-.' . "',

T}lE DESIGn OF A WITH 111ONITORING

CAB IN!I: T FOR USE lOUDSP'i!:A1{ERS.

Heport No. M.012

Investigat ion by:

.H.D. Harwood (part time) R.F. King (part time) D.E. L. Shorter

Report Written by:

D.E.L. Shorter

'Serial No. 1949/39 •

. ,. , I 11" .:'''Coy'- / ...... , ... / . .!.q:-t;. I

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_-r-. (W. Proctor Wilson)

PRIVATE ,AND COtIFIDEl'JTIAL

Research Department Deoember, 1949.

1.

TBE

SUMMARY

Pigs. r~os. M.012.1 and M.012.2

This report gives an account of some experiments on the construct ion of loudspealcer cabinets, commenced just before the outbl"ealr: of Vial' and later carried to the point at1.tllhich a design, produced for use vii th the Parmelco coaxial uui t, was handed over to Designs Department to be incroporated in the new lSU/lO.

, The relative'merits of cloood cabinets, acoustic labyrinths and vented, cabinets are considered. For tho present purpose, the latter are preferred.

INTRODUOTION

In 1939, e:A'"}Jeriments were in progress on the design of a loud­speaker c2.binet for RK uni ts to extend the low frequency range beyond that obtainable with the existing baffle LB/3. Although the vvorlc was interrupted by the outbrenk of Vial', a nUJllber of useful conclusions had been reached.

TOvv8rds the end of the war, when preparations were being made for FM field trials, a need"arose for a \~ide-rang'e 10udsp8t:llCer for cheCking the quality of transmission. The only >vide-range coile units known at that time -wero thos:l made by A.O. Barker, and work on cabinets v,as therefore resmncd >:.i th the immedinte object of producing a housing for these. The B:;:u"kor units latCH proved unsatisfactory (see Research Report 1'1.008), and YiOrO nba ntloned, DU t \wl'lc on cabinets "'as cont inued 'as time po:r'mit'tod,. B;y the [; utumn of 1947. when a special cabinet was required for the ?armeko coaxial loudsPe3ker units then under test, suffiCient data had been' accumulnted to en['i:Jle [1 modol to be produced at short no ticl) : and this design later boccme the basis of the Designs Dopartmont cabinet LE/a \~hich forms part of the complete assembly LSU!lO. Although s aIDO of the 1 ater exp er; Qon1l3 were re ferred to in Research Report M.OOe/2, no complete acco':lnt of the long-term development has as yet been given~ and it is the purpose of this report to r/:jcord some of the Gxperiments which lead to th8 adoption of the fin~l arrangement.

The response of tho LB/3 is much rEJ{luced at low frequencies by

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sound issuing from the open bncl,-" out of phase \\i th the sound from the front. Any extension of the 10\> frequonc;y :rongEl must, therefore, be accomplished either by (al socl ing off the rear of' tho oon'1 by an ellclo~lUre of sufficient V01WllO to nvoid any undue inC1'0.':188 in the mechanical impedance of the system [\ t the lov,est f1'equenc:y of interest, or (b) allowing the s cnnd from the r,::;[Jr of the cone to escapo at the lO'IJi€r f:requencies with its ph8 B €I a.djusted by acouetic net"IJ~orks to re-inforce the front r:::d iation. Tho best }:::no\:,n Elxamples Of (b) nre the so-ca lled labyrinth, and tho v011~od or phaso-invf:lrter system, both of which have the inoidElnt~} 1 nd.va nt8[O of pr0.s8t1Ung to ,the c one a rGlati vely high acoustic im:)u(lL,nce C!t th'2 101,'.81' ond,ot'. the frequency ra:t,;lge so that the Gxcl'.rsion, ~'J.1d heDce JGhG 110n-linoo1' distortion, is reduced.

Al though the fact th3t tho cone moves as Cl V/hole %' t lc;w frequencies rendors tho problem ofcabimt deSign ameruJ'ble .to rouCh ci:'.lculation, cOl,lSi oorable uncertainty exists reg3rdi1l',:'" the valuo so::,' ::mch quanti tios as the absorption of drmpinG mntc),:!'inls and the rndintiGl1 irnped[~nce of cabil18ts of various .shnp<;::s. 'l'hus, nlthoug'h ,~11 the clcvicos rnontioned abov~ are v,ell kncv;n to tho art, tho results of their f.tZrpli'}[,tion to in,di vidual ca.ses cannot bo acc',J.rately predicted. and the P( Gsibili ties of each system woro explored in turn b~T r: serie s of exporiments described in the fol1O\-,i~g sections.

3. ACOUSTIC LABYRIKTH

3.1 The br03d, principle ef opernticn of this device is well Y .. I1mm. The sound from the l'e8r c·f the ccne 01'!Cap8S throl.lgh 3 duct of such a length as to bring the output into apprcximntely .aiding phnse at the lo,'ior part Of the frequency band to be cGvored. The duct is folded

'bnck and forth ,d thin thEi cabinet spnce Dnd. is lined 1\1. th noollstic absorbent material to give attenuation at hi~~hor frequencios at which the requirect phase relat ionship c::.mnot be maintni ned.

