- Email: [email protected]
Investigation of the effects of sample size on sound absorption performance of noise barrier
Applied Acoustics 157 (2020) 106989
Contents lists available at ScienceDirect
Applied Acoustics journal homepage: www.elsevier.com/locate/apacoust
Investigation of the effects of sample size on sound absorption performance of noise barrier Hsiao Mun Lee a,⇑, Zhaomeng Wang b, Kian Meng Lim b, Heow Pueh Lee b a Center for Research on Leading Technology of Special Equipment, School of Mechanical and Electric Engineering, Guangzhou University, 230 Wai Huan Xi Road, Guangzhou 510006, PR China b Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117575, Singapore
a r t i c l e
i n f o
Article history: Received 4 May 2019 Received in revised form 9 July 2019 Accepted 27 July 2019
Keywords: Noise barrier Sound absorption coefficient Sample size Reverberation room
a b s t r a c t A series of tests were performed on a commercial noise barrier in reverberation room in order to obtain the sound absorption performance and to study the effect of sample size on the noise barrier. Two types of noise barriers (metal and plastic) with sound absorptive and reflective surfaces were tested. It were found that for both metal and plastic noise barriers, their sound absorptive surfaces only had better sound absorption performances than reflective surfaces at high frequency range. The metal noise barrier with sound absorptive surface had better sound absorption performance than the plastic noise barrier. It was observed that sample size did not have great effect on sound absorption performance of noise barrier except when the sample size was about 80% smaller than the minimum sample size required by the ISO 354 standard. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction For the past two decades, many type of noise reduction methods were suggested by various researchers over the world in order to solve the worldwide noise pollution problem. One of the most popular method is the use of noise barrier where it is usually built in between a noise source and a receiver. Lambert [1] conducted a case study on a freeway noise barrier where his works included human subjective studies and noise measurements. He found that the freeway noise barrier was able to reduce noise by 10 dB of noise during peak traffic hours and 8 dB of noise during day time. Ise et al. [2] studied the performance of an active noise barrier through physical experiments and basic numerical simulations. They claimed that the active noise barrier was able to enhance noise reduction by about 10 dB to 20 dB with frequencies ranging from 160 Hz to 1.6 kHz. A three dimensional numerical model was developed by Hasebe [3] in order to study the sound pressure behind a T-profile noise barrier. The accuracy of the numerical model was verified through indoor and outdoor experiments. Experimental results showed that the T-profile noise barrier was able to reduce sound pressure level by about 10 dB at 2000 Hz with the addition of the sound absorption material. The performance of a noise barrier with a waterwheel cylinder installed on the edge of ⇑ Corresponding author. E-mail address: [email protected] (H.M. Lee). https://doi.org/10.1016/j.apacoust.2019.07.037 0003-682X/Ó 2019 Elsevier Ltd. All rights reserved.
the barrier was studied by Okubo and Fujiwara [4]. This cylinder could be assumed as an acoustically soft cylindrical surface due to its radially arranged acoustic tubes. Their numerical simulation results showed that the waterwheel cylinder design could improve the noise shielding efficiency of the noise barrier where the noise barrier could achieve 10 dB of noise reduction with frequencies ranging from 500 Hz to 800 Hz. Image receiver model together with Macdonalds diffraction theory were used by Cheng et al. [5] to predict the effectiveness of a noise barrier near the window of high-rise building facades. They conducted a scale model test in order to verify their theoretical results. From the scale model test, the barrier was proven to be able to achieve about 3 dB–5 dB of noise reduction. The performance of a noise barrier coated with sound absorptive material (light expanded clay aggregate) was studied by Palma and Samagaio [6] at A2 highway in Portugal. They found that the noise barrier was able to reduce about 7.8 dB of noise in the frequency range between 125 Hz and 4 kHz. An active noise barrier with unidirectional secondary sources was studied by Chen et al. [7]. In their studies, the secondary source consisted of two closely located loudspeakers with pre-adjusted phase difference. Their experimental results showed that the noise barrier was able to enhance noise reduction by about 20 dB. A noise barrier was constructed by Indrianti et al. [8] in order to reduce noise that generated by an air compressor in the assembly area of a manufacturing company. They designed the barrier based on Pahl and Beitz
