New DSP to measure acoustic efficiency of road barriers. Part 2: Sound Insulation Index

Transcription

1 New DSP to meaure acoutic efficiency of road barrier. Part 2: Sound Inulation Index LAMBERTO TRONCHIN 1, KRISTIAN FABBRI 1, JELENA VASILJEVIC 2 1 DIENCA CIARM, Univerity of Bologna, Italy 2 Univerity of Beolgrade, Serbia tronchin@ciarm.ing.unibo.it Abtract: - Barrier employed for road traffic noie reduction can be characterized by two indice: reflection index for ound reflection and inulation index for airborne ound inulation. They can be meaured following the method decribed in CEN/TS tandard, baed on impule repone meaurement employing a preure microphone. The method mandate for averaging reult of meaurement taken in different point in front of the device under tet and/or for pecific angle of incidence, employing the obolete MLS ignal for performing the meaurement, which can caue evere artefact due to nonlinearity and time-variance of the ytem. Furthermore, the CEN/TS tandard preent ome geometric problem, which could arie if the barrier doe not reach a minimum height or if it ha a very rough (cattering) urface. During the reflection index meaurement on a barrier of limited height, the reflected ound can be contaminated by the ground reflection, compromiing the fairne of the whole reult. Alo the inulation index can be affected by the height of the noie barrier, ince the ound paing above the device under tet can become mixed with the ound paing through it. It ha been noticed how thee practical problem, jointly with the aumption of a urface reflecting pecularly in the final formula, can ignificantly over/under etimate the laboratory value of both the indice. Reult of in itu tet baed on CEN/TS will be hown in comparion with reult obtained through a different approach, baed on ound intenity meaurement, and with the traditional tet performed in the lab. Key word: - Sound Inulation index; Noie barrier; DSP development; Acoutic meaurement 1 CEN/TS tandard CEN/TS tandard decribe a way to calculate two indice, reflection index and ound inulation index, ued to characterize barrier employed for road traffic noie reduction. For both the indice the method mandate for averaging reult of meaurement taken in different point in front of the device under tet (ound inulation index) and/or for pecific angle of incidence (reflection index). Thee indice are computed in one-third octave frequency band; they decribe how much the device under tet reflect a ound wave back toward the ource and how much the device under tet attenuate a ound wave paing through (not above) the barrier. 1.1 Sound inulation index Equation (3) how how to obtain the ound inulation index SI for every one-third octave frequency band. n 2 d k F f t,k d j = k= 1 i SI j 10 log10 n F f i i j [ h ( t) w ( t) ] [ h( t) w ( t) ] 2 t,k df 2 df (1) ISBN:

2 where: n 9, i the number of canning point; f j i the width of the j-th one-third octave frequency band (between 100 Hz and 5 khz); F i the ymbol for the Fourier tranform; h t,k (t) i the tranmitted component of the impule repone at the k-th canning point; w t,k (t) i the time window applied to the tranmitted component (Adrienne window, Figure 1); d k i the geometrical preading correction factor for the tranmitted component at the k-th canning point (Table 2); d i i the geometrical preading correction factor for the reference free-field component (Table 2); h i (t) i the incident reference \\\ of the freefield impule repone; w i (t) Table 1 i the time window applied to the incident reference free-field component (Adrienne window). 1 di d i + 3 di d i + d t b 6 d i + characterize the road traffic noie reduction barrier in it totality: [ ( L SI ) 10 i i ] DL = i= m SI 10 log L ( 10 i ) i= m where: m L i (2) = 4 (number of the 200 Hz one-third octave frequency band); Relative A-weighted ound preure level (db) of the normalized traffic noie pectrum, a defined in EN , in the i-th one-third octave band. Before uing (1) and then (2) it i neceary to calculate h i (t) and all the h t,k (t). CEN/TS tandard ugget employing a meaurement ytem ketched in Figure and Figure. The equipment i compoed of a loudpeaker and it own tand, a panel with 9 predefined poition in which a preure microphone will be hoted and a tand for it, Figure 5. Both the tand need to be a high a the half of the barrier. Figure 3 how how to poition the equipment for the reference h i (t) meaurement: the loudpeaker i perfectly in line with the microphone, placed in the 5-th poition of the panel of Figure, at a well-defined ditance d t. MLS ignal (or Sine Sweep) can be employed to obtain the impule repone. d t b d d i + d di + 2 d i 1.25+t b 0.40 m t b Barrier thickne (m) Once the ound inulation index for all the band ha been calculated, it i poible to obtain a ingle value, in db(a), to Figure 1 Sound Inulation geometrical layout according to CEN/TS 1793/5 ISBN:

3 w i (t) and w t,k (t) repreent an analyticallydefined window (Adrienne window) that ha to be applied repectively to h i (t) and h t,k (t). CEN/TS tandard provide a well documented way to do that. in Figure, eparating thee component can be difficult. A both tand need to be placed at half the height of the barrier, if the barrier height i too mall, the diffracted component ha not enough delay for the tranmitted component to extinguih, before the arrival of the diffracted one. Figure 2 Sound Inulation geometrical layout according to CEN/TS 1793/5 Figure 4. Sketch of an impule repone during S I meaurement. Source: (CEN/TS tandard, January 2006). Figure 3 Reference meaurement according to CEN/TS 1793/5 It i neceary to replicate the whole SI procedure both in front of the element and in front of the pot (if preent). Whenever poible, two ingle-number rating hall be derived to indicate the performance of the product: one for element and the other for pot. 2 Practical problem 2.1 Minimum height for S I Index In Sound Inulation Index, width and height of the barrier are again the limiting factor. In the Sound Inulation Index formula (3), we ee the component of the impule repone tranmitted through the barrier in it numerator. By properly placing the Adrienne window, in principle it hould be poible to inulate that component. An impule repone, meaured by the preure microphone on the right tand, i compoed by a direct component, a tranmitted component, a diffracted component and by paraitic reflection; however, a it can be een Figure how a ketch of an impule repone of a SI meaurement. 1 repreent the tranmitted component, 2 the diffracted component, 3 i the Adrienne window needed to iolate 1. The delay of the component 2 (diffracted component) i related with the height of the barrier. Of coure, the ditance between component 1 and 2 decreae when the barrier become maller (2 move left). The amplitude of 2 behave oppoitely: decreaing the barrier height caue an increae in the amplitude. Thi behavior generate two troubleome cenario: The height of the barrier provide component partially overlapping, with the diffracted component having amplitude higher than the tranmitted one: the operator could cut away completely the diffracted component by hortening the Adrienne window, thu removing alo the tail of the tranmitted one which i overlapped with the ubequent diffracted component. Thi reult in an overetimation of the Sound Inulation Index, a part of the ound paing through the barrier ha been cut away. The operator maintain a tandard length of the Adrienne window, which will include alo the trong peak of the ISBN:

4 diffracted ound, which will be improperly conidered a being part of the tranmitted component. Thi reult in an underetimation of the Sound Inulation Index, a part of the ound diffracted by the upper edge of the barrier ha been erroneouly included in the tranmitted component. The CEN/TS tandard mandate for a minimum height of the barrier equal to 4.0m. Our experience demontrated that thi minimum height can be inufficient for avoiding overlapping between tranmitted and diffracted component, particularly with thoe barrier having a reonating tructure (cavitie, etc.); they often caue a tranmitted impule repone ringing for everal milliecond (even m), which mean that a complete eparation of the diffracted component would require a minimum height of 7 or more meter. Again, when thi overlapping problem occur, the tandard doe not provide clear indication about the real minimum height of the barrier (a the length of the tranmitted component i alway aumed to be horter than the tandard Adrienne Window, which only accommodate a length of approximately 5.0 m). Sound Inulation Index, can either reult i ignificant underetimation or overetimation of the real value. 3 Reult of ome in itu tet Here the reult of ome CEN/TS meaurement will be hown. The firt two barrier under tet are intalled in Grande Viabilità Trietina between Cattinara and Patriciano (Italy). Barrier A i made by metallic panel; it ha a height of 5 meter and a thickne of 0.20 meter. Barrier B i made by wood; it i 2 meter tall and 0.12 meter thick. The reult of the meaurement by the CEN/TS method are preented in Table 2. Table 2. Meaurement reult Barrier A Barrier B DL RI 4.52 [db] 3.41 [db] DL SI (element) 25.6 [db] 18.5 [db] DL SI (pot) 23.1 [db] 19.3 [db] Comparion of the behaviour of the two barrier i hown in the following figure in 1/3 octave band: SI (db) Barrier A (element) Barrier A (pot) Barrier B (element) Barrier B (pot) Figure 5. Barrier that doen t meet minimum dimenion requirement. Furthermore, the tandard doe not define how to proceed when the operator i aked to qualify a noie barrier of limited height, a it often occur in practice, a hown in Figure 5. Thi gap in the tandard can create wrong claification, which, with reference to the Frequency (Hz) Figure 6. S I comparion 3.1 Comparion between laboratory and CEN/TS meaurement Sound inulation Table 2 compare the ingle-rating number DL R and DL SI. DL R repreent the reult in laboratory, the in itu behavior i repreented by DL SI. The fact that the two coefficient can ISBN:

5 deviate i known, proved by other tet and documented [8]. In barrier A thi difference can be afely conidered null: a gap of 0.4 db i phyically inignificant. Barrier B manifet intead a too wide gap between value. A decribed before, thi effect i due to the hort delay between tranmitted and diffracted component: Figure 7 how how the diffracted component fall inide the Adrienne window, becaue of the limited height of barrier B, compromiing the whole reult. Table 3. Sound inulation reult comparion DL R DL SI Barrier A 26 [db] 25.6 [db] Barrier B 29 [db] 18.5 [db] Figure 7. Barrier B : tranmitted and diffracted component both fall inide the Adrienne window. 4 Concluion The ue on CEN/TS to claify the effectivene of barrier ha been hown to provide reult that agree with the laboratory only in ound inulation tet and only when the barrier i very tall and without reonant cavitie: for example, the deviation between laboratory and in itu reult for barrier A i negligible. When thee geometrical and tructural requirement are not met, the reult deviate ignificantly (a it happened for barrier B ). In thi cae, it ha been hown how DL SI can be trongly affected by the diffracted component of the impule repone, if it fall within the Adrienne window. Depending on how the operator deal with thi problem, the final reult of the Sound Inulation rating can either be underetimated or overetimated. Reflection index reult obtained by in itu method did how ytematic underetimation of the effectivene of the device. The final ingle-rating number obtained for both barrier A and B i approximately 4 time lower than the laboratory value. It ha been noticed how cattering urface effect and ground reflection could falify the reult. The cattering problem ha been addreed and a new formula (9) ha been propoed intead of (1). However it ha been noticed how thi new approach could not be ued for real world barrier becaue of the lack of knowledge of the frequency dependent cattering coefficient: thu, thi ytematic error cannot be avoided. In practice, the CEN-TS 1793/5 method revealed to be completely unuable for meauring reflection index; of conequence, an approach baed on ound intenity meaurement ha been propoed for etimating correctly the ound reflection from the barrier, alo for device of limited height and with very rough urface. It ha been hown how, employing thi alternative meaurement method, it i poible to etimate correctly the ound aborbing performance of the barrier, while minimizing the time required for the tet, therefore minimizing the time variance of the ytem. 5 Acknowledgment The author wih to thank Andrea Venturi and Angelo Farina for their very preciou collaboration on thi reearch. Reference [1] A.Farina Simultaneou meaurement of impule repone and ditortion with a wept-ine technique, 110th AES Convention, February [2] S. Müller, P. Maarani Tranfer- Function Meaurement with Sweep, JAES Vol. 49 Iue 6, pp ; June [3] G. Stan, J.J. Embrecht, D. Archambeau Comparion of Different Impule ISBN:

6 Repone Meaurement Technique, JAES Vol. 50, Iue 4, p. 249; April [4] J. Vanderkooy, Apect of MLS meauring ytem, JAES Vol. 42, Iue 4, pp ; April [5] Peter Svenon, Johan L. Nielen, Error in MLS Meaurement Caued by Time Variance in Acoutic Sytem, JAES Vol. 47 Iue 11, pp ; November [6] A. Farina - "RAMSETE - a new Pyramid Tracer for medium and large cale acoutic problem" - Euro-Noie 95 Conference, Lyon march [7] A. Farina, A new method for meauring the cattering coefficient and the diffuion coefficient of panel, Acutica/Acta Acutica, vol. 86, n. 6 pp , December 2000 [8] M. Garai, P. Guidorzi, European methodology for teting the airborne ound inulation characteritic of noie barrier in itu: Experimental verification and comparion with laboratory data, J. Acout. Soc. Am., Vol. 108, No. 3, Pt. 1, Sep 2000 [9] M. Garai, P. Guidorzi, Experimental verification of the European methodology for teting noie barrier in itu: ound reflection, Proc. Inter-Noie 2000, Nice, France, (2000). [10] A. Farina, A. Torelli, Meaurement of the ound aborption coefficient of material with a new ound intenity technique, Pre-print of the 102nd AES Conference, Berlin, March 1997 [1] ISBN: