = 27 db SBSgw-Dnt, w = 27 db 30 db Figure /3-octave band center frequency Standardized level difference, D 2m,nT in octave 1/3-octaves frequency bands of mountings with and without glass-wool in the cavities. the higher frequency-bands, and apparently, had the effect of provoking a reduction of sound insulation in the 1/3-octave frequency bands of 400 and 500 Hz. In these cases, the net result is a reduction in the D nt,w values of 1 and 2 db, respectively. The reduction of sound insulation in these two octave frequency-bands seem consistent and might have been caused by interference mechanisms between cavity-aperture. At the higher frequency-bands, the sound insulation remained approximately the same after the insertion of glass-wool into the cavities of mounting LB5, whereas have provoked a reduction in the sound insulation of mounting LB10. No physical explanation could be found for these mixed results, which may be attributed to the experimental uncertainties. Since one can say that the insertion of the glass-wool into the cavities have an overall beneficial effect, and particularly for mounting SBS (as can be seen in Figure 8), which have resulted in the highest D nt,w value of 27 db of the mountings with apertures, and for the sake of not crowding the plots with too many types of mountings, comparisons from now on will be made amongst mountings with glass-wool, against the closed mounting with the highest sound insulation (closed mounting with the highest D nt,w value). Figure 9 shows the standardized level difference D 2m,nT of these mountings, in octave 1/3-octaves frequency bands. This is a best case scenario because sound insulation of the mountings without glass-wool in the cavities is generally lower. As can be seen in Figure 9, and comparing with the blind facade (LBclosed), the sound insulation of the mountings with apertures are much lower, with differences as high as 25 db in the 160 Hz 1/3-octave frequency-band. In this frequency-band, the D 2m,nT value is only around 3 db for all mountings with apertures. Above this frequency-band and up to the 400 Hz frequency-band, the differences become smaller, but increase again continuously from 400 to 5000 Hz, in different degrees depending on the type of mounting. These frequency-ranges are different for mounting SB5gw. At the higher frequency-bands above around 500 Hz, the mounting SBSgw stands out as the

14 BUILDING ACOUSTICS Volume 19 Number db LBclosed - Dnt, w = 40 db SBSgw - Dnt, w = 27 db LB5gw - Dnt, w = 25 db /3-octave band center frequency LB10gw - Dnt, w = 23 db SB5gw - Dnt, w = 21 db Figure 9. Standardized level difference, D 2m,nT in octave 1/3-octaves frequency bands of the closed mounting with the highest sound insulation (closed mounting with the highest D nt,w value of 40 db) and mountings with apertures with glass-wool into the cavities. most sound insulating amongst the mountings with apertures, whereas at the lower frequency-bands below around 500 Hz, the mounting LB5gw stands out as the most sound insulating. The close D nt,w values of these two mountings of 27 and 25 db does not reveal, of course, this unbalanced frequency-wise sound insulating characteristic. Despite the better sound insulation characteristics of these two mountings with apertures, their % of open area are lower, with values of 12% for the SBSgw and 8% for the LB5gw, whereas the two other mountings with apertures LB10gw and SB5gw, which are less sound insulating, have higher % of open area of 13% and 19%, respectively. Notwithstanding mitigated, there is still a conflict between sound insulation and % of open area, which is decisive factor for the promotion of natural ventilation. 5. RESULTS AND DISCUSSION OF COMPUTER SIMULATIONS OF NATURAL VENTILATION Computer simulations of natural ventilation in facades with apertures were accomplished with methods of computational fluid dynamics - CFD, using the Phoenics software, version 3.6. This software works with three basic modules: Pre-Processor, Solver and Post-Processor. The model-facade (geometry and dimensions) is inserted in the Pre-Processor module, which is assumed being subjected to a uniform windvelocity in one vertical face of the facade this can be understood as a virtual wind-

15 38 Facade Elements for Natural Ventilation and Sound Insulation tunnel model, with the facade in the vertical plane in the wind-tunnel test-section. Grid settings and parameters are also inputted in the Pre-Processor module. The Solver module deals with the numerical solution of the model, and the Post-Processor module presents the results of the simulation in different formats (scientific images and plots). Figure 10 shows scientific images of the CFD simulations results of air velocity across facades with some of the mountings that presented the best sound insulation performance, namely SB5, LB5, and SBS, and for an incident air-velocity of 2.5 m/s. These results can be considered reliable because, according to the experience gained in Velocity 1.300E E E E E E E E E E E E E E E E E + 00 Probe value 0.000E + 00 Average value 3.231E + 00 (a) 3D-view of the SB5 mounting with color visualization of air-velocity in an horizontal plan across the facade Velocity 2.500E E E E E E E E E E E E E E E E E + 00 Probe value 0.000E + 00 Average value 4.703E + 00 (b) Color visualization of air-velocity in an horizontal plan across the facade with mounting LB5 Continued

