STUDIES OF SOUNDPROOFING CHARACTERISTICS OF SANDWICH PANEL WITH HONEYCOMB CORE AND ELASTIC POROUS ABSORBER

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1 STUDIES OF SOUNDPROOFING CHARACTERISTICS OF SANDWICH PANEL WITH HONEYCOMB CORE AND ELASTIC POROUS ABSORBER Tsvetn Nedkov Deprtment of Telecommunictions, Technicl University - Sofi, Kliment Ohridski 8, Sofi ceco@decibel.bg Abstrct In this pper, some experimentl dt on soundproofing chrcteristic of triple sndwich prtitions combined between cermic bricks with holes with honeycomb core, elstic porous bsorber nd hevy weighted gypsum fiberbord is presented. The honeycomb core is widely used in utomotive nd erospce industry in vriety of pplictions to reduce noise nd vibrtions nd to improve sound qulity. On the other hnd gypsum fiber bords nd porous bsorbers re widely used in building constructions nd the exmintion of the properties of combintion between these tree type of cores will be useful. Keywords: soundproofing, sndwich pnel, honeycomb, porous bsorber. INTRODUCTION Sound insultion between two rooms is very importnt problem nd there is lot of well known methods for solving it. However, with the development of new mterils nd construction methods the interest of lighter, thicker nd more efficient constructions grows. Over the yers, gret del of reserch hs been crried out in identifying the trnsmission loss (TL) chrcteristics of different pnel constructions. Unique pproches to chieving high TL within mss limittions include design developed by Wtters nd Kurtze [], the sher wll, nd the coincidence wll developed by Wrnk []. These designs re bsed on understnding of coincidence effects in the interction of the incident sound field with the vibrtion response of the pnel. Moore nd Lyon [3] developed nlyticl model for clculting TL of sndwich pnel with orthotropic honeycomb core. Dimino, Vitiello nd Alibdi [4] developed nlyticl model in trnsporttion vehicles to predict sound trnsmission trough infinite sized triple pnel prtition plced in rigid bffle. They lso developed numericl procedure to evlute the trnsmission chrcteristics of finite prtitions due to n incident diffuse field. The method is bsed on FEM/Ryleigh methodology nd utilizes numericlly clculted sound trnsmission loss of flt multipnel prtitions nd box like cvities with idelized boundry conditions. Bllgh [5] developed low frequency model for triple prtitions tht cn predict the trnsmission loss reltively well up to bout 50 Hz. It is interesting to compre double pnel nd triple pnel system where the overll width nd mss of the system is constrined. Bllgh [5] exmine the behvior of double nd triple prtitions with sme summrized mss nd ir cvity. The results re shown in Figure where it cn be seen tht lthough the triple pnel system hs superior performnce t higher frequencies, its performnce t low frequencies is mrkedly inferior. Fig.. Trnsmission loss comprison between: double prtition ; triple prtition For residentil buildings the most common sound sources re speech, sound generted from TV nd smll home music systems. To chieve stisfctory levels of trnsmission loss of prtitions nd to receive unintelligible speech the

