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1 Vhicl Intrior Nois Evaluation Using Brain Magntic Fild Shunsuk Ishimitsu, Hiromi Nishikawa, Siji Nakagawa 2, Yoshiharu Sota 2, Takuya Hothama 2 Hiroshima City Univrsity 3-4- Ozuka-Higashi, Asa-Minami, Hiroshima JAPAN 2 National Institut of Advancd Industrial Scinc and Tchnology (AIST) ABSTRACT In rcnt yars, th focus of production concpts rgardingg car ngin sound has changd from finding a solution to nois, to dsigning spcific sounds. Althoughh many studis hav bnn conductd to crat subjctivly plasant car-ngin sounds, th psychoacousticc ffcts of th tim-varying rat of acclrating ngin sounds ar still unclar. Th prsnt study invstigatd auditoryy imprssionss of intrnal car nois using psychological and nurophysiological mthods. To rprsnt th half-ordr sound of an ngin, w usd harmonic complx tons that simulat acclration nois with a sinusoidal modl as stimuli. Nuronal activity in th auditory cortx vokd by ths stimuli was masurd usingg magntoncphalography (MEG). Subjctiv valuations wr xamind using paird-comparison tsts.. At th sam tim, w rcordd rsponss in th MEG alpha-wav rang (8-33 Hz), and analyzd thm using th autocorrlation function (ACF). Th rsults of a jury tst confirmd that changs in th lvl of half-ordr sound wr associatd with changs in subjctiv annoyanc, and vrydayy drivrs prfrrd ngin nois that includd a high lvl of half-ordr sound. MEG analyss rvald a rlationship btwn subjctiv annoyanc and th ffctiv duration of th ACF in MEG activity btwn 8 and 3 Hz. Kywords: Vhicl Intrior Nois, Brain Magntic Fild, Magntoncphalography, Sinusoidal Modl. INTRODUCTION Associations xist btwn spcific s products and crtain sounds for most popl. Th focus of production concpts of car ngin sound has changd in rcnt yars, from findingg solutions to nois to dsigning particular sounds. That is, most rsarch no longr focuss on rducing ngin nois, but sks to mak it subjctivly plasant [][2][3]. On problm in th control c of ngin sound is i finding a suitabl mthod for valuating th diffrncs in auditory imprssions. Dsigning such a mthod is difficult, bcaus componnts off ngin sound chang as th ngin spd incrass. Incrasing th volum of ngin sound givs listnrs particula auditory imprssions, such as "sporty" and "acclrating". On rport [4] indicatd that whn th lvl of half-ordr sound of an ngin incrasd, listnrs' prfrnc dcrasd. Half-ordr causd by torqu fluctuation in an ngin, which hav a soundd occurs as a rsult of half-ordr multipl rolling momnts. Ths momnts ar diffrnt working point and a diffrnt phas angl in ach cylindr [5].[ Howvr, th psychoacoustic ffcts of half-ordr sounds hav not bn sufficintly studid using comparison c analysis mthods, and obtaining rliabl rsults with only ths mthods is difficult. Thrfor, a mthodd of valuation that is quantitativ and objctiv is rquird. Most prvious studis hav usd wll-stablishd bioinstrumntation mthods for objctiv sound quality valuations [5]. A numbr of studis hav bn carrid out o to xamin th rlationships btwn alpha activity in th cortx and subjctiv prfrnc and annoyanc masurmnts for f sound filds [6],[7]. Ths studis hav rportd that th subjctiv annoyanc and ffctiv duration of th autocorrlation function (ACF), τ, of th magntoncphalography (MEG) alpha wav wr ngativly corrlatd. That is, th brain rpatd a similar rhythm during xposur to prfrrd sound ishimitu@hiroshima-cu.ac.jp 2 s-nakagawa@aist.go.jp

