Sound insulation performance of Brazilian dwellings: from colonial ages to contemporary architecture Elisabeth de A. C. Duartea, Elvira B. Viveirosb a,b
b
Postgraduate Program in Architecture and Urbanism Acoustics Unit, Laboratory of Environmental Comfort, Dept. of Architecture and Urbanism Federal University of Santa Catarina, Florianópolis, SC, 88040-900, Brazil a
[email protected],
[email protected]
Abstract The 20th century saw a constant search towards lightweight constructions, which tends to go against good sound insulated partitions. Particularly in Brazil this is an important issue, as there is no building regulations concerning acoustic comfort and, therefore, builders target economical goals only. This paper proposes an analysis of the sound insulation performance of Brazilian houses along the centuries. First, a survey of the most important and typical constructions for different periods of Brazilian history is shown. Subsequently, analytical prediction of the sound insulation of their components are performed and evaluated along a timeline. It is demonstrated and quantified the exponential decrease in the building components performance. The aspects that collaborate to keep the situation unchanged are discussed and possible ways out proposed. INTRODUCTION Studies related to acoustic comfort are scarce in Brazil and non-existing when it comes to historical buildings. There are many reasons associated to this lack of investigation. The tradition of studies on theory of architecture related to aesthetic, economical and social aspects is one of the main aspects. In countries that make use of heating and thermal insulation, i.e., where energy conservation is a concern, sound insulation comes along even if it was not the main target in the first place. On the other hand, hot weather countries demand other type of constructions. Therefore, being a tropical country reinforces the need for researches on the subject in Brazil [1]. This paper, in its first part, deals with the historical analysis of Brazilian partitions used in dwellings. Later on, analytical predictions are performed and the process of acoustic degradation along history is quantified. Brazilian partition performances are also compared to international classification schemes.
THE EVOLUTION OF PARTITION WALLS IN BRAZILIAN DWELLINGS Brazilian colonial age is characterized by a mixture of different cultures and influences. In architecture, native and Portuguese techniques are predominant. The constructions are considered as of low quality by many authors [2,3,4]. The building process was based on the workmanship of African slaves, who had no experience on those methods whatsoever. Low resistance materials were applied, such as wood or clay. Stone was available for the upper classes of the society only. This situation was reflected in preferable materials according to different regions. Hence, in colonial ages, is possible to divide the regions according to its building process, as shown in figure 1. They are: Northeast: where oldest techniques are predominant. Since the arrival of Portuguese people, mud brick partitions were used; South coast: the area extends from Bahia to the south of Brazil. The most important difference from other regions is the use of stone walls (LEMOS, 1996); Tableland and outskirts of São Paulo: taipa de pilão is predominant in residences. It was used for almost 300 years. It is a typical technique, with a clay infill in between wooden panels, which is squeezed and knocked into the partition with the aid of an adequate equipment, the so called pilão. In figure 1 is shown a panorama of Brazilian colonial building processes:
Figure 1: Building processes from Brazilian colonial ages according its regions
In the end of the XIX century, the quality of the buildings showed considerable improvement in consequence of the decay in African slavery and the beginning of European immigration. In this period, an important change was the introduction of the so called cooked brick or
ceramic block, as masonry option. [3]. However, even in São Paulo, taipa de pilão was the overspread constructive technique. Only the First World War changed Brazilian architecture completely. At this time, lightweight constructions were made possible with the use of concrete. The peak of Brazilian civil engineering was the creation of Brasília, which allowed new constructions methods to be used in remote regions far from the industrialized cities [5]. Nowadays, despite the levels of noise pollution in urban areas, no concern regarding the acoustic quality of buildings is observed. The building methods are basically the same of the beginning of 20th century, that is to say, the use of reinforced concrete for the structural elements and masonry brick for the partitions. SOME REMARKS ON SOUND INSULATION THEORY According to Sharland [6], thermal, electric or sound insulation is offered when a barrier is in opposition to a natural energy flow, in this case, sound energy. A barrier imposes attenuation to the transmitted energy. It is necessary, therefore, to know which properties of the partition play an important role in sound insulation. This paper presents a brief study about sound transmission phenomena of a solid simple barrier in a diffuse field. A panel presents different effects to the sound transmission according to their properties of stiffness, damping and mass. There are four regions of interest: (1) the low frequencies region, where the stiffness of the partition is of preponderant importance; (2) in a different segment of frequencies, the transmission loss is determined by the damping coefficient. This is the so called resonance region; (3) in frequencies around the double of the first resonance frequency, is the mass region, known as the mass law frequency band, where the transmission loss increases by the rate of 6 dB per octave; (4) the last region starts with a deep in the transmission loss curve, at the so called critical frequency [7]. For each of these regions, there is a specific equation to describe the transmission phenomena. Theoretically, transmission loss is the logarithmic ratio of incident and transmitted sound power, expressed by: TL = 10 log
Wi dB Wt
(1)
The ratio of sound power, transmitted and incident, is the expression of the transmission coefficient, τ , according to: 1 TL = 10 log dB τ
(2)
Equation 2 can be expressed in different ways for each frequency region. Figure 2 illustrates the performance of a hypothetical partition in a diffuse sound field. It is possible to observe how equation 2 changes according to different frequency ranges.
Figure 2: Typical transmission loss curve of a single panel for different frequency ranges in a diffuse sound field.
