Sound power certification of a dodecahedron shaped sound source

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Sound power certification of a dodecahedron shaped sound source
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  Sound power certification of a dodecahedron shapedsound source Gilberto Fuchs de Jesus a , Rodolfo Venegas  b , Marco Nabuco c , Paulo Massarani da GROM Acústica & Automa!o, Rio de Janeiro, "#""#$"%,RJ, 'ra(il  b )e*artamento de Acustica, +niersidad P-re( Rosales, 'ro.n Norte "/#, 0antiago, 1hile, c,d Acoustic 2esting 3aborator4, 5nmetro, )6 de 1a7ias, "8 "8#$#"#, RJ, 'ra(il a; b c; d, Abstract Reference sound sources must be stable, omnidirectional and shall *resent afre9uenc4 s*ectrum as flat as *ossible, .ithin standardi(ed limits6 2he reference soundsources most used on acoustic measurements are constructed as centrifugal fans thatgenerate aerod4namic broadband noise6 2hese sound sources are er4 stable due tomechanical and electrical inertia .hich guarantee an almost inariable rotation s*eedduring its o*eration6 2he4 can be o*erated for long *eriods of time .ith neglectingmodification on their out*ut sound *o.er6 Neertheless, due to constructie limitations,the sound * leel of aerod4namic sources is limited to around /8 d':A;6 <enthough this sound * leel is enough for man4 laborator4 and field a**lications,some tests re9uire more sound * in=ection in reerberation chambers or on fieldmeasurements6 Performance and endurance tests of electronic deices, .hen submittedto high sound leels, and in situ  sound isolation bet.een *artitions are t.o commone7am*les of acoustic tests in .hich usual reference sound sources do not generateenough acoustic *o.er6 2his *a*er describes a test set$u* as .ell as some results of anelectro$acoustic dodecahedron sha*ed sound source tested in a reerberation chamber62he data can be used as a *ossible standardi(ed *rocedure to ealuate this >ind of sound source6 0ome endurance and re*eatabilit4 inestigation data are also *resented as *art of the alidation tests for a ##? 'ra(ilian made dodecahedron6 1.INTRODUCTION )odecahedral louds*ea>ers hae been .idel4 used as omnidirectional sound sources for along time6 0eeral manufacturers *roduce this >ind of source .hich are used in numerousa**lications6 Neertheless, no reference .as found in the contem*orar4 literature of an4attem*t to erif4 .hether these sources could be used as reference sound sources according tothe international standard 50O @/"@/// BC6 2he abilit4 to generate high sound * leelsof louds*ea>ers is normall4 neglected in face of the lac> of confidence on their stabilit46  2he necessit4 of *o.erful and stable sound sources for 5NM<2RODs Acoustic 2ests3aborator4 :3A<NA; and the increasing 9ualit4 of toda4Ds louds*ea>er ins*ired thedeelo*ment of a ##? 'ra(ilian dodecahedron6 2he source model used for the tests re*ortedherein .as initiall4 designed b4 3A<NADs researchers and their first *rotot4*e unit, testedduring this stud4, .as handcrafted b4 5NM<2RODs car*enters in *l4.ood6 As GROM Acústica & Automa!o decided to turn that *rotot4*e into a *roduct, the srcinalfour inch 'ra(ilian made coa7ial louds*ea>er model .as re*laced b4 a similar and more *o.erful one from the same manufacturer6 2he *entagon sha*ed faces .ere them machinedfrom M)F .ood .ith industrial *recision so the sound source could be assembled in a *roduction scale62.o different units .ere inestigated for this stud46 One is the first handcrafted *rotot4*eo.ned b4 5NM<2RO and the other one .as the first unit sold b4 GROM6 2he second unit belongs to 5NP<, the National 5nstitute of 0*ace Research, that >indl4 authori(ed the authorsto use the data collected during the certification tests *erformed .ith their unit62he sound * measurements made for both dodecahedron units tested sered different *ur*oses6 2he tests made .ith the 5NP< unit .ere used to generate an accredited re*ort for that *articular unit, indicating its sound * s*ectrum for a single in*ut oltage6 2hemeasurements made .ith the 5NM<2RO unit .ere more com*lete and coered a .ider rangeof conditions6 2he different conditions .ere tested to inestigate the dodecahedron as a *ossible reference sound source6 2.MEASUREMENT DESCRITION ! TEST SECI"ICATION2.1 #enera$ 2he electrical characteristics of the louds*ea>er, >no.n from the manufacturer data sheet,.ere indiiduall4 chec>ed for each of the " units at the time of the first *rotot4*econstruction6 2he directionalit4 of that unit .as measured b4 FEbio Arruda B"C bac> in ///62he tests *erformed b4 Arruda did not coer the entire fre9uenc4 range *rescribed b4 50O@/"@ and sho.ed the source directiit4 *resented lobular *atterns aboe >(6 2hea**arentl4 inade9uate directiit4 inde7 : N di ; of the handcrafted *rotot4*e ma4 be caused b4the lac> of *erfect s4mmetr4 of the source faces6 5t is e7*ected that the machined constructionof GROMDs model and the use of the *rotection grid ma4 im*roe this characteristiceentuall4 ma>ing N di fulfil the re9uirements of 50O @/"@ for a .ider fre9uenc4 range62here .ere t.o biggest concerns related to the tests6 2he first one .as the *h4sical integrit4of the louds*ea>er units the e7*ected long duration of the tests at high * out*ut coulddegrade the 9ualit4 of the source itself causing ariations on sound * s*ectra during thelong term stabilit4 inestigation6 2he influence of the tem*erature increase at the louds*ea>er coil could cause out*ut ariations eentuall4 oiding the results6 2he second one .as there*roducibilit4 of the test set$u*6 2he choice of the test *arameters that could influence themeasurement results and a .a4 to control them in an a**ro*riate .a4 .as carefull4 discussed b4 the authors before starting the measurements6  2he main goal .as to determine an engineering *rocedure that could be du*licated and under .hich the sound * leel : L W ; emitted b4 the source, as .ell as its s*ectrum could bemeasured b4 a standardised method6 2he tests .ere re*eated at least H times at each source *osition in order to alidate is re*eatabilit4 of the *rocedure and the source stabilit462he tests described in the ne7t sections .ere carried out to determine four basiccharacteristics of the source its sound * s*ectrum at a gien in*ut oltage :so callednominal test oltage;, the relationshi* bet.een the sound * leel and the in*ut oltage,the long term stabilit4 of the sound * s*ectrum and the *ossible influence of the sourceeleation aboe the test room floor on its sound * leel and s*ectrum6 2.2 E%uip&ent and Test Set'up 2he e9ui*ment used for the tests re*orted herein are listed on table 6 2he "HH anal4ser .asused to generate a *in> noise signal that .as then am*lified b4 the * am*lifier to driethe dodecahedron6 2he driing signal .as monitored b4 channel ' of the %# anal4ser toma>e sure the e7citation signal .ould not ar4 during the sound * measurements6 Table 1: List of equipment used for the tests. <9ui*mentModelMultimeter P HI%AFunction Generator P HH"#ARamdom 5ncidence Micro*hones 3arson )ais "88/# channel mico*hone multi*le7er 3arson )ais Model ""#Real 2ime Anal4ser Norsonic R2A %#Real 2ime Fre9uenc4 Anali(er '&J 24*e "HHumidit4 & 2em*erature meter Vaisala MH1alibrator '&J 24*e "HReference 0ound 0ource '&J t4*e "#0trobosco*e General Radio 8@General Radio 8 am*lifier 0tudio R A'0%## 2.(Test Description 2he sound * measurements .ere made using the com*arison method *roceduresdescribed on the international standard 50O HI BHC6 A fi7ed arra4 of si7 micro*hones .as *ositioned inside one of 5NM<2RODs reerberant rooms and a reference sound source t4*e"# .as used for the com*arison measurements6 2hree source *ositions .ere used in eachsound * measurement6For each source *osition the signals from the @ micro*hones in the fi7ed arra4 .ere fed intochannel A of the %# anal4ser through the ""# multi*le7er6 One micro*hone *osition .asta>en as reference and used to calibrate the anal4ser in*ut6 All other micro*hones .ere thenindiiduall4 calibrated and each offset recorded for further correction of the collected data62he reerberant room used for the tests has a olume of /@m H  and three source *ositionscould be mar>ed .ith no need for changing micro*hone *ositions6  2he channel ' of the %F# anal4ser .as calibrated using a & >G( *ure tone :sine .ae;generated .ith the am*litude of #6& V RM0s  b4 the HH&"#A function generator6 2his signal .asfed to the * am*lifier .hich .as ad=usted to delier & V RM0s  on its out*ut6 All oltagereadings .ere made .ith the HFI%A multimeter6 2he am*lified signal used to drie thedodecahedron .as also connected to channel ' of the %F# R2A6 2his channel reference .asset to &# $@  V RM0s 6 After this calibration *rocedure the %FA channel ' indicated &"# d' re &# $@ V RM0s  at the &6#>G( &KH octae band62he 5NM<2RO *rotot4*e used for the tests is e9ui**ed .ith t.ele 0<3<N5+M coa7iallouds*ea>ers model 10AF#F6 2his louds*ea>er model has a *a*er cone and is rated H8L RM0 62he louds*ea>ers are connected in a series$*arallel arrange that results in a nominalim*edance of 86HH Ohms6 2his im*edance .as used to calculate the electrical RM0 * of the dodecahedron based on the indiidual rated * of the louds*ea>er6 2he nominal RM0 electrical * of the louds*ea>ers .as determined b4 the manufacturer according to the 'ra(ilian standard N'R&#H#H6 5n the tests re*orted on the *roduct datasheet, the louds*ea>ers are continuousl4 drien at the RM0 * for t.o hours in a F8 litresealed bo76 Once dodecahedron has t.ele louds*ea>er units assembled in a much smaller olume and the heat dissi*ation could reall4 be a *roblem the driing oltage to thedodecahedron .as limited a fraction of its calculated RM0 oltage6 5t .as e7*ected thatdoing so the tem*erature increase .ould be >e*t .ithin reasonable alues, minimi(ing the *ossible distortions associated .ith the resulting modifications of the electromagneticcharacteristics of the louds*ea>ers6 2he theoretical RM0 electrical * of 5NM<2ROsdodecahedron *rotot4*e is F"# L .hich corres*onds to a driing oltage of FI6H V RM0 65n order to *resere the integrit4 of the sound source and considering a long term stabilit4 *lanned to last u* to F hours and H# minutes .ith continuous o*eration, the driingoltage .as limited to &#V RM06  2his oltage corres*onds to F68? of the rated RM0 electrical * of the dodecahedron6 2he relationshi* bet.een the in*ut oltage and the sound * emitted b4 the sound inestigated b4 com*aring the sound * s*ectrum of the dodecahedron .hen drien b4 four different oltages, sho.n on table "6 2hese oltages .ere set b4 ad=usting the * am*lifier to a fi7ed gain and ar4ing the *in> noise leel on its in*ut6 2hese *rocedure .aschosen due to the difficult4 of controlling the am*lifier gain6 As a dra.bac>, the driingoltage could onl4 be set in fi7ed ste*s on the "&HH anal4ser .hich resulted in the aboeuneenl4 rounded alues of the driing oltage6 Table 2: Input voltages  Nominal Voltage:V RM0 ;Real Voltage:V RM0 ;Voltage leel:d' re &# $@ V;#6@#68/I  115,5 &6@&6@"%  124,2 86@86@"H  135,0 &#/6II"  139,8  )uring each test run the Leq  leels and s*ectra .ere measured for both in*ut oltage andout*ut sound *ressure6 2he Leq  oltage leels aried from run to run .ithin a K$ &d' range62he difference bet.een the nominal in*ut leel and the actual measured leel for each used to normali(e the corres*onding Leq  sound *ressure leel to e7actl4 the same in*utoltage62he 5NM<2RO unit .as also submitted to an endurance test in order to ealuate its long termstabilit46 )uring this test the in*ut oltage leel .as >e*t constant and set to the nominalalue of &#V RM06  2he source height .as set to &6HIm aboe the room floor6 )uring theendurance test the source .as turned on continuousl4 for Fh H#min and four sound * measurements .ere *erformed6 2he time interals bet.een source actiation and eachmeasurement start are sho.n on table H6 Table 3: Measurement intervals Measurement 5nde72ime before measurement start &&# mim"&h &# mimHF hFF h H# mim2he last test *erformed .as the sound * measurement on 5NM<2RODs dodecahedron atdifferent heights6 2he source stand .as ad=usted in three different *ositions aboe the roomfloor6 2he in*ut oltage .as >e*t constant and the sound * .as measured after &#minutes of continuous o*eration for each height6 2he source .as turned off bet.een eachmeasurement to allo. *ro*er cooling6 2he three different height are sho.n on table F6 Table 4: Soure heights 0ource heights #68Im#6/Im&6HIm (. MEASUREMENT RESU)TS 2he 5NP< source .as tested .ith the in*ut signal set to &" V RM0 6 2he source height .as >e*tconstant and the srcinall4 su**lied tri*od .as used to hold the source6 2hree measurementruns at three different *osition each .ere made .ithin t.o consecutie da4s6 2he sound * s*ectra calculated as *er 50O HIF& for each run .ere then aeraged to be *ublished asthe source characteristic s*ectrum6 2he com*arison of this s*ectrum .ith the corres*ondings*ectrum obtained for the 5NM<2RO source is *lotted in figure &62he *lot on figure " sho.s ho. the sound * s*ectrum changed .hen the 5NM<2ROsource .as drien b4 four different oltages at a constant height6 5t is clear that the differents*ectra =ust shift u* .ith the oltage increase, >ee*ing its sha*e unchanged6
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