ACOUSTIC CORRECTION USING GREEN MATERIAL IN CLASSROOMS LOCATED IN HISTORICAL BUILDINGS

ACOUSTIC CORRECTION USING GREEN MATERIAL IN CLASSROOMS LOCATED IN HISTORICAL BUILDINGS
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   Acoustics Australia Vol. 41, No. 3, December 2013 147 ACOUSTIC CORRECTION USING GREEN MATERIAL IN CLASSROOMS LOCATED IN HISTORICAL BUILDINGS Gino Iannace, Amelia Trematerra, Patrizia Trematerra Department of Architecture and Industrial Design, Second University of Naples, Borgo San Lorenzo Aversa, Italygino.iannace@unina2.it INTRODUCTION Due to aesthetic and historic reasons, it is difcult to install sound absorbent panels for acoustic correction in classrooms located in historical buildings. Usually the dimensions of such classrooms (volume and area) are large so the reverberation time is high (over 2 seconds) [1]. Furthermore, classrooms located in historical buildings are not regular in shape and the ceilings are not plane. To improve the acoustic characteristics of classrooms, sound absorbing materials are usually suspended on the walls; however in historical buildings it is not possible to use xed structures [2]. Absorbent panels are generally made with traditional sound absorbent materials such as glass wool, rock wool, polyester or polyurethane foam. In this paper a sustainable “green material” was used as the sound absorbent material. This has the advantage that at the end of its useful life, it can be disposed of without difculty and without damage to the environment. In addition, green materials store carbon dioxide during their growth. In order to evaluate the classroom surface area to be covered with the green material absorbent panels, this study used the architectural acoustic Odeon software in conjunction with a classroom virtual model. Classrooms of the Faculty of Architecture of the Second University of Naples (SUN) were selected for the study. The Faculty of Architecture is located in an ancient building in the city of Aversa near Caserta (Italy). The building was built in the 10th century as a Benedictine monastery. It was then expanded in the 15th century and later converted into a school and nally into a University in 1990. Figure 1 shows the cloister on two levels with arches and columns. The classrooms were irregularly shaped, with vaulted ceilings and smooth plaster walls. The acoustic parameters measured in each classroom were: Reverberation Time (T 30 ), Early Decay Time (EDT), Denition (D 50 ) and the Speech Transmission Index (STI) [3]. Figure 1. The cloister on two levels with arches and columns ACOUSTIC MEASUREMENTS The acoustic measurements were carried out in seven classrooms, using an omnidirectional spherical source fed with a Maximum Length Sequence (MLS) signal. The impulse responses were detected with a measurement microphone GRAS 40 AR endowed with the preamplier 01 dB PRE 12 H connected with a laptop PC through the interface 01 dB Symphonie. The sound source was placed in each classroom at the teacher’s position (height 1.6 m). The microphones measurements were set in different points at a typical ear height of 1.2 m, to obtain average values of the classroom acoustic parameters. The acoustic parameters were measured according to ISO 3382 [4]. Figure 2 shows The acoustic correction inside classrooms located in historical buildings using absorbent panels is difficult for aesthetic reasons. Furthermore, architectural restrictions are often imposed to preserve the historical heritage. The acoustic measurements inside the classrooms show high reverberation time values, which imply an adverse environment for speech reception. In this paper the reverberation time in classrooms located in historical buildings was reduced by installing removable sound absorbent panels. The panels were made with “green material”. The absorbent material was obtained by crushing giant reeds of sweet water, a plant which grows quickly in wetlands. The crushed material was then put in jute sachets, installed in the wooden frames and covered with different colours jute cloth for aesthetics. Acoustic measurements were made in the classrooms with smooth plaster walls, without students. A virtual model of the classroom was drawn with 3D CAD. The surface area covered with green material absorbent panels was evaluated by the software Odeon. After the installation of the absorbent panels, comparisons between the virtual classroom acoustic properties and the real classroom acoustic properties were made to validate the effect of the green absorption panels.  