2 CEMA 4 conference, Sofi 33 frequency region of interest is between 400 nd 5000 Hz. Sound wves striking such pnel re predominntly trnsmitted by trvelling bending wves nd the dmping of bnding wves reduces the trnsmission of intelligible speech trough the pnel by interfering with the norml propgtion of the sound rditing wves. The dmping of bending wves is more effective thn the dmping of other vibrtions with the result tht by plcing the criticl frequency for bending wves t or ner the upper end of the bss region, ll frequencies bove this region re effectively cut off. Cutting off ll of the speech frequencies bove bss essentilly prevents trnsmission of intelligible speech trough the pnel. In this document is presented comprison of trnsmission loss between two types of sndwich prtitions: first consist of hevy nd light fce sheets nd elstic core; second consist of hevy nd light fce sheets nd ddition honeycomb structure plced in the center between two elstic lyers. In both cses, fce sheets nd elstic cores re with sme thickness nd densities. This comprison vlidte tht the introduction of dditionl lyer with high bending stiffness nd reltively smll density hs the potentil to improve trnsmission loss of relted structure in the desired frequency rnge.. ACOUSTIC MODELING OF TRIPLE PANELS For modelling triple wll pnels it is stisfctory to divide the frequency region into low frequency region where lumped prmeter model is stisfctory, mid frequency region where wve motion in the porous elstic bsorbing lyers is importnt, nd high frequency region where structurl coupling between pnels is importnt. At low frequencies, where sound wves hve very lrge wvelengths, it is found tht it is the bulk properties of mterils such s their mss re most significnt. The components in the wll cn be regrded s msses or springs coupled. This is the clssicl lumped prmeter model. Pnels re described with their msses per unit re (surfce mss) nd filled ir gps with porous elstic bsorber re modelled s springs. In its simplest form triple pnel wll would be represented by 3 msses connected by two springs (Fig. ). A simplified system of three msses ttched to ech other by springs with rigidity k nd k, eq. () cn be defined, nd this cn be solved to give the nturl frequencies of the system: m mm3 [ km3 m m k m m m ] [ k k m m m ] () c c With k nd k, where d nd d d d re the thickness of the elstic lyers nd is resonnce frequency. Fig.. Mechnicl scheme of coupled triple pnel For the cse of sound insultion of structure (consisting of tree or more pnels), the trnsfer function of interest is the rtio of the incident sound pressure to the velocity of the rditing pnel. Rindel [6] gives the trnsmission loss s: s R 0log () 4( c ) r nd it cn be seen tht it is rtio of incident pressure <ρ s > to velocity <ν s > of the rditing pnel tht is importnt. By using stndrd Fourier trnsform methods the trnsfer function cn be derived. The sound trnsmission coefficient () is defined s the squre of the bsolute vlue of the rtio of the trnsmitted to incident pressures: / i Where for nti symmetric pnels: t p t p, (3) z' / z ~ z ' / z z' / z ~ z ' / z pi 5 (4) p In eq.(4) z ' nd ~ z ' re impednces of symmetric nd nti symmetric motions in the pnel, z is the modified coustic impednce of the coustic field: z c / cos (5) Where cos is ngle of incidence of sound wve, is the density of the ir nd c is the speed of sound in ir.

3 34 CEMA 4 conference, Sofi In eq. (4) nd 5 re rtios of opertors tht pper s coefficients in the equtions of Dym, Ventres nd Lng [7]. The trnsmission coefficient is function of the ngle of incidence of the sound wves. To ccount for this distribution, n verged form of the trnsmission coefficient is used. Conventionlly this verged form of eqution is: lim 0 lim sin cos d / sin cos d, (6) where being known s the field incidence verged trnsmission coefficient. The limiting ngle lim is tken s equl to 78, bsed on field nd lbortory mesurements. Finlly, the field incidence verged TL is: 0 TL log / (7) 0 0 In the observed triple prtition, the middle pnel is selected to be fbricted from recycled pper honeycomb structure lminted on both sides with elstic porous polyurethne fom. Such mterils hve different stiffness modules in plnes perpendiculr nd prllel to the direction of the cells, nd cn be chrcterized s orthotropic with nine independent stiffness constnts. identicl fce sheets nd homogeneous core mteril. When the wve speed for either motion in the honeycomb pnel exceeds the sound speed, then mtching condition occurs between tht motion in the pnel nd the incident coustic wve tht results in incresed trnsmission trough the pnel. For symmetric pnel motions, these occur due to double wll resonnce nd t higher frequencies where the motion is controlled by bending deformtion in the fce sheets. For nti symmetric pnel motions pnel motions, three regions exist with bending deformtion of the entire pnel cross section: the controlling fctor t low frequencies; the core sher stiffness controls in the mid frequency trnsition region; nd bending deformtion in the fce sheets is the limiting behvior t high frequencies. The wve speed for nti symmetric motions increses monotoniclly with frequency trough the three regions. Where coincidence first occurs is importntly dependent on the sher stiffness in the core. If the stiffness is too lrge, coincidence cn esily be shifted to occur below the mid-frequency region t lower frequencies with the TL dversely decresed over the useful frequency rnge. In Tble re given physicl properties of the exmined mteril: Tble. Physicl properties of solid mterils in observed triple prtition Fig. 3. Section of observed triple prtition: Solid cermic brick with holes; Solid honeycomb structure; Solid 3 lminted gypsum fiber bord nd gypsum bord; Elstic nd PU elstic bsorber The honeycomb pnel TL behviour is conveniently explined in terms of coincidence effects ssocited with motions in the pnel tht re either symmetric or nti-symmetric in chrcter. The decomposition into symmetric nd nti-symmetric motions is exct for symmetric pnel constructions with Type of mteril Density E Modulus Coeff. of Poisson Coeff. of internl loss --- kg./m 3 GP ν η Cermic brick with 655 6,85 0, 0,03 holes Gypsum fiber 30 3,9 0,3 0,0 bord Gypsum bord 680, 0,4 0,0 Honey comb structure PU elstic bsorber 50 0,7 0,35 0,5 The clcultion of TL where done with softwre INSUL tking into ccount ll the prmeters described in Tble.

