, RESEARCH DEPARTMENT

COMPOSITE CATHODE RAY OSCILLOGRAPH DISPLAYS OF ACOUSTIC PHENOMENA. AND TEEIR INTERPRETATION

Investi~tion/by:

. C.L.S. Gilf'ord A.L. Newman T. Somerville

Instrumentation by:

D.G. Beadl. C.G. 1hyo . A.L. Newnan

Report by:

C. L. S. Gilford T. Somerville

Report No~ B.046 Serial No. 1951/13

L-____________ ~ ________________________ _

/~'/"-:;:." /r,~<t,~ ---.-_ ..... , (w. proot-;~mlson)

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•

PR IVATE AND CONFIDENT IAL

Researoh Department September, 1951

1. ,

SUMMARY

Figs. Nos. B~046.l to B.046.l2

COlilPOSITE CATHODE RAY OSCILLOGRAPH DISPlAYS OF'A"BOUSTIC PllEHOfiJENA. j~liiD TJlEIR nr,rERPRET1..TION . -_. ----------~

Since its perfection in 1935 the high-speed level reoorder has. been used. universo..lly for nxmy kinds -of o..coustic meo..surements and has been highly satisfo..otory in most respects. It has h9Viever certain limitations and disadvantages, and it has now been largely superseded in the B.B.C. by direct;..reo..ding logarith-mic displays on a cathode ray oscillogro..ph. This report deals with the application of new methods, developed in Researoh Department, to reverbero..tion time measurement and to the study of struoturo..l vibrations, ana desoribes a new teol~ique in which the behaviour' of the studio over, the desired frequenoy gamut is obto..ined o..s a permo..nent photographio record.

INTRODUCTION

, The invention of the high-speed sound-level reoorder and its development o..s a lo..boratory instrurnentby1feurnn.rm in 1935 rendered previous techniques in architeotural o..ooustic moasurements out';'of­do..te, Hitherto, the study of transieITc effects ino..uditoria had been limited to the laborious determin~tion of separato points on the time-level ourvo or to photographic recording of dooo..y ourvos by the use of a Duddoll oscillograph. The o..bility to CXD..mine deoay curves on a logo..rithmio soalo, even with reverbero..tion times o..s short o..s a fro..ction of a second, c\lthough now common-plo..ce, ho..d not been roo..1ised, o..nd the fine structure of reverbero..tion curves could not be observed.

The high-speod recorder is still the instrument used for the mo..jority of routine measurementsj but it has certain limitations, some of which co..rmot be overcome. In the first plo..ce the m:x:imum writing speed of recorders of this kind is limited. The Neumo..nn Pegelschreibcr which uses mo..gnetically operated friction clutches ' re.s 0.. mo..xinrum "writing speed of 300 db/sec. for 0.. 50 .db soo..le l'ridth. The Brllel o..ndKjo..er recorder(l) recently introduced o..chieves 0.. figure of 1000 db/sec. by the use of direct olectromo..gnetio forces on the reoording pen, in place of' a:n electromeoho..nicnl servo syst~m, but "chis systom introduces oonsiderable extro.. "lileight in the form of permo..nont Inagnets. Moroover in o..ny servo system in whioh displace­nwnt is proportiono..l to oontrol, hunting can be o..voided only by good design o..nd co..reful mo..intemnoo. . ' ,

There are disadvantages also in .the .fcl.ct that the apparatus produces records which require s(lbsoqllel1t o.l"1r.lysis. In m..'l.ny

· applications the ttllD.lysis of records crently incroo.ses the overall time of measuremenb. The porrrnnoncy of -(;110 l'iI!'ittenrocord, though occasiorolly an o.dvan:bage.t is seldom. nocesset.ry for routine moasuro­ments~ ';

The system to bo described, in~hich cathode ray tube displays are used, was produced 1i'rith the .object of overcoming the disadvuntages and limiGet.tions of electromechanical.recording instrumonts.

2 • REVERBEru~T ION NIEASUrIEME1TT

2.1 'History of Direct Revorberation li[oasurenlent . '------------_.. ~--~

The first attempt in tho B.B.O. to de:vilJe a means of giving a direct roading of reV9rQeration time 1VCl.S made by O.G. l.lhyo of this 'Departmont in 1946,2). The principle of his IIHoverboration Meter" was to integrate the sound level against time during ·the

· whole period of tho decay, and to compare this intogral with the initial steady-state. This devico did not fulfil expectations sinco the steady-stD.te ievel deponds upon both the direct and reverberant sounds,livhereas fora correct measurement the rever-berant sound only should be taken into account. Tho meter 'WUs therefore iroccUrD/co unless the microphono V!/D.S romo'Go from the sound sourco,' and an improv9d syS"COXllvVD.S tried. In it the intogral over a'short period of time aTtor the cessation of the source 'Wf).S compared with the integral over the ren1D.inder oftho decay. . The o.ppo.ro.tus lNUS' however -ru.thor complicated and' heavy and was sensitive to supply voltagefluctua.tions. It also had, in common with the rehy mothods cu;rront in the oarly 30' s, the ~imitation thD.t only tho deco.y as 0. whole could be examined con-voniently,. and not in fine structure. Other methods of direct moosui'omont wore therefore cixplored.

Thore aro tvV9 alterr> ... f1tive possibilities of using displays · on a co.thodo ray tvbe. . An increasing gain amplifier hD.s boon doscribed by To.k(3) in which an expoYlOntialdecay may bci displayed as a horizontal lino by suito.ble adjustment ofa time-cons1;.'1.nt. The timo-constcmt setting required to nchievo this' io a measure of thQ reverboration time, but o1nco tho disptay hao'to bo adjus­ted to correspond with a fixod roferonce direction, measurement .

'becomes a IDo.tter· 'of trial and' orror. .

