Sound Transmission Through Hollow Brick Walls

211 Sound Transmission Through Hollow Brick Walls M. Fringuellino 1 and R.S. Smith 2 1 Dept. of Acoustics, Istituto Elettrotecnico Nazionale Galileo Ferraris, Strada delle Cacce 91, 10135 Torino, Italy 2 Dept. of Applied Acoustics, Section 1.42, Physikalisch-Technische Bundesanstalt, Bundesallee 100, Braunschweig, Germany. (Received 13 August 1999 and accepted 23 September 1999) ABSTRACT Hollow bricks and blocks are one of the most common forms of material used for wall construction found in Central and Southern Europe. The principle purpose of the perforations (holes) is to increase the thermal insulation properties. As a result of these perforations the block gross density is reduced significantly and these block walls have high anisotropy. Consequently, the acoustical insulation properties are influenced detrimentally in comparison to solid blocks. Due to the large thickness of some of these types of hollow block thick wall effects can occur such as bending shear waves and thickness resonances. These further reduce the sound insulation at high frequencies. This paper describes the characteristic features of sound transmission through hollow walls. For this study several different types of wall were built of varying thickness and materials and the sound reduction index was recorded. The effects of additional plaster layers is also discussed. It is suggested that the material properties of the block s complex web structure may strongly influence the sound reduction index at the low and high frequencies. 1. INTRODUCTION Perforated bricks or blocks are one of the most common forms of material used for construction in Central and Southern Europe. These type of blocks or bricks are used in walls, floors and roof sections. Figure 1 shows the variation in different types of these blocks. The complexity of these blocks should not be underestimated. Apart from the variation in materials used such as clay, calcium silicate or concrete (of which there are several different types of concrete materials used) other variables include hole (perforation) dimensions, complex structural webs (straight, staggered and cross webs) and presence of grip handles on larger blocks. Some block designs also have the possibility to insert insulation material within the block s larger holes, such as jacket blocks. The use of additional plaster to provide a smooth surface finish and to seal the block surface is common practice for walls and thus provides another variable. The type of

212 SOUND TRANSMISSION THROUGH HOLLOW BRICK WALLS mortar used can be important with regards the ratio of its material properties to that of the blocks. Not all blocks are mortared on their four edges, two horizontal and two vertical, as they sometimes have grooved or keyed joints where no mortar is used. Whilst this speeds up construction it does not favour good sound insulation. The principle purpose of the perforations, or holes, is to increase the thermal insulation properties of the walls. Perforations also make the blocks lighter and easier to handle on site, reduce the total mass on the foundations (in comparison to solid walls) and in general are cheaper than solid blocks. Whilst there is an improvement in thermal properties in comparison to solid walls, the holes reduce the gross density of the blocks and subsequently there is less sound insulation than for solid walls. The complex web structure within hollow blocks has been found to influence the overall sound reduction index. It has been found that staggered webs have poorer sound insulation values than straight webs [1], and that cross webs generally have values between staggered web and straight web. This study focuses on the sound reduction index characteristics of single hollow block walls. The sound reduction index (SRI) was measured for eight different walls and in two walls more detailed measurements were carried out prior to plaster being applied to the wall surfaces. A similar 10mm thick layer of mortar was used in all walls at the vertical and horizontal joints between the blocks. The walls vary in thickness between 0.1m to 0.3m and were made of either leca-concrete or clay. Figure 1. Examples of hollow bricks with various depths and web structures. (a) staggered, (b) cross, (c) straight, (d) staggered and indented grip handle. 2. ACOUSTICAL CHARACTERISTICS OF HOLLOW BRICK WALLS The sound insulation characteristics of hollow blocks are quite complex. Due to the block design structure of the webs and holes a hollow block wall exhibits strong