In figure 2, k is the stiffness of partition, f is frequency, C is damping, ! is the specific density of the medium (air), c is wave speed in air, m is mass, is the loss factor and fc the critical frequency, which is expressed by: fc =
c2 Hz 1.8.h.c l
(3)
where h is the barrier thickness and cl, the longitudinal wave speed along the barrier.
SOUND INSULATION OF BRAZILIAN PARTITIONS As stated before, Brazilian partitions presented a constant search towards lightweight constructions. Figure 2 demonstrated that the panel superficial mass, m, is determinant for the sound insulation in important frequencies ranges. Hence, a partition with a less dense material and/or a lower thickness results in a decreased sound insulation. The superficial density, m, of Brazilian partitions was calculated based on a comprehensive survey that this paper is based on. The results are represented in figure 3. It is notable the decline of density along centuries, which justifies, not necessarily alone, the decrease of sound insulation in Brazilian dwellings partitions.
Figure 3: Superficial density of Brazilian partitions for different periods in History.
At this point LVSRVVLEOHWRHVWLPDWHWKHGHFUHDVHRIWUDQVPLVVLRQORVVû7/H[FOXVLYHO\E\WKH ratio of density reduction, given by: ∆TL = 20 log
m1 m2
(4)
where m1 and m2 are the superficial densities to be compared. According to equation 4, if a stone wall is replaced by a masonry brick and if a masonry brick is replaced by a ceramic block wall it will represented a reduction of 19 dB and 15 dB, respectively. Associating stone wall, the densest partition used for the last centuries, to a wooden panel, the lowest density from 20th century onwards, it is possible to conclude that five centuries of residential architecture meant, in general terms, a loss of around 35 dB in the sound insulation offered by partitions in dwellings. In a detailed analysis, the estimation of sound insulation level over a frequency range requires more parameters to quantify TL correctly, as shown in figure 2. Therefore, the prediction of sound insulation is an elaborated task, mainly because there is not much information about Brazilian properties partition. Using the equations from figure 2, the prediction of TL was made with panels that data were available. The results are illustrated in figure 4.
Figure 4: Analytical prediction of transmission loss of Brazilian partition examples.
For the older elements, as there was no information, the calculations were not possible to complete the estimation. As representative of partitions from earlier centuries, the reference was the masonry brick with 45 cm of thickness. Partitions with no analytical estimation are expected to present a better performance than the masonry brick, as they have higher density, as shown in figure 3, except for the taipa de mão. As observed in figure 4, the performance of masonry brick of 45 cm was higher than all other walls. In all frequencies ranges, except in its critical frequency region, the transmission loss results of this partition are higher than others in about 10dB. It is observed an expected displacement of critical frequency towards lower frequencies as thickness increases. As critical frequency of masonry brick is very low, less than 50 Hz, its performance is better, because above this frequency the transmission loss is greater than 6 dB per octave band of mass law. Also, according to figure 4, it can be pointed out the similar performance among recent partitions. The 3 cm wooden panel is the only exception, which presents the worst levels of transmission loss. This panel is typical of poor people living in urban areas. To discuss the performance of partitions it can be used the single number ratio, instead of frequency bands analysis. To convert all data into a single index, Rw, the process was carried out according to ISO 717-1 [8], and the results are presented in figure 5. To state that a partition is good or not, it is necessary to compare its sound insulation results to a quality parameter. However, in Brazil, there are no legal sound insulation requirements for dwellings. In Europe there are classification schemes that have been used for approximately 50 years now [9,10]. This research results were assessed based on an
European study [9], which compares the sound insulation requirements of 24 countries. The requirements vary over European regulations, but it can be said that Rw’ varies from 50 to 61dB. Based on these levels, figure 5 shows how Brazilian partitions are ranked in relation to these averaged European requisites. It can be observed that no partition that has been used from the 20th century on would comply with European standards.
Figure 5: Rw’ of Brazilian partitions and the required European range.
The level of annoyance for people living in Brazilian dwellings might be even more critic than the results point out. Grimwood [11] affirms that in England many people are dissatisfied even if their homes meet the intended standard. This may be explained by the fact that regulations present minimum levels of sound insulation, which sometimes might not be enough. Also, regulations in Europe are based on 50 years old studies. Cities changed over this period, a great number of new sound sources appears and the demand for better quality and comfort in residential buildings rose.
CONCLUSIONS This paper is part of a research line that has been investigating the evolution of Brazilian dwellings along history, which focusing sound insulation [1,12]. In a first moment, it was done a survey of mainly modifications in constructive processes along the Brazilian architecture timeline. After, it was made a brief investigation of theory of sound insulation. Since then, the levels of insulation of the researched partitions were evaluated.
Analytical methods indicated that sound insulation of partitions, only in reason of the decrease in the superficial density, it arrives at 15 and 19 dB. When compared the densest elements, used among the centuries XVII-XIX, with the least dense of current days, the decline in sound insulation reaches around 35 dB. For the prediction of transmission loss in frequencies, curves demonstrated a considerable decrease of sound insulation along history and contemporary partitions have almost the same variations. Finally, if appraised by international regulations, none of the Brazilian partitions used after the century XX would be in agreement with demanded in Europe, in spite of researches indicates to be now necessary a larger rigidity of criteria already adopted. ACKNOWLEDGEMENTS The authors wish to thank FAPEAL, the Assistance Research Foundation of State of Alagoas, whose financial support guaranteed the adequate continuity of the research.
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