2
H.M. Lee et al. / Applied Acoustics 157 (2020) 106989
approach and the noise in the assembly area was reduced from 106.3 dB to 74.0 dB. Harun et al. [9] investigated the effectiveness of a noise barrier made by fibre cement mortar infill with absorbent material on mitigating traffic noise. They found that the barrier could achieve insertion loss of 5 dBA if the receiver point was located at more than 3.5 m away from the barrier. Zhang et al. [10] developed a novel semi-closed noise barrier (SCNB) to mitigate high-speed railway noise through field measurement and numerical simulations. Their results showed that the SCNB could perform better than the existing 3.15 m high vertical noise barrier with additional noise attenuation close to 6 dBA. Radosz [11] presented a resonant sonic crystal noise barrier that could be applied to stationary but movable noise sources eg. power generators. This noise barrier was proven to be able to obtain sound insulation index value up to 16 dB in the targeted frequency range. Acoustical performance of a T-shaped noise barrier was studied by Sun et al. [12] using empirical formula. They concluded that the main structural parameter that affected the performance of the barrier was the length of the T-shape edge. From these reported works, it can be concluded that noise barrier is accepted by public and researchers as an effective structure to mitigate noise. However, not much data relating to the sound absorption performance of noise barrier can be found in open literature. If the sound absorption coefficient of a noise barrier is high, then little or no noise is reflected back towards the noise source or elsewhere. Sound absorption performance data can provide a set of barrier pre-design constraints for designers in order to design an effective low cost noise barrier. Moreover, many research works [13–16] claimed that the sound absorption coefficient that was measured in a reverberation room could be strongly affected by the sample size. Therefore, the main objective of the current effort is to study the sound absorption performance of a commercial noise barrier with different sample size in a reverberation room in order to improve the understanding of this important parameter which is very useful for the noise barrier design process.
2. Experimental set-up A commercial noise barrier (model: P.E.B., from Wes Noise Control Pte Ltd) which is among the most commonly used noise barrier in Singapore was selected for the present studies. The sound absorption performance of P.E.B. noise barrier is not available from open literature, not even from the manufacturer. Thus, the ISO 354 standard [17] was used to measure the sound absorption coefficient of P.E.B. noise barrier. The whole studies were carried out in a reverberation room. Two types of P.E.B. noise barriers were measured, namely the metal (super galum steel) and the plastic (PVC) noise barriers with surface area of 10 m2. This sample size was selected based on the test requirement from ISO 354 where the minimum sample size required by the standard was 10 m2. In fact, each 10 m2 noise barrier was constructed by combining 10 pieces of metal or plastic panels with each panel having a surface area of 1 m2 (0.5 m2 m, width length). The sample was placed directly on the floor as shown in Fig. 1, which was the type A mounting method as described in ISO 354. According to ISO 354, for measurement of reverberation time, the number of spatially independent measured decay curves should be at least twelve. Therefore in the present studies, the data was recorded using a Larson Davis sound level meter (model: 831) at eight different positions (MP-1a to MP-4b) for each Larson Davis omnidirectional loudspeaker (model: BASOO1) position (see Fig. 1). Two different loudspeaker positions were selected and thus, sixteen decay curves were measured and obtained in the reverberation room. All measurement points (MP-1a to MP-4b) were at least 1.5 m apart from each other and were at least 1 m away from any room surface. The positions of the loudspeaker were also at least 3 m apart from each other. For both types of noise barriers, only one side of the barrier was sound absorptive surface and the other side was sound reflective surface. The sound absorptive surface had a matrix of open slost as shown in Fig. 2. All open slots were filled by sound absorption
Fig. 1. The schematic diagram for the locations of the sample, sound level meter and loudspeaker (top view).
H.M. Lee et al. / Applied Acoustics 157 (2020) 106989
3
Fig. 2. (a) and (b) Sound absorptive and reflective surfaces of the plastic noise barrier, respectively. (c) and (d) Sound absorptive and reflective surfaces of the metal noise barrier, respectively.