16 BUILDING ACOUSTICS Volume 19 Number Velocity 3.500E E E E E E E E E E E E E E E E E + 00 Probe value 1.775E + 01 Average value 7.936E + 00 Z (c) Color visualization of air-velocity in a vertical plane across the facade with mounting SBS Y X Velocity 3.454E E E E E E E E E E E E E E E E E + 02 Probe value 1.755E + 01 Figure 10. (d) 3D-color visualization of air-velocity across the facade with mounting SBS Scientific images of the CFD simulations results of air-velocity in m/s across facades with mountings of the type SB5, LB5, and SBS, for an incident air-velocity of 2.5 m/s. these types of CFD simulations, the residuals were small, which means that the convergence of the data was satisfactory. It can be seen in these images, that the average velocity across the apertures of the SB5 mounting is 3.2 m/s, 4.7 m/s for the LB5 mounting and 7.9 m/s for the SBS mounting.

17 40 Facade Elements for Natural Ventilation and Sound Insulation In all cases, the flow accelerates across the apertures, with subsequent separation as the flow leaves the wall-facade internally. The effect of this local flow acceleration as the air-stream crosses the apertures can still be noticed internally to the room, with regions of flow velocity higher than the incident flow velocity, but that fall continuously as the flow spreads from the wall-facade internally throughout the room. However, the average velocity internally, considering the whole facade area, is lower than the velocity of the incident air-stream. This effect is called load-loss and is due to the dissipation of the flow kinetic energy to overcome the losses that occur of any obstruction imposed to the free flow. The highest average velocity occurs with mounting SBS, where the pressure distribution inside the cavities of this type of block, tends to re-direct the flow upwards. This behavior, associated with the significant flow acceleration provided by this type of mounting, would also contribute to provide some degree of internal natural ventilation. According to Givoni (1994, pp. 38), [...] the absence of ventilation in buildings can lead to high humidity and low oxygen levels. To eliminate the possibility of harmful effects developing, it is essential to introduce fresh air. This is termed as comfort ventilation. It is a direct physiological effect created for example by apertures to let the wind in and thus providing a higher indoor air speed which makes the people inside the building feel cooler [...]. The ventilation ratio and air changes per hour are measures of comfort ventilation. For instance, Bittencourt and Cândido, 2000, recommend a minimum ventilation ratio of 130 m 3 /hour (0,036 m 3 /s) per person, for spaces of short stay and with no activities, in order to ensure user comfort ventilation. The ventilation ratio express the flow rate of outside air brought into a building. When the ventilation ratio is normalized by the volume of the space being ventilated, the result is the air changes per hour. The ventilation ratio VR (in m 3 /hour) and air changes per hour ACH can be calculated by applying the following formulas (Lamberts et. al., 2000) VR VR = A V; ACH = 3600, Vol where A is the effective area of the aperture available for ventilation (in m 2 ), V is the air flow velocity in the aperture (in m/s), and Vol is the volume of the space (37.5 m 3 in the present case). These measures have been estimated by considering the total area of the apertures available for ventilation in a particular type of mounting, and the air flow velocity right after leaving the apertures as given by the computer simulations. Table 3 presents the results thus obtained for the mountings the SB5, LB5, and SBS. The results presented in Table 3 show that the ventilation ratio of the mountings that presented the best sound insulation, would provide minimum recommended ventilation

18 BUILDING ACOUSTICS Volume 19 Number Table 3. Comfort ventilation results for the mountings SB5, LB5, and SBS. Aperture Area of Total area Metrics for comfort ventilation air each of Ventilation Type of velocity aperture No. of apertures ratio ACH mounting V (m/s) (m 2 ) apertures A (m 2 ) VR (m 3 /s) (air changes/hour) SB5 8, ,3 LB5 21, ,273 26,2 SBS 28, ,273 26,2 ratios, for 5 people with the SB5 type of mounting and for 7.6 people with the LB5 and SBS types of mounting. Despite providing the greater total area of apertures, mounting SB5 generated the lowest ventilation ratio of m 3 /s, amongst the three types of mountings. For the mounting types LB5 and SBS, the differences in the total area of apertures have been compensated by the differences in air velocities, which have resulted in equal values for the ventilation ratio of m 3 /s. The corresponding estimated air changes per hour in the test suite are 17,3 with the SB5 type of mounting and 26,2 with the LB5 and SBS types of mountings. Indoor environments naturally ventilated can easily reach air changes per hour greater than two. These smaller air changes are sufficient for the dispersion of odors and other domestic pollutants (Bittencourt and Cândido, 2000). Finally, it is important to point out that the measures of comfort ventilation thus estimated are specific for the experimental suite used for the acoustic tests, considering the area of the facade available for mounting the blocks, the resulting total area of apertures, and the volume of this room. Most important, they were all calculated from the computer simulations results of flow velocities through the apertures, which have been based on an incident wind speed of 2,5 m/s over the facade. Of course, these estimates would vary depending on the specific conditions in which these mountings would be applied to. 6. SUMMARY AND CONCLUSIONS Four types of elements (blocks) were proposed to mount different facades with apertures to promote natural ventilation and sound insulation. In some types of mountings, the block cavities were partially filled with glass-wool, as a mean of promoting dissipation of the acoustical energy in the reverberant field inside the cavities formed by the blocks. The evaluation of sound insulation was experimentally accomplished with measurements in-situ according to the procedures established in the ISO Standard (1998). Two metrics were used for rating sound insulation: the Weighted Standardized Level Difference (D nt,w ), calculated according to the method established

