Niigata University, Japan. Kobayasi Institute of Physical Research, Japan. Niigata University, Japan

Transcription

1 INTER-NOISE 216 Control of resonance penetration phenomenon in double leaf structure for sound insulation by insertion of small Helmholt resonator and porous material Teruo IWASE 1 ; Satoshi SUGIE 2 ; Hiroyasu KURONO 3 ; Yasuaki OKADA 4 ; Koichi YOSHIHISA 5 ; 1 Niigata University, Japan 2 Kobayasi Institute of Physical Research, Japan 3 Niigata University, Japan 4 Meijo University, Japan 5 Meijo University, Japan ABSTRACT Double leaf structure is one of the effective ways to obtain high sound insulation. However, it has well known weak point of decreasing efficiency of sound insulation at the specific frequency by the resonance penetration phenomenon. So, it is very important to suppress the phenomenon. Authors tried to understand the basic vibrating form of plates and the weak point, and to recover the sound insulation efficiency by partial insertion of porous material or small Helmholt resonator into narrow air space between two leafs. Our improving way was based on the idea that acoustic impedance in the air space would be changed from high condition by strong sound resonance to low impedance condition by insertion of small acoustic circuit. Insertion effects were tested by both ways of model calculations and experiments using small anechoic box. Efficiency of sound insulation of double leaf structure was decreased about several to 1 db compared with that of double layer structure at the resonance frequency. Suppression effect on the resonance penetration by inserting partial porous material was slight. On the other hand, the recovering effect by insertion of small resonators was high as sound insulation reaches to that of single leaf structure. Keywords: Sound, Insulation, Transmission, Helmholt resonators, I-INCE Classification of Subjects Number(s): 34.3, INTRODUCTION Separation of living space by plate material from loud sound source is one of the simplest noise reduction methods. To realie high efficiency of sound insulation, many test investigations of the sound insulation characteristics of basic materials and structure as wood plates, gypsum boards, partitioning wall, glass windows, wooden/metal doors and outer concrete walls had been researched. Moreover, many theoretical and practical studies concerning structure and combination way for high sound insulation had been done. Authors also had studied on the same subjects of sound insulation(1,2). These studies made clear that high sound insulation could be obtained by use of double wall or double leaf structure with air space between both walls or light plates instead of use of single type heavy and thick wall. Now a day, double type structures are widely used. One of the typical structure is double wall by light gypsum board for partitioning the space in modern buildings. And, there is double leaf glass window for thermal insulation against hot or cold atmospheric temperature. 1 iwase@cc.niigata-u.ac.jp 2 sugie@kobayasi-riken.or.jp 3 kurono@eng.niigata-u.ac.jp 4 okada@meijo-u.ac.jp 5 yosihisa@meijo-u.ac.jp 3619

