Rasti Method in Cars

8/18/2019 Rasti Method in Cars

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Measurements in Cars using the RASTI Method

by Mitsuo Nakamura, Honda R&D Co. Ltd.Torben Jacobsen, Bruel & Kjasr

IntroductionUntil recently the RASTI method

(Rapid Speech Transmission Index)has mostly been used as a quick andeasy method for evaluating speech intelligibility in rooms and the quality of publ ic ad dr es s and re in fo rc em en t sys -terns [1, 2]. Lately, however, other ap- Flg - L ^aUtatwe interpretation of RASTI

pl ic at io ns such as ev al ua ti on of sp ee chintelligibility in aeroplanes and fieldtesting of speech privacy have also become im por ta nt. Thi s app lica tion note -1 , ^ 1 ^ . \T A1 * c n i-K, OOY»O

.„ ,. , » „ T, . r ,rr ,T possible to complete one measur ement rN Ol Sc 111 v y d l Swil l discuss the use or the RAS 1 1 : , 0 ,,, , . ,. ,,, ,, , in no mor e th an 8 sec onds .method m connection with the evalua- mi xl , . ,.

„ . , x , ,. n , l h e noise in the cabin comes fromtio n ot noise an d th e qua lit y ot spe ech ,_,, _. . orT1T , . ^ _ , TT ,, ,, J r ) A.... . Ih e R A S H system consists of a several noise source s. Usually the dHAcon dit ion s in car s. , ._ , . , . , , , jn , , . n . x. .,,

tra nsm itt er, which is place d at th e level is used in conn ecti on with noiseTT71 . , . -, x , x. spea ker' s posit ion and a receiver contr ol design but becau se a rat herWhy consider noise and the acoustic -, , x f, ,. x , . x. ~ ,, n . r r jn 1T̂ A , ,

,, / , » / . , ! • J r n placed at the li st en er s posit ion. By small difference m the dHA level cancomfort of the interio r ot a ca r: P x , x . . / , . . , , . . . . . .m, , ,, , . mea ns ot thes e two in str um ent s it is subjectively be eva luat ed as a sig mh-Ih er e are several rea sons: the drive r ., , , , , ,.,,,. { . ° x, , , , possible to measu re what happ ens cant difference, most car manu fact ur-an d th e pa ssenge rs are expos ed to ._,, , , . , ,,. ., ...^ , f , , . x. , x. r

r x , i /, , , ,, with the speec h intelligibility betwee n ers also use a subjective eva luat ion ofnoise from the roa d/t yr e cont act , the , , . .̂ ,, . ,, . ,, . ,. . ,, . . , the two posit ions. the noise. It is a tim e-c ons umi ng and

tran smis sion , the engine, wind, etc., , , „ . o rnT° ., ,, /. -, • ?, i-/./» i. r not very exact met hod . R A S H is aand the ref ore it is often difficu lt tor ™ n A omT ^ i , i • , -- i • . n- -i -T, i , ,-. , . .,, , jn lh e R A S H met hod takes into con- measu re tor speech intelligibility, butthem to comm unica te with each other ., ,. ,, rr x P x , P x ., , , ,,. , ,, ,. / x , side ratio n the effects of: m the futur e it can also pe rha ps beor listen to the rad io/ cas set te piaver. , , , , , . t ,.,, . . rm, . , i • i T ^ a- backgr ound noise, i.e. in our case used as a desc ripto r ot the noisine ss of1 his causes tir edn ess and , in addi tion , . £ . ., . , , . ,, , ., „ ,„ ,. ,_,.,, ,, x noise from the tyre/ road , engine, the car and its sound quality, lh er e-the passe ngers on the rear seat must . , x , ,, , r x

r , , ;1 n , wind, etc., and lore several car ma nuf ac tur ers aresit uncomfortably near the front seat , , , . i n , - mi , , . , , . , . , . , , . , ,.„ ,, . , . x .„ jn b. rever bera tion and reflec tions. I he eval uatin g the possibility of using theif they wish to com mun ica te with the , , „ x. . „ A „™T , . * , „ , , • , ■i . T-I ,i r , r reve rber atio n and reflect ions in- R A S H value inst ead ot the subjectivedriver, fur the r, car man ufa ctu rer s ol- . , , , ,. , , , ,

, , jn „ , „ side a car dep end upo n the inter ior evalu ation.ten eval uate how the car sou nds so , . . ,, , „ , ,,n f . . x x , c , tri m, i.e. the type of carp et, thetha t, for insta nce , the frequen cy char- , ,, „ ,. . , ' T .. ,± , . . J ... ., . ,. ,. x , . • ■ , i i seat s, the roof lining and all re- fn conn ecti on with speec h mtell iei-acter istic ot the noise is just how the ™ x. P , .,._, n ,. ,. x . ,, , , °,., .Jf fleeting surfaces . bihty and radio listenin g the prob lemcust omer likes it! . x , . x , , . . ,

is to design the car cabin in such a wayBoth these phen omen a are auto mat - tha t the noise is att enu ate d as much

ically tak en into acco unt when carr y- as possible, wher eas the speec h levelThe RASTI Method ing out the me asu re me nts and no cor- rem ain s as high as possible. Ther efore

recti ons have to be applie d. Anothe r the speech soun d ough t to be reflecte dRAS TI has been developed from ex- advantage is th at mea sure ment s are round to the different seats in the cab-

pe ri men ts on so un d tr ans mis si on sys - pe rf or me d wi th bo th th e te st signal in, wh er ea s th e noise sh ou ld be ab -tems using Modul ation Transfer and backgro und noise prese nt at the sorbed. By mean s of the RAS TI meth-Fun cti on ( MT F) ana lysis, [3, 4]. Th e sam e time so th at the signal and the od it is easy to eval uate the effect of amet hod is sta nda rdi zed by IEC [5]. By noise do not have to be mea sur ed sep- change in the cabin design.mea ns of the RA ST I me tho d it is pos- arately. The RA ST I value is derived

sible to mea sur e speech intelligibi lity from M TF meas ure me nts in 500 Hz In the next sectio n, a few me asu re-on a scale from 0 to 1, see Fig. 1. It is and 2000 Hz octave ban ds. men ts are me ntio ned where speec h in-