Fig. 3. Measurement results of an empty reverberation room. (a) Reverberation time (b) equivalent sound absorption area (c) sound absorption coefficient.
material (Polyester). The sound absorption performances of both absorptive and reflective surfaces for both type of noise barriers were studied. In addition, in order to study the effect of sample
size, another four metal noise barriers with surface areas varying from 2 m2 to 8 m2 were constructed and were tested. For sample size studies, only the performance of the sound absorptive surface
4
H.M. Lee et al. / Applied Acoustics 157 (2020) 106989
Fig. 4. The measurement results of both metal and plastic noise barriers for both sound reflective and absorptive surfaces. (a) Reverberation time (b) equivalent sound absorption area (c) sound absorption coefficient.
was investigated. The sound absorption coefficient (as ) of the noise barrier can be obtained using Eqs. (1)–(4)[17]:
A1 ¼
55:3V 4Vm1 ; cT 1
ð1Þ
A2 ¼
55:3V 4Vm2 ; cT 2
ð2Þ
AT ¼ A2 A1 ¼
as ¼
AT S
55:3V 1 1 4Vðm2 m1 Þ; c T2 T1
ð3Þ
ð4Þ
where A1 ; A2 and AT are the equivalent sound absorption areas of the empty reverberation room, reverberation room with noise barrier and noise barrier, respectively. V is the volume of the empty reverberation room and c is the speed of sound in air. T 1 and T 2 are the reverberation time of the empty reverberation room and reverberation room with noise barrier, respectively. m1 and m2 are the power attenuation coefficients in the empty reverberation room. S is the area covered by the noise barrier. In the present studies, m1 and m2 are assumed having the same value since temperature, humidity and attenuation coefficient (b; m ¼ b=10 lgðeÞ) are the same throughout the whole experiment.
3. Results and discussion The measurement data of the empty reverberation room is shown in Fig. 3. In a standard reverberation room, as expected, the reverberation time reduces with increased of frequency (see Fig. 3(a)) due to shorter sound wavelength at higher frequency. This would directly lead to higher equivalent sound absorption area and higher sound absorption coefficient at higher frequency range. In addition, for an empty reverberation room, the absorption coefficient is almost linearly related to the frequency as shown in Fig. 3(c). The equivalent sound absorption area (A1 ) as shown in Fig. 3(b) is used for the calculation of the equivalent sound absorption area of noise barrier using Eq. (3). Fig. 4 shows the measurement results of both the metal and the plastic noise barriers for both sound reflective and absorptive surfaces. For T 2 (see Fig. 4(a)), regardless of material and absorptiveness of the noise barrier, T 2 reduces with increased frequency. For the plastic noise barrier, the differences of T 2 between reflective and absorptive surfaces are big especially at low frequency range from 200 Hz to 500 Hz. Moreover, T 2 for plastic noise barrier is higher for absorptive surface at low frequency range below 630 Hz. For metal noise barrier, the differences of T 2 between reflective and absorptive surfaces are big at higher frequency ranging from 500 Hz to 1250 Hz. Similar to the plastic noise barrier, T 2 for the metal noise barrier is also higher for absorptive surface at low frequency range below 200 Hz. It can be concluded that the sound
H.M. Lee et al. / Applied Acoustics 157 (2020) 106989
5
Fig. 5. Effects of sample size on metal noise barrier with sound absorptive surface. (a) Reverberation time (b) equivalent sound absorption area (c) sound absorption coefficient.
absorptive material is more effective at higher frequency range where it absorbs more sound energy in higher frequency range and thus, the sound absorptive surface has lower T 2 compare to reflective surface in high frequency range. Direct observation of the sound absorptive performances of the metal and the plastic noise barriers can be found in Fig. 4(c). As reflected by T 2 , the metal noise barrier with sound absorptive surface would have the best performance where it has sound absorption coefficient of 0.29 at 800 Hz. For the plastic noise barrier with sound absorptive surface, its sound absorption coefficient would generally increases with increasing frequency and could reach about 0.18 at 5000 Hz. Fig. 5 shows the effects of sample size on the metal noise barrier with sound absorptive surface. It can be observed that the T 2 of the metal noise barrier reduces with increased sample size. The differences of T 2 between different sample sizes are small for frequencies below 160 Hz. Thereafter, the differences are quite large until 800 Hz and then the differences decrease with increased frequency. It should be noted that for frequencies between 160 Hz and 800 Hz, the effect of sample size becomes more significant for sample size below 6 m2 where in this frequency range, the differences of T 2 between 2 m24 m2 and 4 m2 6 m2 are much larger than that of those between 6 m2 8 m2 and 8 m2 10 m2. For sound absorbtion coefficient (see Fig. 5(c)), it can be observed that sample size does not have great effect on it except when the sample size is too small (2 m2) at frequencies below 100 Hz and above 4000 Hz. In addition, the effect of the sample size is also more