2 INTER-NOISE 216 However, it is well known that there is the weak point in such structure. It makes easy sound transmission/penetration by the sound resonance phenomenon between two plates through narrow air space, and sound insulation decrease at specific frequency. Sound resonance phenomena appear at the specific low frequency in the former structure with wide air space. In the later structure of thin glass plates with narrow air space, resonance phenomena appears at the specific mid frequency in which high sound spectra of human voices and community noise are existing. Treatment to control the resonance phenomena is not easy and limited for the later structure with only narrow air space. Many researches to improve the weak point are continued even now. Our research is, one of such studies, to make problem clear on the double leaf structure imitating glass window and to find out the way to recover the lost efficiency of sound insulation. It can be thought that various examinations and considerations obtained in this study will be also able to be applied to problems in double wall of gypsum board to keep efficiency of the sound insulation. 2. PRELIMINARY CONSIDERATION OF DOUBLE LEAF STRUCTURE At first, authors started the consideration by following simple theoretical simulation on the sound transmission with focusing on the double leaf structure, as described just above. 2.1 Basic acoustical model of double leaf structure Basic acoustic model of the double leaf structure of this study is shown in Figure 1. This is based on the assumption of theory in plane wave field to simplify the consideration by calculation and acoustical measurement. To consider whether the resonance phenomena can be controlled or not by inserting some sound circuits to solve the problem called as "Sound resonance penetration in low frequency area". The acoustical calculation model is introduced as below. (A)Basic model (B) Impedance model (C) Insertion of acoustic circuit Single Plate (m=2.35kg/m 2 ) Air space: d=1mm air Sound incidence γ, c Sound transfer air Single Plate (m=2.35kg/m 2 ) Air space: d=1mm air γ, c insert Sound transfer air Sound incidence back _ m2 front _ m1 front _ m2 back _ m1 for Resonator with opening holes S insert Single plate (Acrylic plate 45x3x2 mm 2 ) Single Plate Framing material ( Aluminum) t S insert insert Treating material (Glass wool) Figure 1 Acoustical basic model (A) and acoustic impedance model for calculation (B) of typical test sample with improving ways for sound insulation of double leaf structure (C) By giving the property values of materials consisting structure or media filled in each part, as the area density of plate m 1, m 2 calculated from density and thickness of materials, sound propagation constant γ and characteristic acoustic impedance c in the media (3) between both plates, each acoustic impedance at representative point from the right sound receiving side to the left sound source side in the transmission path is calculated continuously by Equation (1), (2), (3) and (4). back _ m2 front _ m2 back _ m1 = = = air back _ m2 c front _ m2 front _ m2 + jωm front _ m1 = back _ m1 + jωm1 (4) Then, sound transfer characteristics in sound pressure between each representative point in the sound transmission path from the sound source side to the receiving sound side, that means sound attenuation characteristic, can be calculated based on the Equations (5), (6) and (7) for front side, internal space and rear side by obtained acoustic impedance for each representative point and characteristic acoustic impedance in each sound propagating media(4). Total sound transmission characteristics Tr double_leaf is calculated by Equation (8), multiplications of Equations from(5) to (7). 2 coth( γd) + sinh( γd) + c c sinh( γd) cosh( γd) (1) (2) (3) 362

3 INTER-NOISE 216 Tr1 = Tr Tr mid _ sp Tr2 = double _ leaf back _ m1 air + back _ m2 front _ m1 = exp( γd ) c front _ m2 = TrTr 1 c air mid _ sp Tr 2 (5) (6) (7) (8) When some kinds of porous material or acoustic mechanisms as resonator is inserted into the internal air space, sound loss in the space according to an ideal acoustic circuit can be expected. Then, authors thought that the effects as the sound loss by suppressing treatment on the sound resonance penetration could be estimated by the termination by acoustic impedance Z insert corresponding to the inserted sound mechanism which makes by-passing sound flow loss in acoustic particle velocity. For example of insertion of porous material as glass wool, following Equation (9) can be used for the calculation of insert by giving their propagation constant γ gw, characteristic acoustic impedance c_gw and insertion sies of thickness t insert and surface area S insert. insert c _ gw = coth( γ gwtinsert ) (9) s insert 2.2 Experimental model of test samples Authors actually executed many acoustical experiments to confirm the acoustic model of the double leaf structure. For basic test sample in experiments and calculation, two rectangle acrylic plates with area of 45x3mm 2, density of 1.16x1 3 kg/m 3, thickness of 2mm was used, and narrow internal air space of 1mm was kept by aluminum frame parts at their perimeter. 2.3 Calculation of sound transmission through double leaf structure The sound transmission characteristics of the basic test sample of the double leaf structure were calculated by using equations and by giving physical property values introduced just above. Both additional structures of single plate with the framing parts and of double layer of plate were used for the references of sound insulation characteristics. As the calculated result by Equations (1)-(7), sound transmission characteristics of basic double leaf sample is shown in Figure 2(A). In this calculation, loss component was considered in the area density of plate including complex number part as to be m(1+.5i). A sharp peak appears at about 55H. High peak means loosing efficiency of sound insulation structure. Calculated acoustic impedance back_m1 in the internal space is shown in Figure 2(B). c = 2π ρ( m + m m m d f rmd ) (1) When area density m 1, m 2 of 2.35kg/m 2, sound velocity in the air c, the density of air ρ and width of internal air space d of 1mm were given to Equation (1), sound resonance frequency of the basic test sample can be calculated to be 55H. It can be understood that appeared sharp peak by our calculation way is matched to the theoretical frequency of sound resonance phenomenon in the double leaf structure. Moreover, both calculated results based on single leaf structure and double layer of plate based on mass law theory are shown in the same Figure.2(A). From these calculated results in the figure, it can be understood that sound transmission level of double leaf structure is almost the same to that of double layer structure, with area density of 4.7kg/m 2, in the frequency area lower than sound resonance frequency, and is very low in the frequency area higher than sound resonance frequency, with high efficiency of sound insulation. 3621