2 filds. Howvr, stablishing a similarly clar rlationship is much mor difficult with structurally complx sounds, such as half-ordr sounds. In th currnt study, w invstigatd th possibility of using th rlationship btwn subjctiv annoyanc and alpha activity during xposur to half-ordr ngin sound as an objctiv valuation of auditory imprssions. Morovr, auditory imprssions wr xamind using subjctiv valuation with th smantic diffrntial (SD) mthod, and pairs wr xamind using paird-comparison analysis. In addition, w simultanously masurd MEG activity in th alpha-wav rang (8-3Hz) and analyzd it using th ACF. Th rsults of th scal valus of subjctiv annoyanc and th paramtrs of th ACF of th MEG alpha wav wr compard. 2. SOUND QUALITY EVALUATION 2. Stimuli In an acclrating car, th componnts of th ngin sound chang as th ngin spd incrass. Th tim-varying natur of th componnts can induc auditory imprssions such as "sporty" and "acclrating" in listnrs. In th currnt study, th psychoacoustic ffcts of sound lvl and its modulation wr invstigatd. Acclration nois was masurd using a straight-four ngin and harmonic complx tons that simulatd acclration nois wr usd as stimuli. In a 4-cylindr ngin, th ngin firs twic with ach rotation, gnrating a scondary vibration forc, and ths highr harmonics constitut th main componnt of th nois. Figur shows a spctrogram of th sound of a 4-cylindr ngin, rcordd at th intak. In this figur, th ngin sound consists of th harmonic componnts and othr componnts. On of th main causs of poor sound quality in a passngr car is th high contribution of multipl half-ordr componnts of in-lin 4-cylindr vibration. Half-ordr multipl rolling momnts causd by th torqu fluctuation, and combind torsional momnts act on th cylindr block around th crankshaft axis as intrnal forcs. As such, th torsional vibration shap is xcitd as half-ordr multipls of th ngin rotation [5]. Figur Spctrogram of ngin sound rcordd at th intak. A prvious rport [4] indicatd that whn th lvl of half-ordr sounds in an ngin incrass, listnrs' prfrnc for th sound dcrass. In th currnt study, th rlationship btwn subjctiv annoyanc and alpha activity in rspons to half-ordr ngin sounds wr invstigatd as an objctiv valuation of auditory imprssions. Modl sound sourcs, cratd as harmonic complx tons, wr comprisd of vn-numbrd ordr sounds and half-ordr sounds, as wll as th main componnts of th straight-four ngin nois. Each stimulus of th half-ordr sound lvl was changd by +20 db, +0 db, 0dB, 0 db, and thn liminatd. 2.2 Sinusoidal Modl Artificial acclration noiss wr cratd by changing th frquncis of ach componnt ovr tim using th sinusoidal modl [9]. 2

3 Figur 2 Analysis-by-synthsiss tchniqu that squntially xtracts th paramtrs for ach sinusoidal componnt. Th modll implmnts analysis, transformatiot on and synthsis for with Sinusoidal Modll Sinusoidal modling is an analysis-by-synthsis sound wavs, as shown in Fig.2.. Th paks of th spctrogram in ach analysis fram wr xtractd using short-trm Fourir transform (STFT),, and partial sounds wr dtrmind by th information rgarding instantanous magnitud, frquncy and phas includd inn th paks. In this study a 2.3L straight-four sdan ngin was mployd as ann objct, and th driving condition c was st to full acclration in third gar. Th rcording position is shown in Fig..3. Figur 3 Rcording position 2.3 Exprimnt Sound quality valuation for ach lvl of half-ordrr sound was xamind using Schff's paird comparison tsts. Tn subjcts (20-40 yars old; six mal, four fmal) with normal haringg participatd in this xprimnt. Six subjcts drov mor than four days a wk, whil th othr four drov rarly. Subjctiv valuations and rcordings of brain magntic filds wr conductd at th sam tim in a magntically-shildd room kptt at a comfortabl tmpratur. During th xprimnts, subjcts wr satd in a chair with thir bodis fixd in a vacuum cast. Thy wr askd to clos thir ys and to concntrat fully on th sound stimulus. Pairs of sounds wr prsntd, and subjcts wr askd to judg thir prfrrd acclrating car ngin