The second possibility is thedevolopment 0:1:' 0. logarithmic amplifier 1 giving 0. rectified output, so tb.n.t 0. trace idontical in form with that produced l)y the high-spood lovol rocorder could bo obto.inod on tho co.thode~r[\Y tube. In this method 0. fixod trace can be c ompar od' for purposes of roverboro.tion meo.suremont '\frith an o.djurdio.b10 referenoe slopa, and tllO use of 0. porsistont oso illogro.p·h screon onn bIos tI10 roforonoo lino to bo set ·in ono operation. This method obviously shows promil3o, sinoe no trial and orror isnecesso.ry, and it VJUS therofore follovfod up.

The apparatus for prqducing the requirod log;o.rithm~~ ~ts~lo.y has boen ~oscribec1 in detail in papors by Mnyo and Boo.dle 4 5. Fig. 1 shovrs the oxpcrimonto.lo.ppo.rntus as used for rcverbero.ti\on moo.euro·rnent and' Fig. 2 the oxporimento.l chain. 'l'ho soUrce of sound is 0. loudspeo.kor radiating pulses of l"ro.rbled tone. Tho pulses are usually compo.rQ.blo in length with tho rovorEoro.tion timo, but wide vurio.tions in longth 0.1'0 permissible vvithout o.ffocting the rosult. Other sources such o.e "vrhito noiso" and pistol shots ho.voboen co.rofully inveGtif';f.\ted, s:i.nco thoy o.ro fo.voured for reo.sons of simplioity by Continenml workors and by tho Building Rosoo.rch Sto.tion, bu:b thoy givo results identical with the wnrblo tono reslil{;s, if, nnd only if, they lire used in conjunction with· bo.nd··po.ss filters 1/3 octave vr.i.do or loss. Tho vmrble toneeourco gives equal'results "Frith oither octr.vo or 1/3 octa.vo filters, f1.ucl has .boon found to g;ivo bd;-cor corrola.tion with subjocJcbre assessmonts· -{:;bnn other mothods using octavo· filter s.

Microphone signals arc pa.ssed to tho logarithmic r.mplifior whero, after linoa.r preamplificntion, they 0.1'0 po.ssod thi'oug!J. octo.ve filters connectod botrroon the proo.mplifiero.nd logarithmic sJcages. Tho d.~. outpub of tho latter is applied to the "y" pla.tes of an osc illoscopo throu/!,h a. direct coupled amplifier incorporo.ted in it. The tlXlI·plo.tos provido 0. time-h.'1.se vrith variable speed, triggerod from the finish of the pulse of tono.

A goniomotor, of which Fif~.·3 is a pho"togro.ph, is attached by bayonot fa.stening to the front of the oscilloscopo. A gro.ticulo~ on which o.re oD.gro.vod [~ 'sories of po.rallol reform'lOe linos, is aligned vdth tho tro.co by turning tho slow motion 1010b K, o.nd the reverboration t:ime road diroctly from tho 8co.lo . appropriate to tIle) cD,libro.Jcion.· Throoor four succossive pulses are normo.lly sufficient :Cor o.n individuo.l reverbero.tion time . detormina ti on.

I

Reverbera.tion -Gimes below 0.1 soo. ha.ve 1)oon moa.surod by this a.ppa.ra. tUB. The wr :i. Ging spoed :mn~l bo 'VT,r :led over very wide limits by a. switoh, a.nd it is posoi!)lo to IDen.suro rovorbora.tion times in sm-.. ll scalo models provic1(id due rogard is paid:bo tho .lower frequenoy limit,

2.3 . . , .

The ovora.llca.libro.tion o.nd law of the system is, checked by moons of -Ghe "Dooo.y Ca.libra.tor tl doser ibod by Mo.yo a.nd Bea.dlo (loc. cito). 'rhis instrument provides an a.eouro.toly exponontia.l doca.y a.t 0. suito.blo lovel to be a.ppliod to the logay-.i.thmic o.mplifior. input. If the cbnin is working oorrectly tho osoilloscope tra.o() will be a. stro.ight line 1 the slope of whioh IIlny be oh.'l..D.ged indepon­dently by o.ltering the deorement of the origimtl decay a.nd tho time base speed. Tho docrement, which is a.djusta.ble by a. stud switch to fixed va.luos corresponding to reverbera.tion timos of 0.2, 0.4, 1.0 and 2.0 socs., is so·t; to the mlue judged most suito.blo for the room o.nd the gonionlO-1:i oris set to rea.d thosa.roo fig1..U'o on ono of· tho sco.les. The time roso. is then sd; to bring tho slope of the line pa.ra.llel to the gra.ticule line-s, thus oompleting the ca.libra.tion.

An o.lterno.tivo method of ca.libra.tion which is loss convoniont ~ut co.pa. ble of givine; better o.ccura.cy :rno.kes use of tho frtctthat the tone pUlsar is do ~'lignod to pr ov:i.de o.c quro. te pulse lengths from 1/10 seco_ to 2 secs. A rocta.ngle, ABeD in Fig, 3, is :mnrked centro.lly on the goniomoter· gro.-biculo, a.nd to ca.libra."l:io, "che logo.­rithmic amplifier output a.nd tho oscilloscope timo rose spood a.re independently a.djusted so that (a.) the insertion of a. 50 db nttenm­tion moves "che trace produced by a. ste[~dy -bono from AB to DC I a.nd (b) the length of, sny, the 1 second pulse is oqua.l to AB. The reverbero.tion tiElo mo.y then be reo.d directly from the second scala of the goniometer. Other pulse lengths are used in ca.libro.ting to the other sca.les.

3. THE PULSED GLIDE

3.1 Continuous Vo.ria.tion of It'rermency --.~-~--.------.----....--~:':' ...

In inves-t;igo.-billg tho a.coustics· of rooms it is nooossary to 'kn~ not only the vu.rio.tion ef slopo (),long the timo cleco.y curvo, but o.lso the 'VD.rio.tion of slopo n.s 0._ function of fro'quency. All :methods of moo.surelflmrb so -fttl' ho.vo boon rostrio{;od to tho meo.suromont of revol'boro.tton ·bhae o.t a. fov1 saloc"cod frequencies

·e

-'

or bo.nds, but it is clear .tho:t a complete panorama sh01ving the ororo.oteristios o.t 0.11 frequencies simulto.neously is most desirable. This ros been done for loudspeo.kers by Shorter(6), . but the methods npplioo. ble to loudspeo.kers are useles~in 1'001)].

o..oou~tios where the decay tilles extend to seoonds.