4 INTER-NOISE (re.: Incident S.P.L.) (A) Single Plate(2.35kg/m 2 ) Double layer(4.7kg/m 2 ) Double leaf (4.7kg/m 2 ) Normalied impedance (B) Basic double leaf structure Imaginary part Real part Figure 2 Calculated examples of sound transmission of basic double leaf structure with simple theoretical characteristics on mass law for single and double layer (A) and of acoustic impedance in the air space (B) (re.: Incident S.P.L.) Single Plate(2.35kg/m 2 ) Double leafs GW32K(5mm) -1 GW32K(1mm) Double layer(4.7kg/m 2 ) (A) Basic sample and with GW32K (re.: Incident S.P.L.) Single Plate(2.35kg/m 2 ) Double leafs Single Resonator -1 (5H) Other Treatments Double layer(4.7kg/m 2 ) (B) for Improving Figure 3 Calculated examples on sound transmission of double leaf type test sample and its improving examples with insertion of porous material imitating glass wool (A) or Helmholt resonator/s (B) It can be also understood that sound insulation characteristics around the sound resonance frequency is extraordinarily decreases as lower than that of single leaf structure and looks like in lossless full sound penetration. From the calculated acoustic impedance back_m1 shown in Figure 2(B), it could be understood that the acoustic impedance value at the specific frequency, where the sound resonance penetration would be caused, become very high as to be 1 times in the air. Therefore, it could be thought to control sound resonance penetration by some kinds of treatment as inserting of porous material or resonator which would make sound energy loss or acoustic impedance low in the internal air space. 2.4 Control by inserting of acoustical circuits At first, estimation of inserting effect of porous material at an end in internal air space of double leaf structure was tried. In the calculation, use of glass wool strip with density of 32kg/m 3 of a typical porous material was applied as shown in Figurer 1(C). In these case, their widths of glass wool stripe were selected to be 1, 5 and 25mm. Effect of insertion of the glass wool into whole area of the internal space was estimated also for the comparing reference by changing the values of propagation constant γ and characteristic acoustic impedance c from the values in the air to those of glass wool (2) in the Equation (3) and Equation (5)-(7). The last case is not for practical use, because effectiveness of transparency of plates is lost but for just reference. Calculated examples are shown in Figure 3(A), and these show that the insertion effects on peak level decreasing is limited within 8 db, these are very low compared with the typical insertion effect of thick porous material into double gypsum board wall for general partitioning of space in buildings. Next, the calculation was examined under the condition of the assuming of insertion of Helmholt 3622

5 INTER-NOISE 216 resonator with resonance frequency matched to that of sound resonance penetration of the double leaf structure as shown in Figure 1(C) (5). Figure 3(B) shows calculated examples, and remarkable decreasing of transmission at sound resonance frequency and high level increasing at lower and higher both side of resonance frequency are clearly shown when neck part of the resonator had low resistance. From point of view of over all suppression of sound transmission around resonance frequency, this is not effective example. From the some tries of calculation by changing setting condition of the resonator, it could be found that insertion of the resonator with certain value of resistance at the neck part and multiple insertions of the resonator with different resonance frequency for each were effective to control the sound transmission characteristics as the same description of Suigie about the controlling in double gypsum board wall by insertion of Helmholt resonators (6). 3. EXPERIMENTS OF SOUND TRANSMISSION CHARACTERISTICS AND EXCITATED VIBRATION ON PLATES Authors tried acoustical experiments to confirm the calculated results described just above on the sound transmission of double leaf structure. At first of this stage, basic sound transmission characteristics were observed and analyed. And both basic sound transmission characteristics of the single plate structure and the double layer structure of plate were observed too. For the detailed consideration on the sound transmission of the double leaf structure consisted by light weight plate material, the observation experiment on the vibration response on plate excited by sound incidence were also executed at the same time of sound transmission tests. Many experiments were executed by use of setup shown in Figure 4 and following procedures. White noise sound source was radiated onto test samples from a spherical small loud speaker put upper point with distance of.65m from the surface of the test sample. Small microphone, with diameter of 6mm and length of 3mm, was put on the surface of each test sample for the receiving of reference incidence sound pressure. Several numbers of small microphone were set under the test sample for receiving of transmitted one, and a couple of microphone were installed into the internal air space for the monitoring of sound pressure at important point on the sound transmission path. For the sound receiving only transmitted sound wave through object test sample, simplified small anechoic sound receiving box with multi layer from outside to inside as laminated wooden plate with 12mm thickness, polystyrene form of 1mm, fine sand layer thicker than 5mm, inner thin vinyl chloride plate of 5mm and glass wool layer thicker than 1mm was set under the test sample. White noise sound signal and microphone output signals of incidence sound pressure, transmitted sound pressure and that in the internal air space were simultaneously recorded on a multi channel wave-format type recorder. Sound transmission characteristics were analyed from the recorded files as transfer function by use of FFT software's in a personal computer. Loud speaker.65m Vinyl chloride plate Test sample Reference microphone Laser vibrometer to 11x17 points Air space : d Microphones Glass wool layer Single Plate Loud speaker for Horiontal incidence Baffle plate for Horiontal incidence Wooden box Sand layer Polystyrene foam Figure 4 Outline of experiments on sound transmission characteristics and on excited vibration on surface of plates of double leaf structure 3623