19 42 Facade Elements for Natural Ventilation and Sound Insulation in the ISO Standard (1996), and the Standardized Level Difference, D 2m,nT, calculated according to the ISO Standard (1998). It has been found that the Weighted Standardized Level Difference, as a singlenumber type of metric, does not reveal the differences in the sound insulation characteristics of the mountings that were tested. A more comprehensive picture of these differences could be better evaluated by plotting the Standardized Level Difference in 1/3-octave frequency-bands. These types of plots revealed the following: in general, compared with the blind facade, mountings with apertures severely limit the sound insulation in all frequency-bands, which becomes particularly critical at the lower frequency-bands, below around 400 Hz, with the lowest D 2m,nT value of only around 3 db in the 160 Hz 1/3-octave frequency-band; above this frequency-band, the differences in sound insulation compared with the blind facade are smaller in different degrees depending on the type of mounting; the contribution of the glass-wool in the cavities of the proposed blocks to the sound insulation of the mountings, if any, occurs only at the higher frequencybands above around 400 Hz; overall, mounting SBSgw shows the best sound insulating performance of the mountings with apertures, with the highest D nt,w value of 27 db, but with a % of open area of 12 % (in a tortuous path in this type of block, which is a detrimental effect regarding the ideal straight an unobstructed path), which is about in the middle-range of the % of open areas that were tested; notwithstanding mitigated with the proposed facade elements, there is still a conflict between sound insulation and % of open area of the facade, which is decisive factor for the promotion of natural ventilation. Regarding CFD computer simulations of natural ventilation, it has been shown that the mounting with the best overall sound insulation performance, mounting SBS (with glass-wool), also provides the highest average velocity across the apertures, with a value of 7.9 m/s. However, because of the so-called load-loss effect, the average velocity at the wall-facade internally to the room, considering the whole facade area, is lower than the uniform velocity of the incident air-stream. Because of the tortuous path for the air-stream provided by mounting SBS, the pressure distribution inside the cavities of this type of block tends to re-direct the flow upwards. This behavior, associated with the significant flow acceleration in this type of mounting, would contribute to enhance the internal natural ventilation. This has been confirmed with estimates of measures of ventilation comfort, which shows that the mountings with the best sound insulation characteristics are also capable of providing adequate values of ventilation ratios and air changes per hour, based on the characteristics of the test suite used to measured sound insulation. 7. ACKNOWLEDGEMENT We wish to thank the Fundação de Amparo a Pesquisa do Estado de São Paulo - FAPESP for the doctoral scholarship provided to the first author to conducted this research.

20 BUILDING ACOUSTICS Volume 19 Number REFERENCES 1. Bittencourt, L., Shape-effect of elements with apertures in the resistance against natural ventilation (in Portuguese). In: Encontro Nacional sobre Conforto no Ambiente Construído, III, 1995, Gramado, RS, Brasil. CD-ROM, 1995, p Bittencourt, L. and Cândido, C., Introduction to natural ventilation (in Portuguese). EDUFAL, ISBN , Maceió, Givoni, B., Passive Low Energy Cooling of Buildings. Wiley, 1994, Ventilative Cooling pp ISO (1998) Acoustics Measurement of sound insulation in buildings and of building elements Part 5: Field measurements of airborne sound insulation of façade elements and façades. 5. ISO (1996) Acoustics - Rating of sound insulation in buildings and of building elements Part 1: Airborne sound insulation. 6. Lamberts, R. et al, Thermal performance of edifications (in Portuguese). Universidade Federal de Santa Catarina, Florianópolis, 2000.

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