prominent at the peak coefficient (defined as the maximum coefficient gained when the sample size is equal to the minimum sample size required by the ISO standard which is 10 m2 in the present studies) when the frequency is equal to 800 Hz. These findings are very useful for the condition where if the size of the reverberation room does not meet the requirement as stated in the ISO 354 standard [17], the engineer still can try to test the sound absorption performance of the noise barrier by reducing its sample size by not more than 60% of the minimum size required by the standard. 4. Conclusions A series of tests were performed on commercial P.E.B. noise barriers in a reverberation room in order to study the absorption performance of the noise barriers. In addition, the effects of sample size on the noise barrier were also evaluated during the tests. The tests were conducted according to the ISO 354 standard. Two types of noise barriers (metal and plastic) with sound absorptive and reflective surfaces were tested. It was found that for both the metal and the plastic noise barriers, their sound absorptive surfaces only had better sound absorption performances than reflective surfaces for frequencies above 315 Hz and 630 Hz, respectively. This means that the effect of the sound absorption material was not obvious at low frequency range. Generally, the metal noise barrier with sound absorptive surface had better sound
6
H.M. Lee et al. / Applied Acoustics 157 (2020) 106989
absorption performance than the plastic noise barrier except for frequencies below 100 Hz and above 2000 Hz. It was concluded that sample size did not have great effect on sound absorption performance of noise barrier except when the sample size was too small (2 m2) at frequencies below 100 Hz, above 4000 Hz and at the frequency where peak coefficient occurred (800 Hz in the present studies). Acknowledgement The authors would like to acknowledge the financial support by the Land Transport Authority of Singapore under the LTA Innovation grant (WBS no: R-265-000-542-490) and support from Natural Science Foundation of Guangdong Province (2018A030313878). References [1] Lambert RF. Experimental evaluation of a freeway noise barrier. Noise Control Eng 1978;11(2):86–94. [2] Ise S, Yano H, Tachibana H. Basic study on active noise barrier. J Acoust Soc Jpn (E) 1991;12(6):299–306. [3] Hasebe M. Sound reduction by a t-profile noise barrier. J Acoust Soc Jpn (E) 1995;16(3):173–9. [4] Okubo T, Fujiwara K. Efficiency of a noise barrier on the ground with an acoustically soft cylindrical edge. J Sound Vib 1998;216(5):771–90. [5] Cheng W, Ng C, Fung K. Theoretical model to optimize noise barrier performance at the window of a high-rise building. J Sound Vib 2000;238 (1):51–63.
[6] Palma M, Samagaio A. Acoustic performance of a noise barrier coated with an absorptive material. Noise Control Eng J 2006;54(4):245–50. [7] Chen W, Rao W, Min H, Qiu X. An active noise barrier with unidirectional secondary sources. Appl Acoust 2011;72(12):969–74. [8] Indrianti N, Biru NB, Wibawa T. The development of compressor noise barrier in the assembly area (case study of pt jawa furni lestari). Proc CIRP 2016;40:705–10. [9] Haron Z, Jamaludin J, Darus N, Yahya K, Jahya Z, Mashros N, Hezmi M. A case study of acoustic efficiency of existing noise barrier in reducing road traffic noise in school area, vol. 220. Johor, Malaysia; 2019.. [10] Zhang X, Liu R, Cao Z, Wang X, Li X. Acoustic performance of a semi-closed noise barrier installed on a high-speed railway bridge: measurement and analysis considering actual service conditions. Measurement 2019;138:386–99. [11] Radosz J. Acoustic performance of noise barrier based on sonic crystals with resonant elements. Appl Acoust 2019;155:492–9. [12] Sun W, Liu L, Yuan H, Su Q. Influence of top shape on noise reduction effect of high-speed railway noise barrier, vol. 493. Sanya City, China; 2019.. [13] Bischel M, Roy K, Greenslade J. Comparison of astm and iso sound absorption test methods. Paris, France; 2008. pp. 1669–1674.. [14] Rubacha J, Pilch A, Zastawnik M. Measurements of the sound absorption coefficient of auditorium seats for various geometries of the samples. Arch Acoust 2012;37(4):483–8. [15] Tomiku R, Otsuru T, Okamoto N, Okuzono T, Azechi Y, Yoshida T. Finite element sound field analysis for correction of absorption coefficient in reverberation room. Melbourne, Australia; 2014.. [16] Hughes WO, McNelis AM, Nottoli C, Wolfram E. Examination of the measurement of absorption using the reverberant room method for highly absorptive acoustic foam. In: 29th Aerospace Testing Semina, , Los Angeles, United States. [17] Iso 354: Acoustic-measurement of sound absorption in a reverberation room..