6 INTER-NOISE (A) in Air space Double layer Single leaf Double leaf Figure 5 Analyed examples of sound transmission characteristics (A) and exited vibration on double leaf structure (B) To research the detailed causes of sound penetration at the sound resonance and to find out the recovering way for the lost efficiency of sound insulation or improving way, excited vibrations by sound incidence on the plates consisting test sample were simultaneously observed with the acoustic observation by use of a laser vibration meter. Examples of the obtained measured and analyed results are shown in Figure 5. These are transfer functions of the object sound pressure or vibration velocity against the reference sound pressure just on the surface of incident side plate, and they are seemed to be approximately refereed with the two times value of ideal and theoretical incident sound pressure to the test sample. Therefore, displayed value should be corrected by adding 6dBs to understand truly physical and acoustical phenomena. Analyed results in Figure 5(A) shows that penetrating/transmitting sound level are roughly decreasing by increasing of frequency. For example of single plate structure, levels are as -7dB at about 1H, -22B at 3H and -35dB at 1kH. In the case of double layer structure, they are more decreasing than the characteristics of single structure with difference of several dbs as according to mass law theory. However, the observed sound transmission characteristics have a lot of peak and dip, these do not shows smooth decreasing curves based on simple mass law theory on sound insulation of plate/board material. It can be understood that these characteristics are reflections of a lot of natural vibration mode on board material excited by sound pressure. The analyed result of the penetrated sound pressure through the basic test sample of double leaf structure and observed sound pressure at the important monitoring point in internal air space on the sound transmission path are shown in the same Figure 5(A). In the observed and analyed result on the double leaf structure, distinguished peak can be seen at 48H. Even it is lower than 55H where phenomenon of sound resonance penetration is estimated to be appeared in the double leaf structure as shown in Figure 2(A). Level of the peak at the frequency considered to be caused sound resonance is about -17dB, it is 13dB higher than that of single plate, and 18dB higher than double layer structure. It is also understood that the relative sound pressure level of 1dB at the resonance frequency in the internal air space has been risen more than on the surface of incident side plate. Resonance frequency appeared in the actual experimental result shifted to lower side, because supporting parts of stiff aluminum material at the four sides fixing at perimeter increases effective weight of plates. Moreover, analyed frequency characteristics of vibration velocity on the plates are shown in Figure 5(B). Many peaks appear at many frequency from lower than 1H. And distinguished peak at frequency about 48 H can be seen in the results not only on front incident side plate but also on rear side plate as they are higher than 15dB, even on rear side plate, compared with that of single structure at the same frequency of resonance in the double leaf structure. It can be understood that narrow air space becomes stiff spring between both mass weight of front and rear side plates, and that excited vibration on the incident side plate easily propagates to the rear side plate by crossing the stiff bridge of the internal air space. It also is estimated that natural vibration modes closed to the resonance frequency of double leaf structure are strongly excited. Since above-mentioned results are in the condition of imitating normal sound incident from upper side loud speaker, the observation experiment of imitating horiontal sound incidence to the test Transfer function : db (B) Single leaf Double layer Double Leaf Rear plate Front plate 3624