Sinoethis report VilUS started I!'. tJ. Loeu'VV'~:m (7) ros published o..n interes~ing 100thod of obto..ining continuous reverberation-frequency diagbvflS by the use of 0. level recOrder. In his method, however, the reverbero.tion at eo.ch frequency is represented by the length of 0.. vertico..l line and does not show the srope of the deoo..ycurveo

Attempts i'\fOre thereforo made to show 'o.mplit ude , time o.nd frequency on 0. quasi-threo-dimensional dio.gro..m.

It was first oonsidered trot this might be done by producing successive logarithmic decays on 0. co.thode ray oscilloscope o.t pregressively· increo.sing" frequencios and o.pplying incromerreo.l shirts to the X [',no. Y uxes o.s oach pulse was radio.ted into the room, 0. composite picture being obtained photographically. ' Some experiments wero o.lso coxlducted with a skhtrolfJE todisp'lay the composite picturo, but ·practical diffi­culties eventually led to the adoption of a simpler system. Suocossive tracos on the face of the oathode ro.y tuba aro photo­graphed on a film which is moved by 0. synchronous motor at a speed of 1 mm. per sbo. in the experimental camera shov{U in Fig. 4. Referenoe frequencies o.re marked o.t intervals by

. pressing 0. push-butJeon which brightens the trace for the duration of 0. single pulse.·

Sections from exo.mplos of the photographic records thus. obtained o.re shovrn in Fig. 5. .The level sco.le is logo.rithmio (increo.sing level upwurds), o.nd tho -!:;imc sco.le is linoo.r ~ from loft to right. The frequency increases from oach deco.y curvo to the ono on its right, and as 0. result of the logarithmio soo.le of the tone source, equul distances along the length of the record represent equulfrequency ro.tios. The frequenoies of the brightened mo.rkers o.re indico.ted by the figuros in the margin. These records have beon named nPulsed Glide Displays".

ii

,A spec io.i type of cathode ray tube with an aftorglow time of tho ordor of minutos.

- 6 -

It shQuld be sai~.at the outset that there are many difficul­ties in the detailed interpretation of the pulsed glide displays. The information in these displays is as it were coded, and requires 'elucidation b"th qualitatively and, to some degro;s quantitatively. The families ef displays :make u.p complete patterns, which can 'be regarded as characJc;oristic both of the overall be,haviour of a studio,

,and also as, giving 0. coded pictorial representation of the individual behaviour of the studio under test in spocific frequency damn ins. The fine structure of thoso patterns can be to some extont correln:bod with' subjec-tive effects observed by the listener, and this is proving of great value. An account will bo given here of the progress so

.. far nnde in interpreting such patterns.

3.2 General Characteristics oftho Pulsed Glido Disp1D..ys

The appearanco of the display varies bot\vcen that of Figs. 5(a) in which the decay curves change slowly with frequency giving rise to horizontal "structuros tl

, and Fig. 5(b) in which tho complete changes take place be'bween ono pulse and the ,next. Fig, 5(a) is characte-ristic of small rooms at low frequencios, and 5(b) of large spaces at high frequoncies~ This trend is to bo expected from a conside-ration of the eigentone spacing, the mean value of which varios inversely as the squnre of the froquency and inversely as the volume of the room. At the low'0st frequoncies in a s:rna.ll talks

. stUdio there may be many pulses in the froquency oo.nd betwoen t·vro succoss ive eigentoncs, and consequently the decn.y curve changes slov,rly. A rnathemo:tical study of tho changes in the decn.y curve in the frequency region of separated eigontonos is in progros s.

Confining attention first of all to the low frequency part o~ the display, another important general characteristic is observod. Many of the horizontal features are determined 'by tho position of the microphone, and change completely as the position of the microphone is chn.nged. Suchfen.tures n.rc described as "adventitious il

• others:rrny bo'observed in more than ono micro­phone position,perhaps even all positions, indicat,ing that thoy are "intrinsic" features of the acoustics of thoroom.Fig's. 6(0.) and6(b) which are displays from tvro microphol1o positions in tho same room illustro.te a :number of adventitious i'eatures, and an intrinsic foatur.o at 100 c/s.

3.3 Lov~~oquency Formations

, The processes of build-up and docay of sound i~ 0. room htwo boon descr ibod in an extron10ly useful 'form by 1ilayo (8). Briofly the build-up 1r,~th n. ster..dy source lllc'l.y be represented as the successive ac1di·tion of an in.finito series of d~minishing voctors, random in phase n.t first; but eventunlly' de~enorn.ting into an

.J'

"0

- 7 -

equiangular spiral which roprosont,s the eit~Qntone regimo. (See the sketch in Fig. ,7 (a)). Wlion oche sourco is removed the process is roversed, the sJeoady sta'bo lovol OA taking nG'V; values OB, OC etc. in sllccession. This pictnro facilita.tos the exp1o.mtion of many of the foa:c,lros, paroc:.cularly tho adventitious ones in tho 10w-freq'~~oncy disIlb,ys9 ,

3.3.1 Advontitious J?oaturos

The fluotuations in the upper pad; of each decay curvo represont the random 0 choes from nearby surfaces whereas th6 lower parts:roprosent the regular eigentono po. t'cerns. It ' might be thought that tho carlier echoos would not bo very spnsitive to frequency_ A sil11.ilar diffic'ult:J ,,'VUs noted by I\fuson and lIoir (9), who sought Jeo exp1o.in the frequency variation of short pulso picture s by reference to possiblo changes in the polar 'diagram of °Ghe loudspeaker. Reference to Fig. 7 (a) aGain shows this to he incorrect, h~ccmse the sound levol corrosponding to any point X on the diagram is determinod by OX; any slight cPJlnge of froquency will cause a c~nge in tho angle of. the spiral and honco an appreciable change in lovel.

During tho docay the disappearanc e of the direct sound and the first fow ochoes m.'l.y cause et decroaso in lovel (as in D'ig. 7(b))1 or an incroD,sG as in Fig. 7(C)1 with the rosuH that the decay curvo yrill Sho-I"f an il1.iticcl f[\11 or rise (Figs. 'led) . and (0»). The exto;1:!; and direction of this inHjial changol for any part icular microphone position, var ios with frequency and is a maximum 'or minimum vrhen the direct and reverberant compononts

,are in or out of phase respectively. Examples of tho formD..-tions producoa in this iNUY are shown in Fig. 8(£1,) at, tho points A and B. .

Fig. 8 (b) shoW's the anti-phase caso [\rtificial1y producod by pas~Jing tone pulses through tho eloctric circuit, shown in the insot, to the logarithmic amplifier and cathode ray oscilloscope.

The uppor envelope of the curve thus ropresents the steadY,-state leyol or the roverberant sound levol, whichover is the groo..tor,tho'steady •. state curve itself appearing as 0.

recogniso.blo line which normally coincides with the envelopo, but occasio:t1.JJ.lJ.y drops belo,r it as at B iriFig. 8(0.). Tho frequencies of m,aximum steady-state level normally represont eigontono frequencies.. . Thore is 110 change of slope of the curves associL1:cod vr:i.th the eigontone frequoncies provided tho.t tho room is fairly uniform in [Jound-absorbing propex-bies.

,I '.

Fig. 8 (0) is a disp lay 0 ben. inod fl' om n. micr ophono in ono end of an undamped tube 4 :rc. long, at the other cnd of "\I.-hioh is a loudspeaker. Tho maxira..",corrospondin;o.; to the eigenJeono froquen-' c ies and the unif?rmi-by of slope are illuGtratod.

An extension oftho use of simplo models. to build up s.uch idealised diagrams is proposod, and further '\Iror1: will be carriod out on this in duo course.

3,3.2, Intrins~c. Features a_~_LoW'!.r.os.u(mcies

Three types of intrinsic feature have been recognised. The first consists of a sories of violent fluotuatio~ls in slope illustrated .in Fig. 9(a). This is probably due to ,non-uniform damping ·of differont eigontones. tt has not beon correlated with any subjective effect. The seoond is a phenomenon occur-ring usually woll do'\,vn in oche decay curves, consisting of a SUdden disturbance reo.ching a IlJ.D.ximum evb' a partioulc,r frequency. This is almost inw.riably associated vvith an audible colouration on spoech. Fig. 9 (b) shows such a foo. ture occurring in the display from a talks studio,'whilo for comparison Fig. 9(0) is of the dead-room at Nightingale Square with a bloclc of six Helmholtz rosonators prosent. Examplos ofcorrohtion with subjoctive offects ydll be given in "cho nO:h.-G secti6:1.

The third intdnsic featurc consists of all 0:2m1inl~ out of the curves at a particular frequoncy as shoiim. in Fig.' 9(d) •

. This has often been found to coincido vrith a structural reSo­nanoe, -vihich in soma cases has causod audible colouro.tion.

During the last few months pulsed gli,des Mve boen carried out in 0. number of studios in the London area vviththe object of estab1ish~ng a correspondenco between the appearance of tho displays'and the quality of the studio. Several instances of the intrinsic features described above were found and are illustrated in Figs. 10 (a)' to ;t.0(e).

Egton House 3F (Fig. ,10(a)) MS a colouration 011 speech and a ring on pulsing in the reglon of 175 c/s., 'The cause of the 'colouration is not knoYm,and the indiCAtion in Fig. 10(0.) is not voryrnarkod. Bush House Playback Room (Fig. 10(b)), has n colouration duo to floor vibration, which c,ppcarsas an opening-out of tho display YfhaJGevor 'bhe microphono position. Grafton Thet:J.tre narratort,s studio has o. colourJ:bion in the regiol?- of 1,70 c/s due to radiators in the room. Fig. 10(c)

'.

..

shovis this region. The experimontal talks studio in Nightingale Square (Fig. 10(d)) has a serious colouratj.on at 115 c/s. A Svr.i.ss studio, Bernei 7, is also includod (FiGt, 10(e}) as a sample of serious 6010uration o..rising from the use of e:1."Panded mot~l in the oeilingt,

These examples are fairly typioal of tho findings in reoent surveys.' Pattern s'cructures such as those shown o..re alvvays assooiated with audible coiourations though the oonve~se ~s not so invario..bly true. . .

3.5 High-Freg~enoy Formations

The rapidly-varying nature of the patterns at high frequenoies and in large auditoria makes it impossible to examine the. trends over small frequency bands.' There a.re no olear '!horizontal" struotures. Consequently, a.san initial step we oan only try to apply criteria based on the' appearance or o'bher properi~,Y of the r.ecotd as a whole. Generally speaking any tendenoy to for.m repee..-'ting patterns lTrill indicate a discrete eigentone, and all modern wave acoustics starts from the assumpt~on that this is an undesi­rable condition. . The absence of such a tendenoy will bo indicated by the maximum degree of randomness in the pattern.· 'An attempt to

. represen-e the Q,egree of randomness in a semi-cpmntitative form 1/IIO.S

made in the follmving vmy. .short samples wore seleoted from available glide records varying from the hiijhly-ordored type suoh as Fig. 5(a) Jc;o the random type representod in Fiij. 5(b). From these samples, ton were seleoted, providing an arbitrarily ohosen (by visual jUdgment) series of gradations from one extreme to 'che other. These 'were· numbered from 1 to 10 and assembled together,' as a "sample oardl!. Tenglides, taken in studios Vihose' subjective order of merit had already been decided, were then each divided into ten eqbal frequenoy bands. Each of the bands was assigned an index of randomness by comparison with the sample oard and these· indices Were plotted for each studio as a function of the mid-band frequency. The graphs thus obtained were compared \vith the sub-jective judgments.

The assignment of a subjectivG rank is full of pitfalls for reasons which will be discu~sed in seotion 5 belmv, but for this purpose the assessmontwas the best possible assessment of genoral qua.;tity excluding isolated peculiarities ouoh as colourations. The graphs of, randonmess against frequoncy showed theexpeoted increaso of randomness with frequency and size of a.uditorium. They also show a rough correlation betweon randomnes's and sub­jective,merit, but it vvas not sUfficie:ltly procise for 'practioal purposes.

A comparison '\i.-us also mad,o betwoo:Cl the dj.ffcredj studios at corresponding froq11enc :LOG ch9gon. to [;i ve /dlO samo value of the non­dimensional constall·1.:; p. = fVl, v MgiVOJl by DoH o.:J.d;(oop(lO). Tablo

-c·~ ..-·(i) below,compariIlg; su1)joo·tivo assesGYiLc::;.ts with ·rai:1dom .. '1oGs indox at cor:r:osp~mdi;ng frequonc:i.os, Sfi(J,YS tho degroo of correlation obtainod"

,

Tablo (1) ·Corrolq.tion betvroon P.o.ncl.oFDJ.oSS Index and Qlla1iJGY: , -.--.......... -- . ,-..... ..-.......--..... ,-----...-.-~---.-.-..---.. ~.-.- . ~

studio

A

13

C

D

E

J?

G

Vol. V

(m3 )

1410

3400

2210

327

83

3400

47

11.3

15,0

13.2

7.0

4.3

15.0'

4.0

----------

80

60

70

125

210

60

225

6

10

9

6.,1· 2

6

8

3

Order of Subjectiv~ ' -~ I\I~rif -- -

6

1

4

3

7

2

5

---.. --.----~~---"- .. ~----

It is c 100.1'. from 0. COlnrxl,rison of the· last t-v{O columna that although the bottor studios have, on the 'whole, tl~e higher random­ness ~lldices thoro is no·1.:; camp lote agrooment bety{Oon the order a in the tyro co1Ullllls. '1'ho dogreo of agrooElont mn:y. bo ropresented matho:m.o..tico.11y us 0.. llcorrelo.tioll coef'i'iciontn(ll) caloulatod from the tV{osots of firruros. The correlation oooffio iont is zero if v . .

thoro is no o.greomont, and 1 for cor,lploteagroemontwith a .positive sign if the highest figuros corrospond and a n~gative sign if tho highest figure in one column corrosponds with tho lowdst in the other. ·A coefficient groater than 0.5 is genero.1).y takon.to .indi-oate 0. significant agreemont.

3.5 Br:ij;,htnes_s Va:r.iation Dis;elo..ys

The output of tho logarithmio, amplifior mD.y altorl1rdiivoly, be o.ppliod to tho [;I'id of '!:;hooscilloscopo, thUG varyj.n:,; tho brigh:!:;rioss of tho trace in proportion 'GO the lo(;arith,n of 'bho sound lovel. The Y-dofloction plat9s arc disconnected and tho co.mcl'o .. rotaJced through a suitablo cmglo bo-t;wocm 45° and ~)Qo so that 'bhe X-swoep runs aoross tho film,

~ f = frequency, V = volume of room, C = volocityof sound.

--

... 11 -

11ho .pho·bogra?hic rocord thon C021sists of a sorios of parallel lines, p.s shm-m in Fi0;. ll~ oac11 lirco diminishinfi in brightness along its long'bh as tho sound lovel f:\l1s. This form of display is sirJ.pIorthc1.Il 'bhat doscribod ab·ovo, intonsity being represented as brif,htness on a pInna vihoGO n.xes are fro­quency and Jeino I in 'bhe IIl( 1)mor of the visiblo spoech pattorns of Potter and Stoinborg(12. Very careful control ,of photo­graphic exposnro and process ing is nocossary if r6lia ble

. results 0.1'0 to bo obtained and hence it VlO.S decidod that the Y-di3flocJeion displays should receive first [l·t;tantion. The possibilities of·the brightnoss-variD:tion method have conse­quontly not yet been explored.

4. STRUCTURAL llliS01TAFCES

Sorious colouration can bo caused by struc-burul resomncos, and their presence is USUD.lly indicated in tho pulsod glido dis ..

'plays as described above. Tho HSDf,l cause is a building struc .. t1.1I'e which produces a strong resonanco at ono frequency by virJeue of having uniform properties, O.[je eqUi1lly spaced atmch.l2len'Gs over a . largo n,rea. It can usu.ally be provo:otod by the inJero .. duction of damping or irroguhri-Gy in the design •

. . In detocting such 1'0 sonances a nU):;11)or of :md;hodscan be employed.

(a) It is p08siblo to uso u vibrn.tion pick-up in contac'b with the structure to neasure the frequency of resonance, using a loudspeakor in JGho room as the' sound source. Thero arc D.

number of such pick-ups. on tho mo.rkot, but none of the!fl have proved 'co be successful. in the o.coustio field 0 Gramophone pick-ups havo also 'boon modified J but without succoss. . A moving-coil microphone,-rith a thin conical paper conto,cting point has boon developed and used with reasonable success. Fig. 12 is·a photograph of this dovice mounted on a, lead-screw which is used to adjust its position rolative to the surface undor investigation •

. With. this contact microphone it is possible; to eA"plore the frequency responso of a punel and tOlllOU sure the decay tines inexactly the same TIUll-1J.Gr as the reverberation tiI""o of tho room J

using theoscilloscopo method describod in section 2'. When tho rovorbern.tion tinQ of tho pa:lol Groatly oxcoodc that-of tho room at tho samo froquency 1 serious colouration may bo, expocted.

The £\.cou.stic coupling botvvoon Jeho roon itself o.nd its com .. ponent surfaces YDD.y. cuuse difficulty in making moas'\U"ements on

- 12 ..

the 1attqr. In such cnsos o..ltornativCl El\Jthoc1s 0:[' drivo havo boon used. For examplo, tho surfaco n1.y bo' 0xcj:t;o(1 'u:/ :lloa,is of a simi~ lnr contact microphono, used as fl. vibro.,tor. For o-b11.o1' purposos -1:;he surface may be struck v.rith 0.. wallet, 1)s:i.!J.!; fiH;oX's,Jco soparaJeo tho output of, tho contact nicrophono into llo..::'.ds C1.n.d C1. nicrophono,,:,operntod triggor'to start tho oscilloscope s'iiOoP •.

(b) Another method which does not roquire spec:Lnl oquipment and is very offecti vo is tq 1is~~en oVGr 0.. microphonG v\rhilo various parts of tho s't;ructure arc tappod with 0.. drum s·tick or r;lo..ll()Je vith soft covering. Bosom,ncos can thon be hourd very oasily and c,omparod -vdth 0.. tono source to dotermine the pitch. A refinoment of thi's method is to usoa selective anmlifior 1vhich whon ;l:;unoo. to 0.. fro­quency of reSOIJD.nCO om.blos an ~xac·t; dotormimtion of pitch to be :rnnde. '

5. . SUBJECT I'VE ASSESSI,;IENT S - . One of tho main difi'icultios confronting tho 'workor in archi­

tectural acoustics is the difficulty of doscr:i,bing the subjective attributes ef [1.n auditoriuLl in sufficiently exact torns to render possible any sort of rank corrolo.'cion 1N~:bh 'physical mr;nsurm;lents. Very careful 'listening tas'cs nust therofore bo cnrriod out in parallel with all physical meo..nurOX;lOnGS, and in gonerr,1 in lmittors of proforenco spocin1 tosts involvhlg the public uust bo nado.

Fortunatoly thoro [~ro Govero..l subjectiv~ critoria vihich :rriny be described accurately enough to moan procisoly 'che same thing to all intorostedpooploo Examples of 'chose are lIJiveness" or "deadness ll

, single-frequency "colournti ons U, "boom" or "toppiness". On the other hand some of the attributes of a concert m.11 are described in terms which do not seem to have a universal mea.ning, which may indeed be different aspects of the samothingor even mutUally exclusive. "Definitionll, "blend", "hardness", "singing. tonell,.etc. are examples.

5.~ The first of these two classes of attributes can be measured to a certain extent by roverbero.tion mothod s and the pulsed glido. Thoy nay be a.ssessed subjectively with substantial agroemBnt between observors. The best test hntorhls are speoch and music. Spoech i9 a transient phenomonon -which covers D. wido spootrum and has the advan-t;age that everybody knoTfS hovv it should sound. If the'speaker is knovrn Jeo the listenorG, ·Ghe test becomes very oritical indeed 0 Music on the o-chor hand covers a wide frequency range and very high intensitios are produced. The acoustio con-ditions in a studio 'oan be assessed fairly easily by Gxperienced

"

.. "

13 ..

listeners, but for those vvho are not expert the faultsoan be emphasised ,in the following way. A recording is made from the studio, replayed into the studio and ro .. reoordec1, the, process being repeaJced until the defoc'c becomes obvious by being 'multi­plied at each repeti Mon.

Golourations in speooh from a stUdio rrny bo examined 'with the help of a selective amplifier. Thp output of the mio!'o'" ' phone is fed to a normal amplifier as wol~as to the "se'leotive amp,lifier and their outputs mixed in sueh proportions that the aocontuation produoed by the selective amplifier, is soa:t()ely noticeable. The tuning dial of the latter is thon svropt over the whole frequency spectrum and any oolourations are hoard vory strongly" when they coincide with tho selective amplifier frequenoy •

. The frequencies thus measured rrny be oompared vvith the patterns on the pulsed glide displays. ' ;

5.2 The othor class of attributes is more difficult to investi­gate, par-cly,because thodescriptions given by one individwfl rrny not moan very much to another I and partly bocause one is hero reaching tho boundarybetvroon matters of fact and mattors of preference. The difficulty is 'most obvious when judgipg music studios £1.114 concert halls.' The reactions to th6 Royal FGstivul Hall haVG provided:;;' striking example in r:Gcetrbweeks. Most people agroe in saying that thG 'hall is less revorbero..:nt than it 'should be for the most pleas i:ng sensation, but there aro vridc disagreemonts on the question o'f do'finition. '1\hny people arc oonvinced that IIdefinition lt and lIolarityi! are outstandingly good, but there is an important minority vmich is equally convinced -that it is poor. Again "hardness of t on0 1l or Itsinging tone" rrny not. be independent qualities but rrny be, intir(lat.ely connected, with the reverbcratio'ntime or the shape of the oharacteristio. '

It is these attributes which a study of the high-frequoncy regions of the pulsed glide 1Jlight be expected to show,' sinco the former <?la.ss are already sufficiently reyeale9- by reverberation measurements and '1;;he low-frequency displays. An exact inter­pretation can therefore hardly be exp06toduntil the subjective

'qualities can be mor90xaotly described and. placed in some order of m~rit. '

The ordor of subjective assessment which is sho'wn in Table (i) 1I'I'as based on an overall judgment of qmlity, taking i~o account all the loss definable attributes. 'The fact tl11.l.t there is any appeo.rance of oorr'olation is therefore enoouro.ging but the possibility mus'b not be ruled ou"!:; that the oorrelation mdsts only because both 'sides of the compari'Son are correlated separately ,,-rHih a third, po'ssibly moo.surablo , p!'operty.

.. 14 ..

. ,

CONCLUSIONS

The equipmont doscribed 'in tb.:i.s report Qnablos tho recogn~.sed,~coustiQ to's·bs to bo or~rr:i,od. out more quiokly and oon'Y'en~ontly tlmn.by f'ormcr mo'HlOds. l-b also lnnkos possiblo mea sur om 0 nta , e.g. on'models li{hero much higher 'Writing speeds are needed than hitherto availablo and provides a new type of test in the pulsed glide display. The in~erpretation of these displays .is still o.t an elementary stago, but present progress is cmoouraging. The formations at low froquencios are most tractablo,but the interpretation of the high­frequency patterns is likely to be difficult duo to their random natUre and the laok ofagteemont on the subjective assessment of tho 'acoustic qmlitios with which thQY may be correlated.

REFERENCES

. (1) P. V. Br{\el & U. Ingard \lA New 1I1gh Speed Level H.e~ order n I J.AoS.A. 21 (1949) p. 91.

"

(2) 9.G. Mayo, RoE. Tanner, W. Vlharton British p[1:bent 652,384 •.

(3) W.Tak Philips Technical Review 8 .. (1948) pp. 82 .. 88.'

(4) .C .G. Nhyo ~ P .G. Beadle & W. WhD.rton ,"Equipment for Acoustic Pulse Meo. sur ement ll

1 Electronic Engineering (In publico.tion).

(5) C .G. Nhyo & DoG. Beo.dlo IIEquipmont for Acoustio Moasurement" Electronic Enginoering (In publication).

(6) D .E. L. Shorter "Loudspeaker Trn.nsient Rosponso", B.B. C. Quarterly, Vol. 1, Ho. 3, Octobor 1946,

(7) F. J. Leeuy{On "Een Automatisch werkond. Nn.gn.linmo'ettoesto1 t1 ,

Tijdschrift :van het Nederlo.nds Rldiogenootschap XVI (1st. Jan. 1961) pp_ 13 .. 36.

(8) C. G. ly1o.yo UStanding Wave Pa ttorn8 in Room Aooustic an J

Acustica (In publioation).

(9) CoA. ]/hson& J. Moir "Acoustios of,Cinemo. Anditor:L"\.n, I.E.E. Jourml, Vol. 88, Pt. HI,' Soptembor 1941, p. 183.

.. 15 ..

(10) R.U. Bolt and E.W. Hoop "Ji'roqlloIlCY Hosponse li'luc'buo.tions in. Rooms tl , J~A.S.Ao 22 (rhrch 1950) .pp •. '280 - 289.

(11) 'Yule & KOlldall llIntroduc-l:;ion to the Theory of sto.tfsticsl1

Third Ed. (Griffin, London) p. 209.

(12) R.K.' Po'eter, J.C. Steinoorg et. al. nVisible Speeohll Van Hostrand, 1949.

DIW

THIS PHOTOGRAPH IS THE PROPERTY OF THE BRITISH BROADCASTING CORPORATION AND MAY NOT BE REPRODUCED OR~ CLOSED TO A THIRD PARTY IN ANY WITHOUT THE WRmEN PERMISSION F THE CORPORATION.

FIG. I

APPARATUS USED FOR DIRECT ACOUSTIC MEASUREMENTS

rv o -• Cl> CJ)_V'I • C

VI m

ISSUE 1

20-8- 51

BBC

TONE SOURCE

WHITE:- i NOISE [J GENR. : ,

TONE PULSER

LOUD­SPEAKER

AMPLIFIER

r--- -- - ------.., , , , , STUDIO I I I

I I

MIC. I , 1

, I L... _____________ J

I I

: DECAY I CALlBR, I I

LOG AMPLIFIER

C.R.O.

1 TRIGGER

L' ~-------O\ ._----- :

FIG. 2 r

BLOCK SCHEMATIC DIAGRAM OF APPARATUS

COMPOSITE CATHODE-RAY OSCILLOGRAPH DISPLAYS OF ACOUSTIC PHENOMENA AND THEIR INTERPRETATION 12 SH EETS No, 2

ISSUE 1

- 8 - S1

K-_

FIG.3

GONIOMETER FOR REVERBERATION TIME MEASUREMENT

HODE-RAY ~_~.IL..L.~~ LAYS OF ACOUSTIC PHENOMENA

THEIR . INTERPRETATION

THIS PHOTOGRAPH IS THE THE BRITISH BROADCASTING CORPORA AND MAY NOT BE REPRODUCED OR CLOSED TO A THIRD PARTY IN ANY FORM WITHOUT THE WRITTEN PERMISSION OF THE CORPORATION.

FIG. 4

CAMERA FOR PULSED- GLIDE DISPLAYS

I\) o _ I VI

CD -~ , m ~

THIS PHOTOGRAPH IS THE PROPER~ THE BRITISH BROADCASTING CORPORA AND MAY NOT BE REPRODUCED OR IS-CLOSED TO A THIRD PARTY IN ANY FORM ~--------------i WITHOUT THE WRITTEN ' PERMISSION OF THE CORPORATION. .

NOTE:- BRIGHTENED TRACES ARE FREQUENCY MARKERS

I\)

o Vi . ~ '" (J) C I m

Ut

(0) SMALL STUDIO, LCNI FREQUENCY RANGE, SHOWING HORIZONTAL STRUCTURES

Cb) LARGE AUDITORIUM, HIGH FREQUENCY RANGE, SHOWING COMPLEX NATURE OF PICTURE

FIG.S TYPICAL SECTIONS FROM PULSED-GLIDE DISPLAYS

THIS PHOTOGRAPH IS THE PROPER THE BRITISH BROADCASTING CORPORA " I\ND MAY NOT BE REPRODUeED OR 015- 1-----------------; CLOSED TO A THIRD PARTY IN ANY FORM WITHOUT THE WRITTEN PERMISSION OF THE CORPORATION.