148 - Vol. 41, No. 3, December 2013 Acoustics Australia the omnidirectional sound source in the classroom at the teacher’s position, and the measurement microphone between the tables. During the acoustic measurements the background noise was lower than 50 dBA, the classrooms were empty without students and the furniture consisted of hard chairs and rows of hard tables. The classroom had smooth walls, wooden doors and glass windows [5,6]. Table 1 shows the average geometrical dimensions for the seven classrooms. For these same classrooms, Table 2 shows the STI average values measured, Table 3 shows T30 average values measured and Table 4 shows D50 average values measured. All the acoustic parameters considered show that in the classrooms the quality of speech reception is poor. CASE STUDY Classroom T4, a room with smooth plaster walls, was chosen as the case study. This plan is 9.0 m long and 5.0 m large. The average height is about 5.0 m and the volume is 240 m 3  (Figure 3). In the classroom there are thirty hard chairs and six rows of hard tables; the students’ seating area is 5.30 m × 2.50 m. When the classroom is empty (without students), the measured values of STI, T 30  and D 50  are respectively reported in Tables 2, 3 and 4. The acoustic parameters measured indicate that in this classroom, the speech reception was not good. Furthermore tests administered to students have conrmed that the speech intelligibility was poor. Figure 2. Acoustic measurements in the classroomTable 1. Classrooms average dimensionsClassroomT4T5P3P4S2 S3 T1Volume, m 3 240251741612006261850202Average height, m5.012.15.45.54.67.24.5Base area, m 2 502087721713625745Table 2. STI average values measuredClassroomT4T5P3P4S2 S3 T1STI, measured0.340.380.480.470.470.460.47Table 3. T 30  (s) average values measuredFrequency, Hz1252505001k2k4k T43.432.442.041.761.701.41T54.675.414.234.03.152.14P32.832.672.182.241.811.56P43.222.882.772.442.221.81S22.792.782.692.632.251.74 S3 3.042.732.722.662.242.0T13.493.432.772.372.212.01   Acoustics Australia Vol. 41, No. 3, December 2013 149 Table 4. D 50  average values measuredFrequency, Hz1252505001k2k4k T40.230.160.220.300.270.33T50.220.160.150.230.29 50.50P30.230.300.300.310.340.40P40.230.270.270.310.380.44S20.280.300.230.260.330.41 S3 0.300.340.310.270.250.34T10.220.220.280.320.300.35Figure 3. Classroom plan and section dimensions (in metres) ACOUSTIC PROPERTIES OF THE MATERIAL For the classroom acoustic correction, sustainable materials termed green materials were chosen [7-11]. These materials are generally employed for energy saving purposes (heat insulating materials). Recently they have also been applied in architectural acoustics to replace the traditional sound absorbent materials (glass wool, rock wool, polyester,  polyurethane foam, etc). Sustainable materials have the advantage that they can be disposed of without difculty and without damage the environment at the end of their useful life. The sustainable material used in this study is a giant reed of sweet water (arundo donax). It is a material commonly available in country sides near rivers, lakes and wetlands. This  plant grows very quickly, usually reaching 6 m in height and a diameter of 2-3 cm. The giant reeds were cut, dried and then crushed. They were then shredded into akes of small size, with average dimensions of 40 mm length, 10 mm width and 3.0 mm thickness (Figure 4). The loose grains obtained were  placed in sample sacks made of jute which were tested by the measurement system. To assess the material acoustic properties, the absorption coefcient at normal incidence was measured with a Kundt’s tube, in accordance with ISO 10534-2: 1998 [12]. The Kundt’s tube has an inner diameter of 100 mm, length of 560 mm. With distance between the two measuring microphones of 50 mm the absorption coefcient measurement is accurate in the range frequency of 200 Hz – 2 kHz. Using the distance between the two measuring microphones of 100 mm, the absorption coefcient measurement is accurate in the frequency range of 100 Hz – 1 kHz. Figure 5 presents the average value of the absorption coefcient measured at normal incidence in the frequency range 125 Hz – 2 kHz with Kundt’s tube [13]. This average value is obtained from measurements with four different specimens (thickness 40 mm). The material has a good value of the absorption coefcient at the medium frequencies. The loose materials were inserted in the tube measurement and stopped with a net metal, so the Kundt’s tube was in a horizontal  position. The measured absorption coefcient values are similar to those of limestone chips with the same thickness [14]. The loose material was then inserted in jute sacks in order to obtain a layer of sound absorbent porous material and mounted in wooden frames covered with a jute burlap (Figure 6). Since the  jute burlap has a large mesh and low air resistance, it can be considered as an acoustically transparent material. Figure 4. Loose grains average dimensionsFigure 5. Average values for the absorption coefficient α  measured at normal incidence in the frequency range of 125 Hz – 2 kHz 0.00.20.40.60.81.00500100015002000   α Frequency (Hz)  150 - Vol. 41, No. 3, December 2013 Acoustics Australia ODEON VIRTUAL MODEL A software used for architectural acoustics, Odeon, was used to evaluate the classroom surface area to be covered with absorbent material in order to obtain an acoustic correction. The Odeon software imports a virtual model drawn by 3D CAD [15]. Figure 7 shows the 3D virtual classroom model with the virtual omnidirectional sound source in the teacher position, and the virtual receivers in the student positions. Figure 8 shows the classroom render with the absorbent panels insertion. The Odeon virtual model had 88 corners, 35 surfaces