7 INTER-NOISE 216 samples were done by the following conditions. The baffle plate with length of 4mm was put on the shorter side of the test sample, then white noise sound wave was radiated from the same small loud speaker used for imitated normal incidence introduced before. Sound pressures were observed by the small microphones on the surface of test sample, behind the test samples and in the internal space as the same settings and procedure in the normal sound incidence. See Figure 4. Figure 6 shows examples of analyed results by the same way in Figure 5(A) of the normal incidence condition. It can be understood that they have almost the same characteristics with those of normal sound incidence. Peaks clearly appear at frequency about 48H in both transmitted sound pressures in internal air space and behind test sample. 1 in Air space -1 Double leaf -2 Single leaf -3 Double layer -4 Horiontal incidence Figure 6 Analyed examples of sound transmission characteristics on double leaf structure under the condition of horiontal sound incidence (A) on Single leaf (B) on 2 nd plate Frequency : 8H to 11H Frequency : 16H to 185H (B) on Incidence side plate (C) on 2 nd plate Frequency : 45H to 53H Figure 7 Analyed vibration amplitude distribution on single plate at lowest natural frequency (A), low order mode one on the rear side plate (B), and on both plate around resonance frequency, See Figure

8 INTER-NOISE 216 Phase delay : Degree Double Leaf Rear plate Front plate Phase difference : Degree Figure 8 Example of analyed phase delay of vibration velocity on the both facing points on front and rear plates of double leaf structure (A) and their difference (B), See Figure 5 As shown in Figure 6, phenomenon of sound resonance penetration is caused in double leaf structure even in the horiontal incident condition. It means the decreasing of the efficiency of sound insulation at low or mid specific frequency, and it is one of the common important problem to keep high sound insulation by using double leaf type structure of light weight plate. For the additional information to find out developing sound insulation structure using thin plate or light weight board material, observation and analysis of strength of vibration velocity on the plate ware executed at 187 points with spacing of 25mm. Analyed examples are shown in Figure 7. Figure 7(A) shows distribution at lowest order natural vibration mode on single rectangle plate with fixing aluminum frame parts on its four sides. Figure 7 (B) shows more high and in low order mode distribution on the rear side plate side plate of the double leaf. Both Figure 7(B) and (C) shows the distributions on both front and rear sides of the double leaf around resonance frequency. Divided vibration motion at low several order mode natural frequency and those on both on front and rear sides plates in frequency area of sound resonance are clearly shown in Figures 7(B)-(D). Example of the additional analyses on phase differences between facing both points on front and rear side plates is shown in Figure 8. From the results, it can be recognied that motion form have inverse phase, about 18 degrees, between just facing points on both front and rear plates, and can be understood that both plates vibrate like spherical shells around the frequency of sound resonance phenomenon. 4. EXAMINATION OF SUPPRESSION OF SOUND PENETRATION PHENOMENON In the both results of simulation and actual experiments on the sound transmission through the double leaf structure, sound resonance penetration is clearly shown. It makes decreasing of efficiency of sound insulation, and it is the important problem to be improved. Then, authors examined the insertion of acoustical material or system into a narrow air space, and evaluated the effectiveness of such treatment for the recovering lost efficiency of sound insulation. 4.1 Insertion of porous material into air space. At first, examples on insertion effects of glass wool are shown in Figure 9. Used glass wool samples were stripe type with density of 32kg/m 3, length of 28mm and thickness of 1mm. They were putted into a short side of internal air space of test sample. Insertion widths of the glass wool stripe were selected to be 25mm, 5mm, 1mm. Width of 43mm covering whole area of test plate was selected for the reference. Last example is not practical, because visibility through the structure lose even plate material has clear transparency. Insertion effects read from the results shown in Figure 9 are limited within several dbs around the frequency of sound resonance phenomenon as estimated by the simulation described in 2. It can be confirmed and concluded that the method of insertion of porous material is not effective to recover the lost efficiency of sound insulation Rear-Front 3626