Contents lists available at ScienceDirect
Applied Acoustics journal homepage: www.elsevier.com/locate/apacoust
Investigation of the effects of sample size on sound absorption performance of noise barrier Hsiao Mun Lee a,⇑, Zhaomeng Wang b, Kian Meng Lim b, Heow Pueh Lee b a Center for Research on Leading Technology of Special Equipment, School of Mechanical and Electric Engineering, Guangzhou University, 230 Wai Huan Xi Road, Guangzhou 510006, PR China b Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117575, Singapore
a r t i c l e
i n f o
Article history: Received 4 May 2019 Received in revised form 9 July 2019 Accepted 27 July 2019
Keywords: Noise barrier Sound absorption coefficient Sample size Reverberation room
a b s t r a c t A series of tests were performed on a commercial noise barrier in reverberation room in order to obtain the sound absorption performance and to study the effect of sample size on the noise barrier. Two types of noise barriers (metal and plastic) with sound absorptive and reflective surfaces were tested. It were found that for both metal and plastic noise barriers, their sound absorptive surfaces only had better sound absorption performances than reflective surfaces at high frequency range. The metal noise barrier with sound absorptive surface had better sound absorption performance than the plastic noise barrier. It was observed that sample size did not have great effect on sound absorption performance of noise barrier except when the sample size was about 80% smaller than the minimum sample size required by the ISO 354 standard. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction For the past two decades, many type of noise reduction methods were suggested by various researchers over the world in order to solve the worldwide noise pollution problem. One of the most popular method is the use of noise barrier where it is usually built in between a noise source and a receiver. Lambert [1] conducted a case study on a freeway noise barrier where his works included human subjective studies and noise measurements. He found that the freeway noise barrier was able to reduce noise by 10 dB of noise during peak traffic hours and 8 dB of noise during day time. Ise et al. [2] studied the performance of an active noise barrier through physical experiments and basic numerical simulations. They claimed that the active noise barrier was able to enhance noise reduction by about 10 dB to 20 dB with frequencies ranging from 160 Hz to 1.6 kHz. A three dimensional numerical model was developed by Hasebe [3] in order to study the sound pressure behind a T-profile noise barrier. The accuracy of the numerical model was verified through indoor and outdoor experiments. Experimental results showed that the T-profile noise barrier was able to reduce sound pressure level by about 10 dB at 2000 Hz with the addition of the sound absorption material. The performance of a noise barrier with a waterwheel cylinder installed on the edge of ⇑ Corresponding author. E-mail address: [email protected] (H.M. Lee). https://doi.org/10.1016/j.apacoust.2019.07.037 0003-682X/Ó 2019 Elsevier Ltd. All rights reserved.
the barrier was studied by Okubo and Fujiwara [4]. This cylinder could be assumed as an acoustically soft cylindrical surface due to its radially arranged acoustic tubes. Their numerical simulation results showed that the waterwheel cylinder design could improve the noise shielding efficiency of the noise barrier where the noise barrier could achieve 10 dB of noise reduction with frequencies ranging from 500 Hz to 800 Hz. Image receiver model together with Macdonalds diffraction theory were used by Cheng et al. [5] to predict the effectiveness of a noise barrier near the window of high-rise building facades. They conducted a scale model test in order to verify their theoretical results. From the scale model test, the barrier was proven to be able to achieve about 3 dB–5 dB of noise reduction. The performance of a noise barrier coated with sound absorptive material (light expanded clay aggregate) was studied by Palma and Samagaio [6] at A2 highway in Portugal. They found that the noise barrier was able to reduce about 7.8 dB of noise in the frequency range between 125 Hz and 4 kHz. An active noise barrier with unidirectional secondary sources was studied by Chen et al. [7]. In their studies, the secondary source consisted of two closely located loudspeakers with pre-adjusted phase difference. Their experimental results showed that the noise barrier was able to enhance noise reduction by about 20 dB. A noise barrier was constructed by Indrianti et al. [8] in order to reduce noise that generated by an air compressor in the assembly area of a manufacturing company. They designed the barrier based on Pahl and Beitz
2
H.M. Lee et al. / Applied Acoustics 157 (2020) 106989