sound. Paird-comparison tsts wr prformd for all combinations of pairs p of sound stimuli. Th modl sounds consistd of 0 pairs (N(N-)/2, N = 5) of stimuli. Th intr stimulus intrval (ISI) was st at.0 s. Subjcts wr instructd to rport thirr dcision by prssing on of two switchs aftr thy hard a signal sound, which was an 80-ms khz ton prsntd 2.0 s aftr th pair of sounds. Th scal valus of th prfrnc wr calculatd according to Cas V of Thurston's T thory. Sound stimuli wr prsntd through plastic tubs and arpics (ER-2, Etymotic Rsarch R Co..) insrtd into th ar canals. 2.4 Rsults Figur 4 shows th rlationship btwnn th half-ordr sound lvl and annoyanc rating valu, which was obtaind by subjctiv valuation. Th rsultss rvald that whn th lvl of th half-ordr sound incrasd, th lvl of annoyanc wass incrasd for som listnrs, but dcrasd for othrs. Th thr subjcts rporting incrasd i annoyanc wr vryday drivrs, whras six of th t svn subjcts who rportd dcrasdd annoyanc drov rarly. From th prspctiv of acoustic mtrics, th roughnss of ngin sounds would b xpctd to 3

4 incras as half-ordr sound lvl incrasd. Howvr, it is likly that vryday drivrs had a positiv imprssion of ngin sounds thatt wr clos to thir own car ngin sound, including half-ordr sound. Scal valus of annoyanc no half -0dB +-0dB +0dB Intnsity of half ordr componnt +20dB FD a FD b FD c FD d FD FD f IFD g IFD h IFD i IFD j Figur 4 Scal valus of annoyanc andd intnsity of th half-ordr componnt for subjcts. 3. MAGNETOENCEPHALOGRAPHY MEASUREMENT 3. Mthods Th nuronal activity vokd in th auditory cortx was masurd with MEG to stablish an objctiv valuation indx of th subjctiv valuation of annoyanc. MEG is wll suitd for studis of th auditory cortx [0]. MEG rcordings wr conductd at th sam tim as th subjctiv valuation. Twnty tsts wr conductd for ach pair of sounds. Th intr stimuluss intrval (ISI) was st at 2.0 s. MEG was conductd using a 22-channl whol-had DC suprconducting quantum intrfrnc dvic (DC-SQUID) magntomtr (Nuromag-22TM, Nuromag Ltd.). Nuromag-22TM magntomtrs hav two pick-up coils in ach position, which masur two tangntial drivativs. On coil masurs th orthogonal tangntial drivativs of th magntic fild normal to th surfac of th had along th latitud, whil th othr masurss thos alongg th longitudinal axis, as a shown in Fig.. MEG dataa wr sampld at 400 Hz aftr bing band-pass-filtrd btwn frquncis of 0.03 and 00 Hz. Figur 5 Exampls of rcordd MEG. 3.2 Analysis In MEG, alpha activity is typically dfind as fluctuations at frquncis btwn 8 and 3 Hz that can b dtctd on th occipital ara. a Similarr oscillatory activity, obsrvd ovr th auditory cortx, is rfrrd to as thτ rhythm. In this study, w xtractd th alpha wav by filtring activity btwn 8 and 3 Hz, and 52 channls that wr locatd around th tmporal and th occipital ara slctd for th ACF analysis. Each rspons corrsponding to on stimulus was analyzd a by ACF for ach subjct. W invstigatd th rlationshipss btwn th dgr of prfrnc and avragd τ valus att 26 sits, masurd at two tangntial drivativs. Th autocorrlation function (ACF) is a mathmatical tool for dtcting priodicitis in pattrns, 4

5 and provids th sam information as th powr spctral dnsity off a signal. Figur 6 (a) shows s an xampl of a masurd ACF. As shown in Fig. 6 (b), a straight-lin grssion of th ACF can only b producdd using th initial dclining portion. In most cass, th nvlop of dcayy of th initial part of th ACF can b a straight lin. Th valus of τ wr analyzd for 2.02 s aftr th prsntation of th sound stimuli. Th valus of valu, τ max. τ In this study, th α wav was analyzd, which appard -3 s aftr stimulus prsntation. Th ffctiv continuanc tim τ also a calculatd. wr normalizd within ach subjct with rspct to th maximum Figur 