9 INTER-NOISE without Glass wool Glass wool width 5mm 1mm Glass wool full filled Figure 9 Comparisons among analyed examples of sound transmission of test sample and those with insertion of glass wool stripe with different width (A) with 3 resonators Double layer Double leaf with Resonator Single leaf Normalied impedance Figure 1 Calculated examples on sound transmission of double leaf type test sample and its improving examples with inserting of Helmholt resonator/s (A) and comparison of acoustic impedance in the internal air space obtained for each calculation with the different property of inserted Helmholt resonator (B) (C) 4.2 Insertion of Helmholt resonators From the results of tried simulation in early stage shown in Figure 2, authors had been able to understand the important condition as that acoustic impedance in the internal air space of basic double leaf structure had very high value, then efficiency of sound insulation would be lost. And it also had been understood that the condition of too low acoustic impedance by insertion of some kind of Helmholt resonator would not be effective to recover the lost efficiency of sound insulation. Considerations by more detailed simulation calculation and experiments on the insertion effects of small Helmholt resonator/s in to narrow internal air space were executed. At first, tests were started by basis of simplified idea. For example, resonance frequency of Helmholt resonator was tuned to be neat or matched to the frequency of phenomenon of sound resonance penetration in the basic test sample of double leaf structure. Figure 1(A) shows differences among calculated examples of transmitted sound pressure under the different condition. Figure 1 (B) and (C) show calculated acoustic impedance values in the internal air space including the sound by-passing through insert of Helmholt resonator and show lowerings of acoustic impedance in the air space comparer with the high value of original basic model as shown Figure 2(B). As shown in Figure 1(A), use of only resonator with certain sound resistance makes peak level 1 db lower. And use of multiple resonators with different resonance frequency makes more low acoustic impedance as shown in Figure 1 (C), and makes level decreasing more than 1dBs. Authors actually tried confirming experiments on the effectiveness of inserting Helmholt (B) with Resonator Real part Imaginary part (C) with Three resonators Real part Imaginay part

10 INTER-NOISE 216 resonator/s by basis of these considerations and our design way about the resonator (5). At first, actually tested result of insertion effect were compared among three kinds of Helmholt resonator with the same resonance frequency tuned to be the same frequency of resonance penetration and with each different resistance at their opening holes. The Helmholt resonator with one opening hole with neck-length of 12mm and wide diameter of 5mm on the surface area, S insert in Figure 2, of 28x1mm 2 has lowest acoustic resistance. Other resonators have three opening holes with diameter of 3mm on the surface plate with thickness of 1.5mm. The kind of resonator with opening holes covered by thin cloth has highest acoustic resistance among them. Pre-measured, in sound tube with diameter of 1mm, acoustic impedance of the sample with nine opening holes of diameter of 3mm on the plate of 1.5mm thickness and covered by thin cloth is shown in Figure 11(A) and sound absorption coefficients in (C), and insert estimated from (A) and equivalent sound absorption coefficient are shown in Figure 11(B) and (C) for the insertion sample model of the small resonator. By comparing these results, it can be thought that certain value of the acoustic resistance brings suppressions on the sound penetration through double leaf structure. In the case of effective example of later two, peaks disappear at about 48H in the results. Then, it could be confirmed that resistance of the opening holes of Helmholt resonator made influences on the suppression of sound penetration trough double leaf structure. It was concluded that more effective way would be use of multiple resonators with each different resonance frequency Normalies impedance 5-5 (A) (B) (C) Real part Imaginary part Z insert 1-1 Real part Imaginary part Figure 11 Measured acoustic impedance (A) and sound absorption coefficient(c) of the Helmholt resonator having thin cloth covered nine opening holes with diameter of 3mm on the aluminum surface plate with thickness of 1.5mm and with back air space of about 2mm in the sound tube with diameter of 1mm, and estimated Z insert (B) for the value evaluation Sound absorption coefficient from Normalied impedance.2 from Z insert 2 1 with Single resonator -1-2 Double leaf structure one Opening hole -3 with Resonator Thin cloth on opening holes Figure 12 Comparisons of measured sound transmission among double leaf type test sample and its improving examples with inserting of Helmholt resonator with different acoustic properties 3628