approach and the noise in the assembly area was reduced from 106.3 dB to 74.0 dB. Harun et al. [9] investigated the effectiveness of a noise barrier made by fibre cement mortar infill with absorbent material on mitigating traffic noise. They found that the barrier could achieve insertion loss of 5 dBA if the receiver point was located at more than 3.5 m away from the barrier. Zhang et al. [10] developed a novel semi-closed noise barrier (SCNB) to mitigate high-speed railway noise through field measurement and numerical simulations. Their results showed that the SCNB could perform better than the existing 3.15 m high vertical noise barrier with additional noise attenuation close to 6 dBA. Radosz [11] presented a resonant sonic crystal noise barrier that could be applied to stationary but movable noise sources eg. power generators. This noise barrier was proven to be able to obtain sound insulation index value up to 16 dB in the targeted frequency range. Acoustical performance of a T-shaped noise barrier was studied by Sun et al. [12] using empirical formula. They concluded that the main structural parameter that affected the performance of the barrier was the length of the T-shape edge. From these reported works, it can be concluded that noise barrier is accepted by public and researchers as an effective structure to mitigate noise. However, not much data relating to the sound absorption performance of noise barrier can be found in open literature. If the sound absorption coefficient of a noise barrier is high, then little or no noise is reflected back towards the noise source or elsewhere. Sound absorption performance data can provide a set of barrier pre-design constraints for designers in order to design an effective low cost noise barrier. Moreover, many research works [13–16] claimed that the sound absorption coefficient that was measured in a reverberation room could be strongly affected by the sample size. Therefore, the main objective of the current effort is to study the sound absorption performance of a commercial noise barrier with different sample size in a reverberation room in order to improve the understanding of this important parameter which is very useful for the noise barrier design process.
2. Experimental set-up A commercial noise barrier (model: P.E.B., from Wes Noise Control Pte Ltd) which is among the most commonly used noise barrier in Singapore was selected for the present studies. The sound absorption performance of P.E.B. noise barrier is not available from open literature, not even from the manufacturer. Thus, the ISO 354 standard [17] was used to measure the sound absorption coefficient of P.E.B. noise barrier. The whole studies were carried out in a reverberation room. Two types of P.E.B. noise barriers were measured, namely the metal (super galum steel) and the plastic (PVC) noise barriers with surface area of 10 m2. This sample size was selected based on the test requirement from ISO 354 where the minimum sample size required by the standard was 10 m2. In fact, each 10 m2 noise barrier was constructed by combining 10 pieces of metal or plastic panels with each panel having a surface area of 1 m2 (0.5 m2 m, width length). The sample was placed directly on the floor as shown in Fig. 1, which was the type A mounting method as described in ISO 354. According to ISO 354, for measurement of reverberation time, the number of spatially independent measured decay curves should be at least twelve. Therefore in the present studies, the data was recorded using a Larson Davis sound level meter (model: 831) at eight different positions (MP-1a to MP-4b) for each Larson Davis omnidirectional loudspeaker (model: BASOO1) position (see Fig. 1). Two different loudspeaker positions were selected and thus, sixteen decay curves were measured and obtained in the reverberation room. All measurement points (MP-1a to MP-4b) were at least 1.5 m apart from each other and were at least 1 m away from any room surface. The positions of the loudspeaker were also at least 3 m apart from each other. For both types of noise barriers, only one side of the barrier was sound absorptive surface and the other side was sound reflective surface. The sound absorptive surface had a matrix of open slost as shown in Fig. 2. All open slots were filled by sound absorption
Fig. 1. The schematic diagram for the locations of the sample, sound level meter and loudspeaker (top view).
H.M. Lee et al. / Applied Acoustics 157 (2020) 106989
3
Fig. 2. (a) and (b) Sound absorptive and reflective surfaces of the plastic noise barrier, respectively. (c) and (d) Sound absorptive and reflective surfaces of the metal noise barrier, respectively.
Fig. 3. Measurement results of an empty reverberation room. (a) Reverberation time (b) equivalent sound absorption area (c) sound absorption coefficient.
material (Polyester). The sound absorption performances of both absorptive and reflective surfaces for both type of noise barriers were studied. In addition, in order to study the effect of sample
size, another four metal noise barriers with surface areas varying from 2 m2 to 8 m2 were constructed and were tested. For sample size studies, only the performance of the sound absorptive surface
4
H.M. Lee et al. / Applied Acoustics 157 (2020) 106989
Fig. 4. The measurement results of both metal and plastic noise barriers for both sound reflective and absorptive surfaces. (a) Reverberation time (b) equivalent sound absorption area (c) sound absorption coefficient.
was investigated. The sound absorption coefficient (as ) of the noise barrier can be obtained using Eqs. (1)–(4)[17]:
A1 ¼
55:3V 4Vm1 ; cT 1
ð1Þ
A2 ¼
55:3V 4Vm2 ; cT 2
ð2Þ
AT ¼ A2 A1 ¼
as ¼
AT S
55:3V 1 1 4Vðm2 m1 Þ; c T2 T1
ð3Þ
ð4Þ
where A1 ; A2 and AT are the equivalent sound absorption areas of the empty reverberation room, reverberation room with noise barrier and noise barrier, respectively. V is the volume of the empty reverberation room and c is the speed of sound in air. T 1 and T 2 are the reverberation time of the empty reverberation room and reverberation room with noise barrier, respectively. m1 and m2 are the power attenuation coefficients in the empty reverberation room. S is the area covered by the noise barrier. In the present studies, m1 and m2 are assumed having the same value since temperature, humidity and attenuation coefficient (b; m ¼ b=10 lgðeÞ) are the same throughout the whole experiment.
3. Results and discussion The measurement data of the empty reverberation room is shown in Fig. 3. In a standard reverberation room, as expected, the reverberation time reduces with increased of frequency (see Fig. 3(a)) due to shorter sound wavelength at higher frequency. This would directly lead to higher equivalent sound absorption area and higher sound absorption coefficient at higher frequency range. In addition, for an empty reverberation room, the absorption coefficient is almost linearly related to the frequency as shown in Fig. 3(c). The equivalent sound absorption area (A1 ) as shown in Fig. 3(b) is used for the calculation of the equivalent sound absorption area of noise barrier using Eq. (3). Fig. 4 shows the measurement results of both the metal and the plastic noise barriers for both sound reflective and absorptive surfaces. For T 2 (see Fig. 4(a)), regardless of material and absorptiveness of the noise barrier, T 2 reduces with increased frequency. For the plastic noise barrier, the differences of T 2 between reflective and absorptive surfaces are big especially at low frequency range from 200 Hz to 500 Hz. Moreover, T 2 for plastic noise barrier is higher for absorptive surface at low frequency range below 630 Hz. For metal noise barrier, the differences of T 2 between reflective and absorptive surfaces are big at higher frequency ranging from 500 Hz to 1250 Hz. Similar to the plastic noise barrier, T 2 for the metal noise barrier is also higher for absorptive surface at low frequency range below 200 Hz. It can be concluded that the sound
H.M. Lee et al. / Applied Acoustics 157 (2020) 106989
5
Fig. 5. Effects of sample size on metal noise barrier with sound absorptive surface. (a) Reverberation time (b) equivalent sound absorption area (c) sound absorption coefficient.
absorptive material is more effective at higher frequency range where it absorbs more sound energy in higher frequency range and thus, the sound absorptive surface has lower T 2 compare to reflective surface in high frequency range. Direct observation of the sound absorptive performances of the metal and the plastic noise barriers can be found in Fig. 4(c). As reflected by T 2 , the metal noise barrier with sound absorptive surface would have the best performance where it has sound absorption coefficient of 0.29 at 800 Hz. For the plastic noise barrier with sound absorptive surface, its sound absorption coefficient would generally increases with increasing frequency and could reach about 0.18 at 5000 Hz. Fig. 5 shows the effects of sample size on the metal noise barrier with sound absorptive surface. It can be observed that the T 2 of the metal noise barrier reduces with increased sample size. The differences of T 2 between different sample sizes are small for frequencies below 160 Hz. Thereafter, the differences are quite large until 800 Hz and then the differences decrease with increased frequency. It should be noted that for frequencies between 160 Hz and 800 Hz, the effect of sample size becomes more significant for sample size below 6 m2 where in this frequency range, the differences of T 2 between 2 m24 m2 and 4 m2 6 m2 are much larger than that of those between 6 m2 8 m2 and 8 m2 10 m2. For sound absorbtion coefficient (see Fig. 5(c)), it can be observed that sample size does not have great effect on it except when the sample size is too small (2 m2) at frequencies below 100 Hz and above 4000 Hz. In addition, the effect of the sample size is also more
