Keränen, Jukka; Airo, Erkko; Olkinuora, Pekka; Hongisto, Valtteri Validity of ray-tracing method for the application of noise control in workplaces

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1 Powered by TCPDF ( Thi i an electronic reprint of the original article. Thi reprint may differ from the original in pagination and typographic detail. Author(): Title: Keränen, Jukka; Airo, Erkko; Olkinuora, Pekka; Hongito, Valtteri Validity of ray-tracing method for the application of noie control in workplace Year: Verion: Final publihed verion Pleae cite the original verion: Keränen, Jukka; Airo, Erkko; Olkinuora, Pekka; Hongito, Valtteri. Validity of ray-tracing method for the application of noie control in workplace. Acta Acutica united with Acutica 89 (), Right: S. Hirzel Verlag. Reprinted with permiion. Thi publication i included in the electronic verion of the article diertation: Keränen, Jukka. Meaurement and Prediction of the Spatial Decay of Speech in Open-Plan Office. Aalto Univerity publication erie DOCTORAL DISSERTATIONS, /. All material upplied via Aaltodoc i protected by copyright and other intellectual property right, and duplication or ale of all or part of any of the repoitory collection i not permitted, except that material may be duplicated by you for your reearch ue or educational purpoe in electronic or print form. You mut obtain permiion for any other ue. Electronic or print copie may not be offered, whether for ale or otherwie to anyone who i not an authoried uer.

2 Vol. 89 () 8 87 Validity of Ray-Tracing Method for the Application of Noie Control in Workplace Jukka Keränen, Erkko Airo, Pekka Olkinuora, Valtteri Hongito Finnih Intitute of Occupational Health. : Laboratory of Ventilation and Acoutic, Lemminkäienkatu - 8B, FIN- Turku, Finland. jukka.keranen@ttl.fi. : Arinatie A, FIN-7 Helinki, Finland Summary Building acoutical noie control i uually very expenive to implement. Therefore, it hould be optimized and argued preciely. Recommended practice for the deign of low noie workplace have been publihed, but in order to elect the mot cot-effective olution, the effect of the potential olution on ound preure level hould be reliable. Acoutical modeling baed on ray-tracing i uppoed to be the bet propective tool in predicting the inertion lo of noie control meaure. The purpoe of thi tudy wa to how how well ray tracing can predict noie level and inertion loe in real application when applied by an educated and experienced uer. Acoutical modeling wa ued in ix operating workplace, where major room acoutical noie control meaure were implemented. The modeling reult were compared to the reult of meaurement which were performed uing an omnidirectional ound ource. Inertion loe of noie control meaure were determined uing both predicted and meaured ound preure level in the octave band Hz to Hz. Accuracie of the predicted ound preure level and the predicted inertion loe in the ix workplace were determined a difference between meaured and predicted ound preure level. By following our general guideline for the modeling of large workplace in the ix workplace, the workplace-averaged accuracy of SPL prediction wa. The inertion lo for noie control meaure wa predictable to an accuracy of. The accuracy of the model wa, thu, adequate. The obtained reult were acceptable conidering the reaonable amount of work ued in the modeling per cae and the uncertainty of in itu meaurement. However, fine adjutment of the model i alway recommended by backing comparion with predicted and meaured SPL. PACS no...ka. Introduction Accurate prediction of the inertion lo (IL) of noie control meaure i important becaue their cot i uually very high and, therefore, they hould be optimized and argued preciely []. Recommended practice for deign of low noie workplace are preented e.g. in ISO 9 [,, ]. In part, noie control trategie, and in part, noie control meaure, are given. In part, a general approach to noie level prediction in workplace i preented. Some cae tudie and reference to the article that deal with noie prediction method are included. However, they do not anwer the quetion: How to ue modeling efficiently to predict the effect of noie control meaure in a workplace with reaonable accuracy? Efficient modeling i a key quetion in practical deign, becaue the planning expene hould be kept a low a poible and reaonable compared to the expene of the intalled noie control meaure. Received 8 June, accepted March. During the lat ten year, the Finnih Intitute of Occupational Health ha ued an acoutical modeling oftware (ODEON. and.) to predict the IL of noie control meaure, whenever it would have been valuable for the noie control project. Mot often, the modeling tool ha been experienced a too heavy in mall workplace, even though it ue would be valuable. The reaon for thi