NOTE:- BRIGHTENED TRACES ARE FREQUENCY MARKERS

FIG.6 DISPLAYS FROM TWO MICROPHONE POSITIONS IN SAME ROOM

THESE ILLUSTRATE THE '<ADVENTlTIOUS" HORIZONTAL FEATURES WHICH CHANGE WITH MICROPHONE PO AND THE OUTLINED "INTRINSIC" FEATURE AT IOOc/s WHICH DOES NOT CHANGE WITH POSITIO

ISSUE 1

20-8-51 c

------A ---

F

. --~ ----- .~

-.... -

(q) SUMMATION OF SUCCESSIVE REFLECTION VECTORS

c~ 1\ -' A --.-' -----....... ~B D t

db

A

rIME

, , ,

~ ) DIRECT AND REVERBERANT SOUND (d) DECAY CURVE CORRESPONDING

"IN PHASE" TO (b)

\

'I I1 t A E B

11;1 !hJ

, A ,

\ \

\ , o

ll{-'t (c) DIRECT AND REVERBER.~NT SOUND

11 J "IN ANTI PHASE

~ J . It r.. FIG.7

\

TIME -40-"

(e) DECAY CURVE CORRESPONDING TO , (c)

.sSJ.la &.1 [I ILLUSTRATION OF THE BUILD-UP AND DECAY OF SOUND

CATHODE - RAY 0 SCll L OG RAPH~..,---.-r-r-~~~-=-------=:'=~ ISPLAYS OF ACOUSTIC PHENOMENA AND~~

THEIR INTERPRETATION

ISSUE 1

20-8 -51

BBC

NOTE:- BRIGHTENED TRACES ARE FREQUENCY MARKERS

(a) EXAMPLES OF PATTERNS PRODUCED BY ABRUPT CHANGES OF LEVEL ON CESSATION OF THE DIRECT SOUND

A. INITIAL FALL CAUSED BY COLLAPSE OF DIRECT SOUND . IN PHASE WITH REVERBERANT SOUND

B. INITIAL. RISE CAUSED &Y COLLAPSE OF DIRECT SOUND IN ANTIPHASE WITH REVERBERANT SOUND

C

INPUT R OUTPUT

(b) ANTIPHASE CASE PRODUCED ARTIFICIALLY BY ELECTRICAL CIRCU(T SHOWN IN INSET

(C) PATTERN PRODUCED BY EIGENTONE FREQUENCIES IN A FOUR FOOT CLOSED TUBE

FIG. 8 . .

PULSED GLIDES ILLUSTRATING CANCELLATION PHENOMENA AND EIGENTONES

COMPOSITE CATHOpE-RAY OSCILLOGRAPH DISPLAYS OF ACOUSTIC PHENOMENA AND T EIR INTERPRETATION

RESEARCH OEPT

ISSUE 1

20-8-51

".

BBC

NOTE:- BRIGHTENED TRACES ARE FREQUENCY MARKERS , ;

(a) GRADIENT FLUCTUATIONS. PROBABLY DUE TO NON-UNIFORM DAMPING

(c) FEATURE SIMILAR TO (b) PRODUCED BY HELMHOLTZ RESONATORS IN A DEAD ROOM. DOTTED CURVE SHOWS

THEORETICAL RESPONSE OF RESONATORS

FI .9

100 120 c/s

(b) CHANGE OF SLOPE ' AT LOW LEVEL (IN OUTUNED AREA) BUILDING UP TO A MAXIMUM AT A PARTICULAR FREQUENCY AND

NVARIABLY ASSOCIATED WITH AUDIBLE COLOU AT'ON

(d) OPENING-OUT OF CURVES DUE TO UNDAMPED RESONANCE IN SOME PART OF STRUCTURE OF ROOM

USUALLY GIVING RISE TO AUDIBLE COLOURATION

.. .. TYPICAL INTRIN .. IC FEATURES

COMPOSITE CATHODE-RAY OSCILLOGRAPH DISPLAYS OF ACOUSTIC PHENOMENA AND THEIR INTERPRETATION

RESEARCH OEPT

ISSUE 1

20-8-51

".

NOTE:- BRIGHTENED TRACES ARE FREQUENCY MARKERS

280 300 c/s

(0) STUDIO 3F, EGTON HOUSE (165-175 c/s). CAUSE UNKNOWN

(b)BUSH HOUSE PLAYBACK ROOM (255-260 c!s) DUE TO FLOOR VIBRATION

(c) GRAFTON THEATRE NARRATORS STUDIO (170c!s). DUE TO RADIATORS

(d)EXPERIMENTAL STUDIO,NIGHTINGALE SOUARE QI5c/s). DUE TO CEILING RESONANC E

(tl) BERNE STUDIO 7 (135 c/s). CAUSED BY EXPANDED METAL IN CEILING

FIG.IO PATTERNS (SHOWN OUTLINE ASSOCIATED WITH AUDIBLE COLOURATION

COMPOSITE CATHODE-RAY OSCILLOGRAPH DISPLAYS OF ACOUSTIC PHENOMENA AND THEIR INTERPRETATION

o

THIS PHOTOGRAPH IS THE PROPERTY OF THE BRITISH BROADCASTING CORPORAE N AND MAY NOT BE REPRODUCED 0 S- ~--------,:----------4 CLOSED TO A THIRD PARTY IN ANY WITHOUT THE WRITTEN PERMISSION OF THE CORPORATION.

100 1.10 120 130 140 150 160 170 180 190 C/S

NOTE:-BLACK TRACES ARE FREQUENCY MARKERS

FIG.11

PORTION OF BRIGHTNESS- MODULATED DISPLAY

~ -. '" Cl) - '" • C ~ m

THIS PHOTOGRAPH IS THE PROPERTY OF THE BRITISH BROADCASTING CORPORATION AND MAY NOT BE REPRODUCED OR C::lOSED TO -A THIRD PARTY IN ANY WITHOUT THE WRITTEN PERMISSION OF THE CORPORATION.

FIG. 12

CONTACT MICROPHONE FOR MEASURING STRUCTURAL VIBRATIONS

I\.)

o -• lit

(J) .... lit • C m ~