SOUND TRANSMISSION THROUGH HOLLOW BRICK WALLS 213 anisotropy. There may be several different stiffness values for the principle directions depending on the block design. The maximum and minimum bending stiffness for one block may vary by a factor of seven. As a result the critical frequency f c dip or coincidence valley is much broader. Plaster-render layers are often applied to walls to seal the face of the blocks. This can increase the total mass of a wall and increase the air flow resistance leading to improved sound insulation properties [2]. Figure 2 shows the sound reduction index (SRI) of a thin hollow block clay wall of 12cm thickness with an additional plaster layer on each side of the wall, 1cm thickness. The volume ratio of the holes per block is 45%. Also shown in Figure 2 for comparison is the SRI of a solid leca-concrete block, 12cm thick with additional 1cm plaster layer on each side. The gross densities, including plaster, of the hollow brick wall and solid brick wall are 978kg/m 3 and 1883kg/m 3 respectively. Figures 3a and 3b show the structure of the blocks. It can be seen that as a result of the decreased gross density of the hollow wall the SRI is lower. Figure 2. Measured sound reduction index of two walls of same thickness. Solid block wall; n hollow block wall. The SRI of the solid wall has a sharp dip at its critical frequency of 160Hz, whereas the hollow wall has a broader f c dip, 160Hz to 315Hz. At higher frequencies there is a second dip at 2500Hz for the hollow brick wall but which does not appear for the solid wall. This dip is suggested as being a coincident frequency effect associated with the thin surface layer of the hollow blocks and plaster on each side of the web structure, which has a thickness of 18mm.

214 SOUND TRANSMISSION THROUGH HOLLOW BRICK WALLS Figure 3. Type of hollow and solid blocks with additional plaster 1cm each side. (a) 12cm hollow block (clay), (b) solid leca-concrete 12cm block, (c) 10cm hollow clay block, (d) 8cm hollow clay block. Figure 4 compares the sound reduction index for the 12cm hollow block wall, with two other blocks walls, with decreased thickness, 10cm and 8cm respectively. The hollow clay blocks are shown in Figures 3a, 3c and 3d. As expected, when the wall total thickness decreases the critical frequency increases and the overall sound insulation decreases. Similar dips appear at the higher frequencies due to the thin surface layers of the block and plaster. All three types of hollow block came from the same manufacturer, using the same clay material. It has been found that the design dimension and layout of the webs and holes within hollow blocks can influence the overall sound insulation properties [1,3,4]. Figure 1 shows staggered and non staggered (straight) block web layouts. From previous experimental work [1] staggered webs are less favourable for sound insulation. This may be a function of the difference in bending stiffness properties in the principle directions for these two types of block. Straight webs within the block interior result in a higher bending stiffness than staggered webs. Furthermore, the complex interior web layout may also effect the inplane and flexural wave motion in these walls. Cross webs are on average better for sound insulation than staggered webs but poorer than straight webs. Again a function that may be related to bending stiffness properties of the block internal structure and complex wave behaviour in these walls. Figure 5 shows a block of similar thickness and material to the cross web block in Figure 3d, but with a staggered web layout. A wall was built, using the staggered web

SOUND TRANSMISSION THROUGH HOLLOW BRICK WALLS 215 blocks, in the test facilities with an aperture of 3.6 x 2.8m, to compare the SRI with the block shown in Figure 3d. Plaster of 1cm was applied to both faces of each wall and both types of block had similar gross densities and were made of clay. Figure 6 shows the measured SRI for each wall shown in Figure 3d and Figure 5. It can be seen that both blocks are orthotropic in their behaviour, with a wide critical frequency valley, 250Hz to 500Hz. As frequency increases so the cross web wall has a higher SRI than the staggered web wall. Again both walls have dips at higher frequencies. The staggered web wall has a less pronounced dip at 3150Hz. Figure 4. Measured sound reduction index of three cross web hollow blocks of varying thickness with additional plaster: n, 14cm;, 12cm;, 10cm. Figure 5. Clay hollow block of 8cm thickness with staggered webs across its depth