6 (a) Exampls of normalizd ACF of MEG activity btwnn 8 and 3 Hz. (b) Exampls of dtrmining thτ 3.3 Rsults W xamind th rlationship btwn subjctiv valuations of o annoyanc and MEG activity, particularly th ffctiv continuanc tim τ of th alpha wav masurd ovrr th occipital rgion. Fig. 7 shows th rsults of vryday drivrs,, and Fig. 8 shows th rsults of popl who driv rarly. Th rror bar rprsnts standardd rror of th man ACF. For both vryday drivrs and rar drivrs, w obsrvd significant diffrncs in annoyanc ratings in som subjcts (for FD a FD b FD d IFD h IFD i, p < 0.0. For FD c, p < 0.05). Among subjcts xhibiting a significant diffrnc in annoyanc ratings btwn avrag and strong stimuli, FD d showd a shortnd τ in rspons to sounds ratd as lss annoying. On th othr hand, for FD i, τ was lngthnd. Ths rsults suggsts that subjctiv annoyanc rlatd to diffrncs in half-ordr sound lvl affcts th ffctiv continuanc tim, τ, of th alpha wav dtctd ovr th occipital rgion... t/t man.05 FD a FD b FD c FD d FD FD f lowst annoyanc middl annoyanc highst anoyanc Figur 7 Rlationships btwn subjctiv annoyanc and valus forr FD subjcts. 5

6 . / man IFD g IFD h IFD i IFD j 0.9 lowst annoyanc middl annoyanc highst annoyanc Figur 8 Rlationships btwn scal valus of subjctiv annoyanc and valus of τ, for subjcts who infrquntly driv (IFD subjct). 4. CONCLUSION In this study, w invstigatd th nural corrlats of sound quality valuation of intrnal car nois using MEG. Th rsults rvald a rlationship btwn subjctiv prfrnc and MEG alpha activity in rspons to acclrating car nois. Spcifically, w found that th ffctiv duration ( τ ) of th ACF of MEG activity btwn 8 and 3 Hz was lngthnd in rspons to th prsntation of prfrrd acclrating ngin sounds. In both vryday drivrs and rar drivrs, w obsrvd significant diffrncs in annoyanc ratings btwn subjcts. Ths rsults suggst that subjctiv annoyanc rlatd to diffrncs in half-ordr sound lvl can influnc th ffctiv continuanc tim, τ, of th alpha wav dtctd ovr th occipital rgion. Similar mthods could b usd in th valuation of sound dsigns for a rang of industrial applications. REFERENCES [] S. Ishimitsu H. Kobayashi, Study on Instantanous Corrlation Analyss of Acclration Car Intrior Nois using Wavlts and its Subjctiv Evaluation., Transactions of th Japan Socity of Mchanical Enginrs, sris C, Vol. 72, No. 79, (2006). [2] T. Toi, Rcommndation of Comfortabl Sound Dsign and its Mthodology., (in Japans) Machin Dsign, Vol. 48, No. 2, (2004). [3] Y. Ishi, K. Noumura, Evaluation tchniqus and thir applications for intrior sound quality of vhicl., Transaction of Socity of Automotiv Enginrs of Japan, Vol. 60, No. 4, 2-7 (2006). [4] T. Hashimoto, Sound Quality of Car Intrior Nois and Its Dsign Paramtrs., Th Journal of th Acoustical Socity of Japan, Vol.64, No.9, (2008). [5] T. Nakada and H. Tonosaki, A Study of th Excitation Mchanism of Half-Ordr Vibrations in an In-Lin 4-Cylindr Intrnal Combustion Engin., Th journal of th Japan Socity of Mchanical Enginrs, C, Vol. 60, No. 577, (994). [6] Y. Ishi, T. Yamashita, Y. Araga, Snsory Evaluation of Road Nois with Organic Raction using Brain Wavs., Honda R&D Tchnical Rviw, Vol. 4, No. 2, 8-88 (2002). [7] Y. Sota t al., Magntoncphalographic Rsponss corrsponding to individual subjctiv prfrnc of sound filds., Journal of Sound and Vibration, 258, (2002). [8] Y. Sota t al., Magntoncphalographic rsponss corrsponds to individual annoyanc of bandpass nois., Journal of Sound and Vibration, 277, (2002). [9] P. Dpallftal., Procdings of 993 Intrnational Computr Music Confrnc, San Francisco, (993). [0] F. Grandori, M. Hok, and G. L. Romani, Auditory Evokd Magntic Filds and Elctric Potntials, Vol. 6 of Advancs in Audiology, (Kargr, Basl, 990). 6