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telligibility between the seat next to asphal t and with no wind. The rpm (2) with a thic knes s of 25 mm feltthe driver and the rest of the seats in was const ant during the meas ureme nt. placed on the ceiling andthe car is evaluated. The idea is not to (3) with felt placed on the lower par tshow final results for the car design R e s u l t s a n d d i s c u s s i o n of the four doors and on the floor.

bu t ju st to show how it is possible to In Fig. 2 t he re su lt s for th e diffe rentwork with the RAS TI method. positions in the stati onary car are By making exper iment s like these it

shown. is possible to optimize the att enu ati onA _ of the noise and reflect the speech, soM e a s u r e m e n t s The difference in RASTI-value is th at the speech intell igibil ity is as high

only small between the posit ions for as possible.The mea surem ents described below 1000 and 2000 rpm, whereas it is much

were perfo rmed a t Honda Research grea ter for higher rpm s, especiall y for Fig. 5 shows resu lts from the cruis-and Deve lopment, Tochigi Center, Ja- 3000 rpm . The best resu lts were ob- ing car. The car was mou nte d with pan. ta in ed for DS left because th is is th e normal radial ty res and was driven

measurement, point neare st to t he with the auto mati c transm ission in po-M e a s u r e m e n t C o n d i t i o n s tra nsmitt er. DS is at the righ t side of sition D-2 on a dry road.

The meas urem ents were performed the car.in a sta nda rd passenger car, a Hond a It can be seen tha t the speech intel-Accord, 2.0 Si, using the Briiel & Kjasr Fig. 3 shows the result s from two ligibility is much bett er between th eType 3361 Speech Tra nsmi ssio n Me- posit ions at the rear seat for each of driver and the front passenge r com-ter. The tra nsmi tte r was placed on the the two octave bands , 500 Hz and pared to the rear-sea t passengers.front passenger seat pointing towards 2000 Hz. The result s are bett er (higherthe windscre en and at a heig ht level RASTI-va lues ) in the 500 Hz band forwith the passenger 's head . Thi s posi- 3000 and 4000 rpm for RS left probation was used during all the measur e- bly because of the directivity patte rn C / O n C i U S l O I lments . of the tran smit ter, which is equal to

the directivit y of a hum an being. The meas urem ent s performed haveTh e receiver was plac ed in five dif- shown th at it is possible to eval uate

ferent posit ions, 0,20 m to the left of The resul ts from the driver 's seat the acoust ics in a car cabin bv mea nsthe driver's ears (DS, left) and 0,2 m (DS, right) show much higher RAS TI- of the RAS TI method . By experi ment-to the right (DS, righ t) and on the left, values for th e 2000 Hz band th an ing with abso rpti on mate ria l and itscent er and right rear seat s (RS left, those for the 500 Hz band, per hap s posit ioning the speech intelli gibilit yRS center and RS right). Measure- due to reflections from the windscreen can be optimized.ments were performed in a stati onary (result not shown in the figures).car at different rpms and in a cruisingcar at different spe eds. The car was Fig. 4 shows an example of mea sure-mount ed with studless tyres and driv- ments made with different kinds ofen with the automati c transm ission in absorption in the cabin. All the results

position D- 2. Th e me as ur em en ts in are from positi on DS , righ t inthe cruising car were perfo rmed on dry (1) the standar d cabin,

Fig. 2. RASTI results from measurements in a stationary car Fig. 3. RASTI results for the rear seats from Fig. 2 shown separately for 500 Hz and 2000 Hz octave bands

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•j 1 , , , , 1— RP M -| , , , , , ,1000 2000 3000 4000 5000 6000 40 53 70 90 107 km/h

870442 880128,

Fig. 4. Results with different kinds of absorption in the cabin for DS Fig. 5. RASTI results from measurements in a cruising car

right and only for 500 Hz

References[1] ANDERSON, J. & JACQBSEN, T.:

"RASTI Measurements in St.Paul's Cathedral, London", Application Note Briiel & Kjser, Denmark 1985

[2] HORR ALL , T.R. & JACOB-SEN, T.: "RASTI measurements:

Demonstration of different applications", Applica tion Note Briiel& Kjaer, Denmark 1985

[3] HOUTGAST, T. & ST EE NE -KEN , H.J.M.: "A review of the

MTF concept in Room Acousticsand its use for estimating speechintelligibility in auditoria", J.Acoust. Soc. Amer. 77(3),1069-1077 (1985)

[4] HOUTGAST, T. & ST EE NE -KEN , H.J.M.: " The modulation fig. 6. The transmitter (Type 4225) of the Speech Transmission Meter placed at the fronttransfer function in Room Acous- passenger seattics and RASTI: A tool for evaluating auditoria", Technical Review,Briiel & Kjasr, 1985 (3)

[5] Draft IEC publication 268: SoundSystem Equipment - Part 16: "Re

port on the RASTI method for theobjective rating of speech intelligibility in auditoria"

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Rasti Method in Cars