216 SOUND TRANSMISSION THROUGH HOLLOW BRICK WALLS and additional 1cm of plaster applied to both faces, total thickness 10cm. Figure 6. Measured sound reduction index of two blocks of similar thickness, hole volume ratio and material but with different web design: cross web; n staggered web. The wall thickness can generally vary between 0.1m to 0.4m depending on internal or external wall application and load bearing requirements. As a result of these larger dimensions the blocks experience thick wall effects which reduce the sound insulation. Two principle mechanisms occur for walls with large thickness, shear waves [5] and thickness resonances. Normally for thin walls it can be assumed that the wall undergoes pure bending and the bending wavespeed increases with frequency. Whilst this can be assumed for thick walls, at low frequencies, as frequency increases one must take account of shear deformation and rotary inertia [5, 6]. The limit of regarding the wall as thin occurs where the bending wavelength is six times the plate thickness [7], given by f = c L / 20h for homogenous plates. It has been suggested more recently that the limit may extend to three times the plate thickness [8], an increase of two octave bands. For thick walls as frequency increases the group and phase wavespeeds tend to the same value and appear less frequency dependent at the high frequencies [9]. Generally the bending wavespeed is slightly lower than the transverse wavespeed [5] where the difference is related by poisson ratio [6]. As a result a plateau type effect occurs for the SRI curve for thick walls at high frequencies. The bending wavespeed at these high frequencies may be termed the corrected bending wave [6]. Figure 7 shows the various wavespeeds for a 20cm thick concrete solid block.

SOUND TRANSMISSION THROUGH HOLLOW BRICK WALLS 217 Figure 7. Wavespeed for a 20cm thick solid wall., longitudinal; --- ---, transverse;..., bending (thin plate); +, corrected bending (thick plate),. asymptote of high frequency thick plate max. value. The second mechanism is thickness resonances which can sometimes cause pronounced dips in the SRI curve at high frequencies. These effects have been discussed in detail by Ljunggren [8] using a theoretical analysis. The frequencies at which these dips occur may be approximated by f = c L /(2h), where c L is the longitudinal wavespeed and h is the wall thickness [10]. Most of the previous work on thick walls has been related to solid walls. Nevertheless, it has been shown previously that thick wall effects can significantly lower the SRI curve for hollow block walls within the frequency range 100 to 5000Hz [4,11]. Figure 8 shows a leca-concrete hollow block. A wall was constructed with this 30cm thick block with additional plaster, 1.5cm thick, applied to both sides and the sound reduction index was measured. Figure 9 shows the SRI for this block. It can be seen for frequencies above 1250Hz that the SRI curve levels off until 3150Hz. This is suggested as being caused by thick wall effects.

218 SOUND TRANSMISSION THROUGH HOLLOW BRICK WALLS Figure 8. Dimensions and plan view of leca-concrete hollow block 30cm thick with 1.5cm of plaster on each side. Figure 9. Measured sound reduction index of a 30cm hollow block wall with additional plaster. 3. EFFECTS OF PLASTER LAYERS AND WEB STRUCTURE More detailed measurements were carried out on a series of walls. For the purpose of this paper two walls are discussed. Test wall A was made of concrete leca blocks, 20 cm thick as shown in Figure 10, built in a test aperture 3.6m x 2.8m, between two rooms, source and receiving of volumes, 60m 3 and 66m 3 respectively. The gross density of these blocks with mortar is 1127kg/m 3. Test wall B was the same wall but with additional plaster-render layers of 1.5cm each side. The density of test wall B, with the plaster, increases to 1214 kg/m 3.

SOUND TRANSMISSION THROUGH HOLLOW BRICK WALLS 219 Figure 10. (a) 20cm thick leca-concrete blocks with complex webs used in test walls A and B. (b) position of accelerometers when measuring c L across the block depth. During the period of measuring the sound reduction index of these walls, other factors were also measured, these include the total loss factor (TLF), the longitudinal wave speed of the blocks and the acceleration level difference on either side of the wall for an airborne source. The sound reduction index, R, was measured in accordance with ISO 140 [12]. The total damping (TLF) of the wall, h T, was calculated using the measured standard decay rate of the wall, where the source was an acoustic hammer. An average of 6 positions were recorded. The longitudinal wave speed, c L, of wall A was measured by taking the fly-time of the wave between two accelerometers placed parallel and in plane to the wall surface, with a known separation distance, and recording the time interval on a two channel oscilloscope. This measurement on wall A gave a result of c L = 2705 ms -1, (+/- 50ms -1 ) measuring horizontally across the wall. Measurements of c L were also carried out in the vertical direction on the wall surface and values were recorded of 2980ms -1, with similar variance. Similar values were also achieved when measuring these directions on a single block. The increased value of c L in the vertical direction is perhaps due to the holes, within the wall, being aligned in the vertical direction and thus the wall face thickness is more constant. As shown in Figure 10 the blocks have a surface thickness of 2cm on each. The horizontal c L value may be affected by the numerous webs attached to the 2cm surface face of the wall. By measuring on the face surface of the wall it may not be representative of the wave behaviour within the complex web structure. For this reason further measurements were carried out to determine the wavespeed across the depth of the blocks (across the webs). For the longitudinal wave to travel across the wall depth the compressions and rarefactions of this wave may be influenced by the complex web structure, as there is no direct path. Measurements of longitudinal wavespeed were carried out on a single

220 SOUND TRANSMISSION THROUGH HOLLOW BRICK WALLS block across its depth. The accelerometers were positioned across the depth of the block as shown in Figure 10b. The block was excited on its face by a light hammer and the accelerometers were mounted parallel and inplane to the y-axis. By measuring the shortest possible distance for the wave to travel between the two accelerometers through the web structure and the recorded time interval the c L value was calculated as 1150ms -1. It may be argued that this measurement may not result in a true longitudinal wave travelling between the accelerometers, as a result of the complex web structure. However, the accelerometers were positioned inplane and the measurement technique would record the fastest wave, which is normally the longitudinal wave. This experiment was repeated several times and similar values recorded. Figure 11. Measured total loss factor for wall without plaster, wall A ( ), and wall with plaster, wall B (n). Internal loss factor is also shown, h = 0.01. Figure 11 shows the recorded total loss factors for test wall A, without plaster and test wall B with plaster. It can be seen that the addition of the plaster has caused a considerable increase in the loss factor value. This may be due to the plaster layers increasing the coupling to the supporting walls at the test wall boundaries, thus increasing the total loss factor. The high values of h recorded at the low frequencies may be affected by filter ringing or low modal behaviour within the wall. Both walls have a low mode count [6] below 200Hz. Figure 12 shows the measured SRI for both walls together. It can be seen that the addition of the plaster has affected the SRI in a similar quantity to that of the loss factors shown in Figure 11. For example the difference in the measured loss factors at 315Hz is equivalent to 7dB and the difference in the SRI is 6.7dB. At the higher fre-

SOUND TRANSMISSION THROUGH HOLLOW BRICK WALLS 221 quencies the SRI curves for both walls become very similar and differences in the loss factors do not translate to similar differences in the measured SRI. At these higher frequencies thick plate effects may occur within the wall. Figure 12. Measured sound reduction index for both test walls A and B. n with plaster; without plaster. If the wall was solid and had a longitudinal wavespeed of 2850ms -1, the previous measured surface velocity, the expected first thickness resonance might occur around 7000Hz. Furthermore, the SRI values only increase by 6dB over two octave bands, between 1000 and 4000Hz and it might be expected that SRI curves would be more steep in their gradient. Assuming that the block depth inplane velocity measured earlier is representative of the wave behaviour in the web structure this would result in the first two thickness resonances at 2800Hz and 5600Hz. As can be seen in Figure 12, test wall A does experience a small dip at 3150Hz and a more pronounced dip at 5000Hz. The results of the same wall with additional plaster, test wall B, do not show such a pronounced dip at 5000Hz. 4. PLATEAU EFFECT Ljunggren [8] proposed a simple expression to determine the transmission loss at high frequencies for homogenous plates to account for the plateau effect. This equation is based on the familiar formula for transmission at normal incidence of a longitudinal wave at the intersection between different materials. The transmission loss may be expressed by:

222 SOUND TRANSMISSION THROUGH HOLLOW BRICK WALLS where r 0 is the density of air, c 0 is the wavespeed in air, c is the block longitudinal wavespeed and r is the gross density of the wall. Another expression was proposed by Ljunggren to account for the wall total loss factor, where a reference value was used. In Figure 11 it can be seen that at high frequencies the loss factor tends to the measured internal loss factor value for these blocks, h = 0.01. As a result the reference value would be the internal loss factor of the wall and as the values are similar no additional expression is used in this case. This simplification may cause small errors as not all walls may have similar values of the total loss factor tending to the internal loss factor. Figure 13 shows the measured SRI for test wall A and calculated value for the plateau effect for a solid wall using Equation 1. It can be seen that when the surface wavespeed is used the plateau is expected to be higher. But this expression is for solid walls and thus it may not be appropriate to use the surface wavespeed for these hollow brick walls. The measured longitudinal wavespeed across the webs may be a more accurate function of the block properties across its thickness. Using the measured wavespeed across the block depth (as shown in Figure 10b) gives good agreement for the plateau effect of these hollow block walls shown in Figure 13. 5. DISCUSSION The desire to obtain high heat insulation values for walls by the use of hollow blocks has several disadvantages on the measured sound reduction index values. The higher the bending stiffness across the thickness of the blocks the better the sound insulation. But to achieve high bending stiffness across the thickness of the blocks would generally require straight webs. The disadvantages of straight webs for heat insulation is that one directly connects the front and back of the block with a web which then forms a cold bridge effect and reduces the heat insulation in comparison to staggered webs. In one EC country alone there are over 300 different types of hollow block manufactured for the construction industry. To determine the optimum block design that meets the requirements of good heat insulation and sound insulation will require more indepth study. In addition, not only the type of block must be investigated but also the whole wall structure, with mortar and additional plaster, which can influence the sound reduction index. 6. CONCLUSIONS This study has summarised some of the principle acoustic characteristics of hollow block walls which are used in central and southern Europe. The sound reduction index is detrimentally influenced in several ways. Variation in bending stiffness and block material parameters cause a wide coincidence valley at low frequencies. This is the principle frequency range which causes most problems for noise disturbance in domestic buildings. As a result of the higher coincidence frequencies within the wall, than with a solid wall, the mid frequencies of the sound reduction index are also influenced. (1)

SOUND TRANSMISSION THROUGH HOLLOW BRICK WALLS 223 At high frequencies those walls with a thickness as small is 19cm may experience thick wall effects within the main building acoustical frequency range for ISO 140, possibly due to the wave motion being affected by their complex web structure. The results presented here are preliminary and further measurement work is ongoing for the possible use of prediction methods for evaluating the sound reduction index of hollow block walls. Figure 13. Comparison of measured sound reduction index (SRI) for test wall A with the expressions for the plateau effect for thick walls., SRI; n, plateau effect using web measured wavespeed;, plateau effect using surface wavespeed. ACKNOWLEDGMENTS The authors wish to acknowledge the technical assistance provided by Dr Andrea Pavoni- Belli and Francesco Russo of IEN Galileo Ferraris, Italy, during the measurements and Andreas Meier of PTB, Germany, for the helpful discussions during this work. REFERENCES 1. W. Scholl, L.Weber (1998). Influence of perforation pattern on direct and flanking sound transmission of cored block masonry (results of a literature study). Bauphysik 20, Heft 2. (in German) 2. R. MacKenzie, B. Ma, R. Wilson, M. Stewart (1988). The effect of cement render upon the sound insulation of dry lined concrete party wall. Proceedings of the Institute of Acoustics, 10, Part 8, p21-28. 3. L. Weber, A. Buckle (1998). Sound reduction of hollow blocks new results. Bauphysik 20, Heft 6. (in German)

224 SOUND TRANSMISSION THROUGH HOLLOW BRICK WALLS 4. W. Maysenholder (1998). Masonry, an acoustical diffraction grating? Computation and visualisation of sound transmission loss. Bauphysik 20, Heft 6. (in German) 5. R.D. Mindlin (1951). Influence of rotatory inertia and shear on flexural motions of isotropic, elastic plates. Journal of Applied Mechanics, 18, 31-38. 6. R.J.M. Craik (1996) Sound transmission through buildings using statistical energy analysis. Gower, UK. 7. L. Cremer, M. Heckl, E.E. Ungar (1973). Structure-borne sound. Springer Verlag, Berlin, Germany. 8. S. Ljunggren (1991). Airborne sound insulation of thick walls. Journal of the Acoustical Society of America, 89 (5), 2338-2345. 9. J.H. Rindel (1994). Dispersion and absorption of structure-borne sound in acoustically thick plates. Applied Acoustics, 41, 97-111. 10. L.L. Beranek and L.L. Ver (1992). Noise and vibration control engineering. Principles and application. J. Wiley and Sons, New York, USA. 11. J. Lang (1993). Measurement of flanking transmission in outer walls in test facilities. Applied Acoustics, 40, 239-254. 12. ISO 140/2 (1991). Measurement of sound insulation in buildings and of building elements.