prominent at the peak coefficient (defined as the maximum coefficient gained when the sample size is equal to the minimum sample size required by the ISO standard which is 10 m2 in the present studies) when the frequency is equal to 800 Hz. These findings are very useful for the condition where if the size of the reverberation room does not meet the requirement as stated in the ISO 354 standard [17], the engineer still can try to test the sound absorption performance of the noise barrier by reducing its sample size by not more than 60% of the minimum size required by the standard. 4. Conclusions A series of tests were performed on commercial P.E.B. noise barriers in a reverberation room in order to study the absorption performance of the noise barriers. In addition, the effects of sample size on the noise barrier were also evaluated during the tests. The tests were conducted according to the ISO 354 standard. Two types of noise barriers (metal and plastic) with sound absorptive and reflective surfaces were tested. It was found that for both the metal and the plastic noise barriers, their sound absorptive surfaces only had better sound absorption performances than reflective surfaces for frequencies above 315 Hz and 630 Hz, respectively. This means that the effect of the sound absorption material was not obvious at low frequency range. Generally, the metal noise barrier with sound absorptive surface had better sound
6
H.M. Lee et al. / Applied Acoustics 157 (2020) 106989
absorption performance than the plastic noise barrier except for frequencies below 100 Hz and above 2000 Hz. It was concluded that sample size did not have great effect on sound absorption performance of noise barrier except when the sample size was too small (2 m2) at frequencies below 100 Hz, above 4000 Hz and at the frequency where peak coefficient occurred (800 Hz in the present studies). Acknowledgement The authors would like to acknowledge the financial support by the Land Transport Authority of Singapore under the LTA Innovation grant (WBS no: R-265-000-542-490) and support from Natural Science Foundation of Guangdong Province (2018A030313878). References [1] Lambert RF. Experimental evaluation of a freeway noise barrier. Noise Control Eng 1978;11(2):86–94. [2] Ise S, Yano H, Tachibana H. Basic study on active noise barrier. J Acoust Soc Jpn (E) 1991;12(6):299–306. [3] Hasebe M. Sound reduction by a t-profile noise barrier. J Acoust Soc Jpn (E) 1995;16(3):173–9. [4] Okubo T, Fujiwara K. Efficiency of a noise barrier on the ground with an acoustically soft cylindrical edge. J Sound Vib 1998;216(5):771–90. [5] Cheng W, Ng C, Fung K. Theoretical model to optimize noise barrier performance at the window of a high-rise building. J Sound Vib 2000;238 (1):51–63.
[6] Palma M, Samagaio A. Acoustic performance of a noise barrier coated with an absorptive material. Noise Control Eng J 2006;54(4):245–50. [7] Chen W, Rao W, Min H, Qiu X. An active noise barrier with unidirectional secondary sources. Appl Acoust 2011;72(12):969–74. [8] Indrianti N, Biru NB, Wibawa T. The development of compressor noise barrier in the assembly area (case study of pt jawa furni lestari). Proc CIRP 2016;40:705–10. [9] Haron Z, Jamaludin J, Darus N, Yahya K, Jahya Z, Mashros N, Hezmi M. A case study of acoustic efficiency of existing noise barrier in reducing road traffic noise in school area, vol. 220. Johor, Malaysia; 2019.. [10] Zhang X, Liu R, Cao Z, Wang X, Li X. Acoustic performance of a semi-closed noise barrier installed on a high-speed railway bridge: measurement and analysis considering actual service conditions. Measurement 2019;138:386–99. [11] Radosz J. Acoustic performance of noise barrier based on sonic crystals with resonant elements. Appl Acoust 2019;155:492–9. [12] Sun W, Liu L, Yuan H, Su Q. Influence of top shape on noise reduction effect of high-speed railway noise barrier, vol. 493. Sanya City, China; 2019.. [13] Bischel M, Roy K, Greenslade J. Comparison of astm and iso sound absorption test methods. Paris, France; 2008. pp. 1669–1674.. [14] Rubacha J, Pilch A, Zastawnik M. Measurements of the sound absorption coefficient of auditorium seats for various geometries of the samples. Arch Acoust 2012;37(4):483–8. [15] Tomiku R, Otsuru T, Okamoto N, Okuzono T, Azechi Y, Yoshida T. Finite element sound field analysis for correction of absorption coefficient in reverberation room. Melbourne, Australia; 2014.. [16] Hughes WO, McNelis AM, Nottoli C, Wolfram E. Examination of the measurement of absorption using the reverberant room method for highly absorptive acoustic foam. In: 29th Aerospace Testing Semina, , Los Angeles, United States. [17] Iso 354: Acoustic-measurement of sound absorption in a reverberation room..