i not in calculation time, becaue the calculation uing computer with Pentium III proceor for a typical indutrial workroom could be over in le than quarter an hour. In general, the mot laboriou tak i creation of the model and verification of the modeling reult, e.g., comparing predicted ound preure level (SPL) to meaured SPL. To create a model, there are everal detail to be collected, e.g., geometry and dimenion of the workplace, ize and hape of the fitting, ound aborption and cattering propertie of the urface, poition, orientation and directivity of noie ource and meaurement poition. Epecially, etimation of the ound aborption and cattering coefficient for the urface demand experience or, alternatively, everal trial and error. During mot of the modeling project, meaurement were carried out to validate the prediction, becaue, until recently, there wa no firm evidence that the accuracy c S. Hirzel Verlag EAA 8

3 Vol. 89 () Keränen et al.: Ray-tracing method for noie control in workplace of an acoutical model could be adequate without confirmation uing erie of SPL meaurement. After deign project, a general guide for the practitioner wa publihed []. The main point of the guide, that conider the acoutical modeling of workplace, are ummarized alo later in thi article (the modeling procedure). Recently, the application of the ray-tracing method in indutrial noie control ha been ued in everal cae tudie. Hodgon ued ray tracing in a machine hop, a foodproduction hall and a nuclear power-generating tation to predict reverberation time (RT) and lope of ound propagation curve [, 7, 8]. The predicted lope were ued to calculate the expected SPL in the workroom for variou combination of acoutical treatment and ource control meaure. Hodgon ued fitting zone to imulate machine, creen and other fitting. A good accuracy in the prediction could be obtained uing omewhat arbitrary value for the fitting denity. Hongito et al. ued ray tracing to predict RT and SPL in a workhop in five different cae of acoutical treatment [9]. The predicted RT and A- weighted SPL map were preented. However, the calculated reult were compared to the meaured one only in the initial ituation. Sorainen and Kokkola ued ray tracing to predict the effect of noie control meaure on SPL in a carpentry plant []. The difference between predicted and meaured mean SPL wa preented. In all thee tudie, the prediction were accurate, on average, (Aweighted). Although real noie ource were ued, the accuracie of the tudie were tolerable, becaue the ound power level of the noie ource were determined uing accurate meauring method e.g. ound intenity meaurement. However, their reult can not be applied generally becaue only one environment wa tudied. The purpoe of thi tudy wa to how, how well ray tracing can predict IL in real workplace. Thi wa made uing a ufficiently large number of workplace. The main parameter were SPL and IL of noie control meaure, which are the mot ueful and intereting output of model. The reverberation time (RT) were meaured before and after the noie control in five cae. The RT were ued to etimate ound aborption in the workplace in the initial ituation (before the noie control meaure). The accuracy of RT prediction wa alo determined, though it wa not the main parameter in the noie control deign.. Material Thi tudy i baed on work done in ix workplace where acoutical computer modeling wa ued in noie control deign. The modeling oftware wa ued to predict the IL of noie control meaure. The noie control meaure were implemented and their IL wa meaured accurately. Technical evaluation of the noie control meaure wa beyond the cope of thi tudy. The workplace with aigned reference number are preented in Table I. D-view of the acoutical model of the workplace are preented in Figure. A hort decription of the workplace and the implemented noie control meaure i given below.. Engineering work Initially, the workplace wa fitted by helve and hard creen of height to meter, and a couple of -meter high large machine. A -meter high noie creen wa built around one noiy work tation. The inner urface of the noie creen wa partially covered with mm mineral wool. The outer urface wa profiled teel. (Thi wa not a good noie control olution but it wa not deigned by u.). Weaving factory Initially, there were large weaving machine (height meter) which were modelled a boxe uing urface with medium aborption. The open end led to another large hall, which wa not included in the model. The ide wall and the ceiling joit were covered with mm mineral wool.. Engineering work Initially, the workplace wa very fitted by obtacle of different ize (hand-held tool, machine, etc.). Several large machine of variable height were modelled a boxe uing urface with medium aborption. The ceiling wa partially covered with ound aborbing pray-on material of thickne 8 mm. (The material performance did not fulfill the pecification. That i why the obtained noie reduction wa quite mall.). Engineering work Initially, there were everal noie ource of different ize (hand-held tool, machine etc.). There were everal hard creen and a couple of oundaborbing noie creen around worktation (average height meter). A few - to -meter high large machine were modelled a boxe uing urface with low aborption. The curved ceiling wa modelled uing plane urface tilted ymmetrically along the center line of the ceiling (ee Figure ). The ceiling wa covered with mm mineral wool.. Electronic work / landcaped office Initially, the room wa empty. The noie produced in the packaging area propagated to the office area. The work tation in the office were eparated by -meter high ound- aborbing office creen. The ceiling wa covered with oundaborbing pray-on material of thickne mm.. Knitting factory Initially, there were 8 large knitting machine (height meter) which were modelled a boxe uing urface with medium aborption. The worktation wa in the interection of the two large hall. A large wall of area m wa built between the two hall. However, an opening of area m wa left for traffic, o that ound tranmiion through the opening wa the main path between the hall. The urface of the wall were covered with mm mineral wool on both ide.. Method.. Meauring method A imple meauring method baed on an omnidirectional ound ource wa ued to determine the IL of noie control meaure []. The ound ource conited of a pink noie ignal generator (Neutrik MR), a power amplifier (QSC W USA) and an omnidirectional loudpeaker 8

4 Keränen et al.: Ray-tracing method for noie control in workplace ACTA ACUSTICA UNITED WITH ACUSTICA Vol. 89 () Table I. General information about the workplace and acoutical model ( = number of ource, = number of meaurement point). Work place Type of indutry Volume (m ) Surface in model Number of ray engineering work 9 78 / weaving factory 7 / engineering work / engineering work /9 electronic work/ office 8 / knitting factory / Figure. D-plan view of the acoutical model of the ix workplace (not in relative cale). (Brüel&Kjær 9). The ound power level of the ource wa meaured in the laboratory according to ISO 7 []. The noiiet production machine were topped during the meaurement, or the meaurement were performed after working hour in order to reduce the effect of the background noie. The omnidirectional loudpeaker wa placed in elected poition() (uually cloe to a noiy machine). The output of the ound ource wa et to uch a levelthatatleatdifferencebetweentetignaland background noie wa achieved in all meaurement point. The SPL produced by the loudpeaker wa meaured at elected meaurement point (e.g. other work tation) uing a ound level meter (Brüel&Kjær ). Typically, the height of the loudpeaker and meaurement point wa. meter. The meaurement were performed before and after the noie control meaure uing the ame meaurement point and ource poition. The determination of IL wa baed on meaurement of SPL caued by a calibrated ound ource. While the omnidirectional ound ource produced the noie at the poition, the SPL at the meaurement point wa before and after the noie control meaure. The ound attenuation between one ource and one mea- 8

5 Vol. 89 () Keränen et al.: Ray-tracing method for noie control in workplace urement point wa, repectively, The ue of the ound attenuation a the primary quantity permitted the ue of different ound power level of the ource (different loudpeaker) in ucceive meaurement. Naturally, the ound power level of the ource wa contant during meaurement in a ingle workplace. However, the time gap between meaurement and wa typically one year o that one could not guarantee contant. The mot important parameter in noie control deign i IL becaue it decribe the performance of a certain acoutical remedy in a certain environment. In our opinion, it i very eldom that abolute SPL prediction are needed in workplace application becaue all ound ource are eldom exactly known and they operate in different cycle. The inertion lo (IL) wa determined uing the equation The mean inertion lo wa determined a an average over the number of ource m and the number of meaurement point. The RT were meaured uing a pitol hot a ound ource in workplace to. The RT were determined uing a decay of. The preented RT value are arithmetic average of everal RT meaurement... Modeling method The acoutical computer model were created uing the ODEON. oftware which i baed on geometrical acoutic [, ]. The earlier verion (ODEON.) of thi oftware wa one of the three mot accurate program included in the Vorländer round robin tet for a peech auditorium [, ]. A detailed decription of the ray tracing method i preented elewhere [7, 8]. The ODEON ue hybrid method which include both image ource and econdary ource method. However, it i poible to witch off the image ource method and ue only econdary ource method which i baed on the principle of ray tracing method. In the econdary ource method the ray are treated a tranporter of energy [9]. Each time a ray hit a urface, a econdary ource i generated at the colliion point. The energy of the econdary ource i the energy of the primary ource divided by the number of ray and reduced by the reflection involved in the ray hitory up to that point. The econdary ource i conidered to radiate into a hemiphere a an elemental area radiator. The radiated direction (of the reflection) i calculated a a random direction following the angular Lambert ditribution of ideal cattered reflection. In thi tudy, the modeling wa carried out uing the econdary ource method, becaue ray tracing method ha () () () proven to be the mot uitable method for prediction in a fitted encloure of arbitrary hape []. The number of ray depend on the geometry and volume of the workplace and the number of modelled urface in the workplace (ee Table I). Unfortunately, it i not preciely explained in the oftware manual, how the recommended number of ray i calculated. Therefore, the number of ray recommended by the ODEON wa doubled in order to improve the reliability of the reult. Two important parameter for calculation were impule repone length and maximum reflection order which mut not be confued with number of ray. Thee determine the top criterion for the ray tracing. The maximum reflection order wa et to highet allowed value of o that the impule repone length wa the parameter that defined the end of each run. The impule repone length wa et approximately to / of the average of meaured RT at the octave band... Hz. The modeling procedure wa a follow (explanation below) [].. The room dimenion of the workplace were meaured.. The fitting were divided into three clae by ize: (a) Large olid obtacle like wall, creen and large machine with a ize of over three meter. (b) Medium-ize fitting with a ize of. to. meter, epecially the obtacle near noie ource or worktation, where the noie control i implemented. (c) Small fitting like helve, air duct, table, mall machine etc.. Data on the urface material were collected and the aborption and cattering coefficient of the urface were etimated.. The poition for the omnidirectional ource and the meaurement point were elected o that they benefited the noie control deign, not jut thi tudy.. The ound attenuation and RT in the workplace wa meaured.. The firt (initial) acoutical model wa created uing the collected data on the workplace and SPL, and RT were predicted. 7. Predicted and meaured SPL and were compared and, if neceary, the aborption coefficient of relevant urface were lightly modified in order to obtain higher conitency (ee Table II, modified material). The initial ituation wa recomputed. 8. The econd model including the noie control meaure wa created. The SPL after noie control meaure, and RT were predicted. 9. The noie control meaure were implemented in the workplace. Thi took /...year.. The and RT wa meaured after the noie control meaure.. The IL of the noie control meaure wa calculated uing both of the modelled and meaured value. Wall, floor, ceiling, noie creen and large fitting (clae A and B) were modelled a plane urface. The 8

6 Keränen et al.: Ray-tracing method for noie control in workplace ACTA ACUSTICA UNITED WITH ACUSTICA Vol. 89 () Table II. Collection of the aborption coefficient ued in the acoutical model. *: material could be covered uing perforated teel with perforation ratio % or higher. Typical material work place Hz Hz Hz Hz Hz Hz mooth concrete rough concrete, brick work window, double glazing mineral wool mm, on urface*, mineral wool mm, on urface*,,, aborptive creen lightweight creen or wall indutrial machinery platic curtain,, fitting, low aborption fitting, medium aborption Modified material or pecified aborption coefficient work place Hz Hz Hz Hz Hz Hz ceiling, no aborption treatment, ceiling, aborption treatment ceiling, no aborption treatment ceiling, aborption treatment ceiling, aborption treatment ceiling, aborption treatment brick wall, window area %, brick wall with fitting open end of the room urface boundarie were determined a polygon on the plane. The modelled urface were given cattering and aborption coefficient, and they produced reflection and aborbed ound energy. Small fitting (cla C) were taken into account by applying higher cattering and aborption coefficient on the urface on which they were lying. Thi method i poibly le accurate than uing iotropic or floor zoning method (fitting zone), but it i more accurate than including too many mall urface in the model []. The fitting zone method, where the area i preumed evenly fitted with obtacle, i decribed elewhere []. Becaue ODEON doe not include method for uing fitting zone in calculation, mall fitting were taken into account applying higher aborption and cattering coefficient. Large obtacle of clae A and B were modelled a they were in itu. In our opinion, additional fitting zone were not abolutely neceary, becaue the aborption of the fitting and the random reflection from the fitting were imulated with previou action. The model of the workplace conited of 8... urface. It wa impoible to meaure their aborption coefficient. Therefore, literary databae were ued in the firt place (ee Table II) [,, ]. Typically, the initial RT modelled with literary aborption coefficient wa longer than the meaured RT. One reaon for thi i that all of the urface are not included in the model. Therefore, the aborption coefficient were lightly modified in the initial ituation, if large difference between predicted and meaured exited. (The model wa not repeatedly adjuted until bet fit wa obtained, but the aborption of fitting on the large urface wa taken into account.) The hard ceiling or the floor wa uually totally covered with mall fitting (cla C) having non-zero aborption, epecially at low frequencie, which jutified ome addition to aborption coefficient() in the initial model. Another phenomenon i that the rate of room diffuion can vary a lot and, thu, affect the balance between the horizontal and vertical ound field. For example, the effective aborption coefficient of a ceiling aborbent can be much higher in a fitted room than in an empty room, becaue the empty room doe not ufficiently feed ound energy to the vertical ound field. Thi jutified ome reduction to aborption coefficient() when room were not fitted. In the ODEON, the cattering coefficient i etimated by a value of range... The value i ued to control the degree of random direction in reflected ound ray generation according to Lambert law. The cattering coefficient of a urface wa not only applied a a property of the urface material, but wa alo ued to mimic the mall fitting cloe to the repective urface []. The cattering coefficient for the urface were etimated taking advantage of the guideline preented in earlier tudie and previou experience in modeling indutrial workplace [,, 7]. Typically, the cattering coefficient in the model were between...7dependingonthecomplexityoftheurface. Our election rule of the cattering coefficient are preented in Table III. The omnidirectional loudpeaker wa poitioned in the model a a point ource with an omnidirectional directivity pattern and relevant ound power level. The mea- 87

7 Vol. 89 () Keränen et al.: Ray-tracing method for noie control in workplace Table III. Collection of the cattering coefficient (in ODEON) ued in the acoutical model. Scattering coefficient Decription of the urface.,...,.9 large, plain urface.,...,.9 large partially fitted urface.,...,.9 mall or fitted urface.,...,.89 large denely fitted urface.9,...,. mall denely fitted urface urement point were poitioned in the model a point receiver according to the meaurement ituation. The decription of point receiver calculation in ODEON i preented elewhere [8, 9]. Similarly, the RT meaurement were modelled uing ray tracing. The environmental variable (relative humidity of the air and temperature) were kept contant before and after noie control. The number of ray recommended by ODEON wa not ignificantly different between the firt and the econd model, o that the number of ray ued wa left unchanged. The only ignificant change between the model were the noie control meaure. They were modelled uing additional urface or changing urface parameter (aborption and cattering coefficient). The urface that repreented noie creen, new wall or acoutical treatment on ceiling or wall are haded in Figure. The aborption and cattering coefficient of the urface are preented in Table II and III... Validation method The accuracy of SPL prediction wa determined in two way. The firt decribe the general accuracy of SPL prediction, the econd decribe the trend of the error in SPL prediction. The accuracy of SPL wa determined by averaging the repetitive difference between the predicted and meaured SPL or. The accuracy of the SPL prediction,, wa defined by the following equation: where before or after the noie control, = number of ource and = number of meaurement point. wa determined for each workplace before and after the noie control meaure. An arithmetic average i ued for implicity in thi tudy. Logarithmic average are not preented becaue, in thee cae, the maximum difference between arithmetic and logarithmic average wa le than., typically below.. Equation () preent only the abolute error becaue of the magnitude operation. It doe not tell whether the error wa poitive or negative. Therefore, the average accuracy reult,, are alo given eparately without magnitude operation. In thee reult, negative error cancel poitive error but the trend of the error can be een. () Another quantity i the accuracy of IL prediction. It wa defined by the difference between predicted mean inertion lo and meaured mean inertion lo. The workplace-averaged accuracy of the model,, wa defined a an average of in the ix workplace. Repectively, the workplace-averaged accuracy of IL prediction,, wa defined a an average of in the ix workplace. Thee figure are ued a a general etimate of the achievable accuracy of ound level prediction uing ODEON. The accuracy of the RT prediction,, wa defined by the difference between the predicted and meaured RT. () RT RT (), the workplace-averaged accuracy of RT prediction, wa defined a an average of in the workplace to.. Reult The predicted and meaured RT before and after the noie control meaure are preented in Figure. The accuracy of the SPL prediction before and after the noie control meaure in each workplace i preented in Table IV and alo in Figure. The arithmetic average accuracy of the SPL prediction without magnitude operation,, i preented in Table V. The predicted and meaured mean inertion loe in each workplace are preented in Table VI. The accuracy of the inertion lo prediction,, i preented in Figure. The workplace-averaged accuracy of SPL prediction,, the accuracy of IL prediction,, and the accuracy of RT prediction,, are preented in Figure with tandard deviation.. Dicuion The varied from.9 to 7. in ix workplace at octave band to Hz (ee Table IV). In ingle prediction, and at ingle frequency band, varied coniderably, where noie creen were ued. The tandard deviation of were... and, on average,.8 in the ix workplace at octave band to Hz. Similarly the tandard deviation of were.8... and,on average,.. In general,all model lightly overetimated the SPL at high frequencie (Table V), which may indicate that there wa too little aborption in the model, although higher aborption coefficient than uggeted in the literature were ued for floor and ceiling urface. The fitting apparently aborb more ound than expected in the model. Thi may indicate that the fitting hould be modelled uing fitting zone, additional arbitrary ound aborbing urface or ignificantly higher aborption coefficient on modelled urface repreenting fitting. 88

8 Keränen et al.: Ray-tracing method for noie control in workplace ACTA ACUSTICA UNITED WITH ACUSTICA Vol. 89 () RT (meaured) RT (modelled) RT (meaured) RT (modelled) Figure. Meaured and predicted reverberation time (RT) before and after noie control meaure in workplace. Some detail of eparate model of the workplace are dicued in the following. In workplace and, the acoutical remedy did not increae the room aborption ignificantly. Thu, the workplace remained quite reverberant and good accuracie were obtained. Only in the workplace, the average accuracy,,wale than before and after the noie control meaure. In workplace, the accuracy wa poor at frequencie... Hz, which wa not expected. However, it wa expected that the large obtacle could caue error in predicted SPL. In thi workplace, the ceiling wa aborptive and a large aborptive wall wa intalled between four of the ound ource poition and the meaurement point. The large aborptive wall wa the main reaon for the poor accuracy in the econd model which included the implemented noie control meaure., the workplace-averaged accuracy, wa. at to Hz (Figure ). Therefore, the accuracy i atifactory. Typically, the accuracy wa wort at the lowet ( Hz) and the highet ( Hz) octave band. Thi wa achieved uing typical aborption and cattering coefficient in the modeling (Table II and III) and exactly known ound ource. The accuracy evaluation could be improved uing more ound ource location and meaurement point, which would decreae the uncertainty of meaurement. Naturally, thi increae the amount of work and time conumed in the project. Depite the high tandard deviation of, the accuracy of IL prediction,,wa. at all octave band (Figure ) and., on average, which i a good reult (Figure ). However, the model ytematically overetimated the IL (Table VI). Standard deviation of wa.7..., which i good. The IL were not very high, within...9, othattherelative errorincertainwork- place wa unacceptable. Obviouly, the tandard deviation of can behigherthan the IL if thelatter i mall (e.g. workplace ). However, thi doe not imply that mean IL prediction would not be reliable. Noie creen caued the highet tandard deviation in value, epecially when the meaurement point or the ound ource wa cloe to the creen. One of the reaon for thi wa that the prediction did not take into account diffraction of the ound. It i poible to approximate diffraction by uing tranparent diffuely-reflecting 89

9 Vol. 89 () Keränen et al.: Ray-tracing method for noie control in workplace Table IV. The abolute accuracy of the SPL prediction,. The average and tandard deviation of the accuracy are preented in before and after ituation. ( = number of ource, = number of meaurement point). The firt column for every frequency band depict average value (boldface number), the econd column tandard deviation. workplace Hz Hz Hz Hz Hz Hz before (N=) after before (N=8) after before (N=9) after before (N=7) after before (N=) after before (N=) after Average Table V. The accuracy of the SPL prediction, (without magnitude operation). The average and tandard deviation of the accuracy are preented in before and after ituation like in Figure. ( = number of ource, = number of meaurement point). workplace Hz Hz Hz Hz Hz Hz before (N=) after before (N=8) after before (N=9) after before (N=7) after before (N=) after before (N=) after Average and partially tranmitting urface around the noie creen []. Thi approximation improve the accuracy of prediction even though the frequency dependency of diffraction i not taken into account. However, the diffuion coefficient may then need to be different at low and high frequencie. The modeling of ingle barrier in an empty encloed pace ha been tudied by Dance et. al. []. A comparion of three different diffraction modeling technique demontrated that a diffraction area around a barrier which randomly redirect ray, give accurate reult at low frequencie. The RT were meaured in five of the workplace and they were alo predicted. The aborption coefficient in the initial model were modified in order to obtain a tolerable fit with meaured SPL not to obtain a good fit with RT. Therefore, the accuracy of the RT prediction i not a good a it would be if the model were optimized to RT prediction. During a typical noie control project, it i rarely poible to fine tune the model to the maximum accuracy, o that the workplace-averaged accuracy., at to Hz octave band, i acceptable. 87

10 Keränen et al.: Ray-tracing method for noie control in workplace ACTA ACUSTICA UNITED WITH ACUSTICA Vol. 89 () A_IL A_SPL (before) A_SPL (after) Figure. The accuracie of the inertion lo prediction (circle) and the accuracie,, of the predicted SPL before (tar) and after (triangle) the noie control meaure in workplace. The mot probable reaon for the ytematic error in and wa, naturally, the uncertainty of SPL meaurement. For example, the uncertainty of the ound power level determination of the ound ource (in the laboratory) wa. at to Hz octave band []. Other thing to conider were the uncertainty of in itu SPL meaurement, i.e. uncontrolled or untable background noie; though the production wa interrupted, ome noie event could not be avoided. Diffraction could alo be caued by the meaurer or mall nearby object and interference effect at low frequencie can alo caue different SPL even with mall change of location. Additionally, the SPL calibration of the ound level meter may vary within. during the year of the noie control project. The accuracy of the predicted SPL depend trongly on randomne of calculation. Scattering and number of ray produce variation in the computed SPL in the model. However, the repeatability of calculation wa quite good []. The repeatability wa tudied uing lightly different number of ray in a workplace and determining the average and tandard deviation of the predicted SPL. The maximum difference between predicted SPL wa.. It wa alo found that increaing the number of ray did not remarkably improve the accuracy of prediction. Thi phenomenon i related to the modeling algorithm of the ODEON and ha been reported in cae tudie by Rindel and Chritenen [9, 8]. In practical noie control deign, poorer accuracy can be expected when real noie ource are ued intead of a known omnidirectional ound ource. One hould keep in mind that the uncertainty of laboratory ound power level meaurement i. at to Hz, if the ound power level i determined according to ISO 7 or ISO 9- [, ]. If the ound power level i determined according to ISO 7 (urvey method), which i often the the only practical method in itu, the uncertainty i for the A-weighted ound power level and undefined for octave band []. One hould alo take into account the ize and directivity of the real noie ource in 87

11 Vol. 89 () Keränen et al.: Ray-tracing method for noie control in workplace Table VI. Meaured and predicted inertion lo, IL. workplace Hz Hz Hz Hz Hz Hz predicted meaured predicted meaured predicted meaured predicted meaured predicted meaured predicted meaured Table VII. The accuracy of the RT prediction. workplace Hz Hz Hz Hz Hz Hz before after before after before after before after before after Average Standard deviation the model. In addition, the ue of ound power level of real noie ource include the potential for large error, becaue real noie i rarely contant and unvarying. Thee are the reaon why real ound ource could not be ued in thi tudy. If the number of noie ource with approximately the ame ound power level i large, the noie control of a ingle ource ha a negligible effect on SPL. In uch cae, the IL value obtained uing ingle ource meaurement hould not be preented to the client a the abolute ound attenuation reult, becaue they are overetimate and do not repreent the true attenuation of noie. Thi i natural, ince the inertion lo i determined for a ingle noie ource and the noie control meaure are implemented conidering that ource only. The inertion lo method i uitable for ound propagation and inertion lo meaurement for a ingle ource when background noie can be reaonably reduced. The Maximum Length Sequence technique (MLS) could be ued in workplace in any condition, becaue it place practically no eriou requirement on background noie level []. However, the method ha ome problem. There hould be no patial variation of temperature or of relative humidity of air when the MLS technique i ued. The meaurement hould be done uing fixed meauring point() and noie ource(). Movement of microphone are not allowed. Therefore, the ue of the MLS technique hould be tudied more thoroughly in large indutrial workplace. Although the meaurement were carried out by three different peron in different workplace and the model were created by two different peron, the workplace-averaged accuracy,, wa acceptable. Therefore, it could be aid that the difference between operator i not a major concern if the guideline preented in the method are followed. The temperature and the relative humidity were not meaured and conidered in the model. However, it i known that the relative humidity ha an effect on ound aborption of air. Thi effect hould be noted epecially in large pace and at high frequencie. It i probable that, in workplace, the change in relative humidity caued poor accuracy at high frequencie. In thi workplace, the firt meaurement were performed in September (typically high relative humidity, weak air aborption) and the 87

12 Keränen et al.: Ray-tracing method for noie control in workplace ACTA ACUSTICA UNITED WITH ACUSTICA Vol. 89 () (a) (c) (b) Figure. The workplace-averaged accuracy of the model,, (circle), the workplace-averaged accuracy of inertion lo prediction,, (quare) and the workplaceaveraged accuracy of reverberation time prediction,, (triangle). Standard deviation i denoted by bar. econd meaurement in February (typically low relative humidity, trong air aborption).. Concluion By following our general guideline for the modeling of large workplace in the ix workplace, the workplace-averaged accuracy of SPL prediction wa...atoctavebandto Hz. Standard deviation of at octave band were. The accuracy of the model wa, thu, adequate. Similarly, the inertion lo for noie control meaure wa predictable to an accuracy of (Figure, ) at octave band to Hz in typical workplace. Thi i adequate conidering the reaonable amount of work needed in modeling and noie control deign and the accuracy of ound preure and ound power level meaurement in itu. Good accuracy of IL prediction i mot important becaue modeling i typically ued for comparion of different acoutical remedie. The calculation of abolute SPL i eldom ued in workplace deign application becaue ound power of noie ource can not be reliably determined and they operate in different cycle. The accuracy of the RT prediction wa at to Hz octave band. Thi i acceptable, though reult could be more accurate if the model were optimized in predicting RT. Acknowledgement The author would like to thank Clau Lynge for providing the valuable information about the Odeon oftware. The Finnih Work Environment Fund, The Finnih Intitute of Occupational Health and The Minitry of Social Affair and Health are appreciated for the financial upport given to the reearch project during 998. Reference [] H. Nykänen, E. Klamka, L. Lamula, E. Räiänen: Experience of the acoutical deign of working environment uing computer modeling baed on ray-tracing technique. Proceeding of Internoie 9, Sydney, Autralia, 99, 8. [] ISO 9-: Acoutic Recommended practice for the deign of low-noie workplace containing machinery Part : Noie control trategie. Genève, Switzerland, 99. [] ISO 9-: Acoutic Recommended practice for the deign of low-noie workplace containing machinery Part : Noie control meaure. Genève, Switzerland, 99. [] ISO 9-: Acoutic Recommended practice for the deign of low-noie workplace containing machinery Part : Sound propagation and noie prediction in workroom. Genève, Switzerland, 997. [] V. Hongito, J. Keränen, E. Airo, P. Olkinuora: Acoutic computer modeling in the planning of noie control meaure (in Finnih). Työ ja Ihminen tutkimuraportti, Helinki, Finland,. 87

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