in the room and a total surface area of 254 m 2 . The rst operation is the acoustic model calibration which consists of setting the absorbent coefcient values for all virtual model surfaces and setting the scattering coefcients. The scattering coefcient s does not depend on frequency, but on the surface geometrical properties; so the desks and chairs were simulated as at planes, with a scattering coefcient  s =0.5 for the unoccupied condition. The calibration operation is stopped when at each octave  band frequencies (125 Hz – 4 kHz) the value of reverberation time (T 30 ) calculated is equal to reverberation time (T 30 ) measured. Figures 9, 10 and 11 respectively show the comparison between the acoustic parameters values T 30 , EDT and D 50  both measured and calculated using the Odeon software, when the classroom with smooth walls is empty (without students). After the calibration operation, the acoustic absorbent panels with surface area 11 m 2   with the values of absorbent coefcient given in Figure 5 were inserted in the virtual model. The values of the absorbent coefcient at 4.0 kHz were obtained by extrapolating the measured values. In this conguration, the calculated value for speech transmission index (STI) is 0.45. For the EDT, the difference between measured and calculated values is negligible, while for the STI and the D 50 , the calculated values are higher than the measured values. FULL SCALE ACOUSTIC MEASUREMENTS Figure 12 shows the green material absorbent panels located in the classroom covering an area of 11 m 2 . The absorbent panels are installed in a temporary manner due to aesthetical and historical reasons, and in accordance to Superintendence of historical heritage architectural restriction. During the acoustic measurements, the omnidirectional sound source and the receivers were put in the same initial positions (teacher and students positions). The comparisons between measured and calculated values are presented in Figures 13-15 for reverberation time (T 30 ), early decay time (EDT) and denition (D 50 ), respectively. In this conguration, the calculated value STI = 0.56, while the measured STI is 0.55. The difference between measured and calculated values for the T 30  and EDT parameters is negligible, while for the STI and D 50  the calculated values are higher than the measured values. CONCLUSIONS This paper demonstrates the possibility to obtain a good acoustic correction inside classrooms using a green material corresponding to the giant reeds, properly shredded and re-assembled into panels. The green material is inserted in jute bags where the jute has a large mesh and a low air resistance and as such is an acoustically transparent material. Using sound absorbing panels of different Figure 6. Sound absorbent panelsFigure 7. 3D virtual model by Odeon, with the sound source and receiver positionFigure 8. Render of 3D virtual model by Odeon with the absorbent  panels insertion on the walls colours makes this material aesthetically acceptable. The wide availability reduces the cost of production of the panels, and more importantly, the material used is completely recyclable. It was also found that the Odeon software provides a good prediction of the room acoustic correction. The model calibration with the software Odeon was shown to produce acceptable results for the reverberation time T 30  and the EDT.   Acoustics Australia Vol. 41, No. 3, December 2013 151 Figure 9. T 30  measured and calculated valuesFigure 10. EDT measured and calculated valuesFigure 11. D 50  measured and calculated valuesFigure 12. Sound absorbent panels inserted in a classroomFigure 13. T 30  measured and calculated valuesFigure 14. EDT measured and calculated valuesFigure 15. D 50  measured and calculated values 01020304050125250500100020004000    D   5   0   (   %   ) Frequency (Hz) measuredcalculated 012 3 4125250500100020004000    T   3   0   (  s   ) Frequency (Hz) measuredcalculated 012 3 4125250500100020004000    E   D   T   (  s   ) Frequency (Hz) measuredcalculated 01020304050125250500100020004000    D   5   0   (   %   ) Frequency (Hz) measuredcalculated   012 3 4125250500100020004000    T   3   0   (  s   ) Frequency (Hz) measuredcalculated 012 3 4125250500100020004000    E   D   T   (  s   ) Frequency (Hz) measuredcalculated REFERENCES [1] G. Iannace, L. Maffei and G. Ciaburro, “Effects of shared noise control activities in two primary schools”,  Proceedings of the 39th International Congress on Noise Control Engineering:  Inter-Noise 2010 , Lisbon, Portual, 13-16 June 2010[2] G. Iannace, L. Maffei and M. Masullo, “Are classrooms in historical buildings compatible with good acoustics standards?”,  Proceedings of Acoustics’08 Paris , Paris, France, 30 June - 4 July 2008[3] G. Iannace, L. Maffei and P. Trematerra, “On the use of “green materials” for the acoustic correction of classrooms”,  Proceedings of Euronoise 2012 , Prague, Czech Republic, 10-13 June 2012[4] International Organization for Standardization ISO 3382-2:2008:  Acoustics – Measurement of room acoustic parameters – Part 2:  Reverberation time in ordinary rooms  
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