4 CEMA 4 conference, Sofi EXPERIMENTAL PART For the experiment ws built soundproofed chmber with test opening with dimensions of 85 x 3 cm nd volume of 9.96 m 3. The receiving room is with volume of 65 m 3. The sound reduction index Rw of the prtitions of the chmber, build from two lyers of concrete bricks with ir gp between them, filled with minerl wool is 65 db. Tested specimen is seprted from the other prtition elements with 0 mm rubber stripe. In the source chmber is plced dodechedron sound source connected with genertor of "pink noise". One microphone is plced in the source chmber, connected with sound level meter nd frequency nlyzer. In the source, room t distnce 00 cm from the specimen is plced condenser microphone, connected with sound level meter nd frequency nlyzer. The generted sound pressure in the source room is SPL = 94 db. As the smllest dimension of tested prtition is 35 cm the results for frequencies with length of wve below hlf of this dimension (below 500 Hz) re considered to invlid. 4. RESULTS As obvious from Fig. 4 there is significnt difference between theoreticl clcultions nd experimentl results. Fig. 4. Comprison of clculted nd mesured TL of triple pnel: mesured TL ; clculted TL Resonnce frequency is determined well, but the sound pressure level vries in rnge of 5 db for low frequencies to up to 5 db for high frequencies. Coincidence region from is quite short compred with the mesured. It cn be tke into ccount tht the flnking pths determine the continuous horizontl prt of trnsmission loss curve for mesured results. It s benefit tht the strt of coincidence region is from 50 Hz nd is well subscribed until 500 Hz fter tht the mplitude of bending wves is reduced. On Fig. 5 is presented comprison of TL between triple prtition with middle solid of honeycomb, triple prtition with middle solid of gypsum fiber bord nd double prtition where middle solid is removed. Fig. 5. Comprison of TL between: triple prtition with honeycomb: ; triple prtition with gypsum fiber bord: ; double prtition with removed middle solid lyer: 5. CONCLUSION For triple nti symmetric prtitions the usge of middle orthotropic honeycomb solid pnel benefit TL behviour. In comprison with hevy solid middle lyer from gypsum fiber bord TL is improved up to 7 db nd the coincidence region is with smller mplitude of bending wves. This phenomenon is provided by different bending stiffness in directions nd highest internl loss of honeycomb structure. When prtitions re used for blocking intelligible speech with frequency rnge of interest from 400 to 4000 Hz the ppliction of honeycomb structure is beneficil. Theoreticl model for prediction is poor presented so for future work will be useful to be developed model for predicting TL of triple nti symmetric prtitions with orthotropic middle solid lyer. 6. APPENDIX AND ACKNOWLEDGMENTS This pper ws supported by Technicl University - Sofi inner progrm to support PhD reserch projects under Contrct 45 PD007-07: "Development of lgorithms to study the cousticl chrcteristics of covering mterils for recording studios nd concert hlls".

5 36 CEMA 4 conference, Sofi References [] B. G. Wtters nd G. Kurtze, New Wll Design for High Trnsmission Loss or High Dmping, J. Acoust. Soc. Am. 3, , 959. [] G. Wrnk, US Ptent No. 3, 4, 9, Sound Attenuting Wll for Blocking Trnsmission of Intelligible Speech, 969. [3] J. A. Moore nd R. H. Lyon, Sound Trnsmission loss chrcteristics of Sndwich Pnel Constructions, J. Acoust. Soc. Am. 89, , 99. [4] I. Dimino, P. Vitiello, F. Alibdi, Sound Trnsmission Trough Triple Pnel Prtitions, Recent Ptents in Mechnicl Engineering, 00-5, 00. [5] K. H. Bllgh, Sound Trnsmission trough Triple Pnel Wlls - Low Frequency Model, XXI st Biennil Conference of the Acousticl Society of New Zelnd, 0. [6] J. H. Rindel, Sound Rdition from Building Structures nd Acousticl Properties of Thick Pltes, XXI st BCOM- MET-SAVOIR Course CSTB, 995. [7] ISO EN , Acoustics. Mesurements of sound insultion in buildings nd of building elements. Prt : Requirements for lbortories.