11 INTER-NOISE (A) with a Resonator with 3 Resonators with a Resonatorwithout Resonator with 3 Resonators in Air space (B) without Resonator Single leaf with 3 Resonators Double layer Figure 13 Measured sound transmission of both improved examples with inserting Helmholt resonator (A) and their comparisons including different type structures of single leaf and double layer (B) Transfer function : db without Resonator with Triple resonators with Single resonator Figure 14 Comparisons of measured excited vibration on plate surface among basic test sample and improved samples with inserting of Helmholt resonator/s Next, Authors tried experiment of using multiple Helmholt resonators set for expecting higher effect estimated in the simulation calculation as introduced above. Resonance frequencies of Helmholt resonator were designed to be 35H, 5H and 6H, those were a little lower or higher than the resonance frequency of basic double leaf structure and to be the same. In Figure 13(A), analyed results of sound pressure transmitted to rear side of test sample and those into internal air space are shown for the confirming and consideration. Figure 13(B) shows analyed results to make sure of their actual effect of improving examples of inserting Helmholt resonators by comparing with those of the different basic test sample of single leaf and double layer type structure. It can be recognied that insertion effect of multiple Helmholt resonators is about 1 db for decreasing sound transmission or recovering sound insulation in different meaning as reaches to the efficiency of single layer type structure at the same specific frequency. Observations of vibration excited on the surface of the plate material were executed in parallel with these acoustic experiments. Observed and analyed results are also shown in Figure 14, and then it can be understood that insertion effects is in several db. It seemed that suppression effect is not enough as lower than compared with the efficiency of sound insulation of double layer structure at the same frequency still now. However, more detailed consideration makes clear that the effect of multiple insertion of Helmholt resonator is not only suppression ten dbs on the high sharp peak level at the specific frequency of 48 H but also level decreasing of about 5 db in lower frequency area around 3H. 3629

12 INTER-NOISE 216 Both consideration on the insertion effect of Helmholt resonator into the double leaf structure on the sound transmission characteristics and excited vibration described just above shows possibility to control strong phenomenon of the sound resonance penetration, even above-mentioned confirming experiments was in the stage of repeating trial and error by limited numbers of sample. Finding out of the best improving ways by insertion of Helmholt resonator is the future subject of this study. Many experiments by set of systematic conditions based on the conclusion after the parametric simulation on the double leaf structure and on the Helmholt resonator should be continued. 5. SUMMARY AND CONCLUSION This study work is summaried and concluded as follows. (1) The resonance penetration phenomenon of the double leaf structure was set as the main subjects of this study. Sound penetration/transmission and vibration excitation on the plate or board were considered based on the model calculations and the confirming experiments. (2) A strong sound transmission and vibration excitation by the resonance penetration phenomenon were clearly observed. These were appeared as deterioration of the efficiency of sound insulation. (3) The method of insertion of porous material and small Helmholt resonator into the narrow internal air space was considered to suppress the resonance penetration phenomenon and it was confirmed by experiments. Then, certain effects were recognied. (4) Confirmed effects on this improving way to recover the lost efficiency of sound insulation was not good enough as near that of single leaf structure and lower than that of double leaf structure at the same frequency still now. (5) Finding of direction and concrete ways to recover lost efficiency of sound insulation is expected. Consideration by the settings of systematic condition based on the numerical simulation and its quantitative evaluation are main subjects in next stage of this study. ACKNOWLEDGEMENTS This research work was executed as a part of JSPS KAKENHI Grant for Scientific Research. REFERENCES 1. T. Iwase, N. Chikamatsu and M. Hosokawa, AN EVALUATION METHOD OF SOUND INSULATION CHARACTERISTICS OF A PLATE USING SMALL AREA TEST MATERIAL AND TRANSFER FUNCTION ANALYSIS. proceedings of internoise97; August 1997;Budapest, Hungary, p Teruo IWASE, Satoshi SUGIE, Junichi YOSHIMURA, KOICHI YOSHIHISA. On sound propagation in light weight double leaf wall with porous material. Proceedings CD-ROM of internoise27; August 27; Istanbul, TURKEY, 27 in7_485.pdf 3. C. ZWIKKER, C. W. KOSTEN. SOUND ABSORBING MATERIALS. ELSEVIER,; NEW YORK: Hand book of noise control engineering, Tokyo, Japan, Gihodo publishing; 21. p , edited by Japanese 5. Teruo Iwase, Satoshi Sugie, Masayuki Abe, Hiroyasu Kurono, Shinya Nishimura, Yasuaki Okada, Koichi Yoshihisa. Modeling and verification of perforated plate structure for high sound absorption at low frequency with extending parts behind holes into shallow air space. proceedings Flash Memory of internoise August 215; San Francisco, California, USA 215. in15_546.pdf 6. SUGIE Satoshi, YOSHIMURA Junichi, IWASE Teruo. Effect of inserting a Helmholt resonator on sound insulation in a double leaf partition cavity. Acoust. Sci. & Tech., 29;3(5);