3.2 An experimentsl Cabiui':,t \,[lS constructod, in v,hich both the lene:th .and the cross-sectional Dl'ea ef the cluct I":ore cnrnble of variotion, and varicus absor';Jent iTIDterif'ls '"ore used os linings. Aa expecteu., tho Output rose in the neighbourhood. of the qunrter-wave mode of the duct: but i tt.as difficult to arrango 11 Hninc tc: give sufficient absorption, because of the condition of grczing i]1cidence, tc suppress the higher modes and the froqueilc3i rosponse curve was far from smooth. Fig. 1 (a) shov.s the axial frequency rosponse ~-f a typical e:gporirn.ental arrangement t using a lining of 3 la;;o1'8 of ~II felt, supplemented inpll~ces by rock wool. Tho me8suromC'..nt \,L: s made out of doers ,,;i th a velocity microphone sc oriented as to eliminate most of the 'effects of ground. rof'loctions. Tho response Gxtonds down to 45 c/s but there is a gonor,,:J. falling off in output toward.s the bass cnd the ClJxve sl1C'1!VS serious i:rrl_;[~lari t;l in the neighbourhood of l20 c/s, a featuro· ,;hich WDS vel'S notico[;ble ~~hen

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1i stoning to programme. In addi tian, objectionabl e colouration, believed to be ,due to structural resonance of the cabinet, was heard; but the technique for t'he meas1..1roment of such effects had not then been developed. and no objective stud.y of them could therefore be made.

3.3 The un treated volume of the labyrinth to which Fig. 1 refers was 6 cubic :feet but there were indi6ations that improvement ~n performance would; require an inm'ease in this figu,re. In view of this and of the complicated construct ion involved, it v,as decio.ed to defer further work 011 the labyrinth lmtil other possibilities:hnd been examined.

4.1 In principle, the design of a cl0 sed cabinet involves the provision of; a sufficient volume to place the fundamental resommce of the system ~t about the lowest frequency of interest (assuming that the fundamental resonance of the cone unit alone is not a.lready above this frequency). In addition, sufficient damping must be provided to suppress internal air resonances.

4.2 The compliance of the cone suspension is effectively in ser~es wi th the ac Oll,St ic capaci t;y of the cabinet. Some economy in volume can there fore be effected oy increasing this complinnce. The cone uni ts used in the experiments on closed ct:!binots were mo dified by cutting the spider and slnckening the somey,h:'t taut leather surround. and the compliance thus increased to 2i tim~s the 0 riginal fig'Ul'e.

4.3 Although several experimental closed. cabinets \~ere constructed, only the last of these will be described hore. The cabinet referred to had a volume of 8 cubic feet which, cllov;ing for the space occupied .by the acoustic treatment and. by the cone unit itself, was equivalent· to 0.16 e.g.s. "acoustic fnri1.ds". T110 compliance of the modified cone unit, expiDssed as an acoustic capncit;y.is equivalent to 0.60 c.g.s. units so that the t1iW acoustic c2p3citios in series t1re equivalent to 0.12 c.g.s. unit's. The offoc'!iive m'1SS of the cone,. which inc ludes the n)D.c'i;i VD component of the radiation impodElnce is equivalent to about 10...4 c.g.s. "acoustic honries ll

, and this resonates with the two nOQustic capacities in serios at 45 c/s~ An unmod.ified cono unit gives n complinnco equivalent tC) an acoustiC' capncity of only 0.24 c.g.s. units. Hence, to produce the same resonance frequency vd,th suc:r, a unit, [} ,cabinet caracity of 0.24 c.g.s. units would hnve been required. This in turn Vvould hnvenecessi tated a total volume of 12.3 cubic feet, an increase of 54%.

4.4 Internal air-column reson:'nces are easily suppressed in amnII cabinets by lining the interior Vi ith 3coustically absorbent material.· In the lm'BBr type of cnbinet under consideration, hO\',ever, Borne of

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the modes occur at frequencies too lav,; for the absorbent to be effective. The trouble coul d be mitigated by making the c[lbinet c'lb:lcnl Etnd so minimising the dimensions for [" givcJn C8lX}cHy: but fer C'. net internal volume Of 8 cubic feet, this forEl \,Quld be clumsy and \,~';Stoflll of floor space. 'Moreover, for good high frcqu(;!TIcy distribution, t110 centre of the cone unit should be at lC2st 3 feet fr cm the floor; [.nel ns a oompromise thorefpre, the o=cperimentnl closed cnbinot !:11t ,t'cI,'/ roferred to was mado 4 feet high. Fig. l(b) shOViill'tho axial fr(jque.lJ.o~T T(Jsponse of this cabinet, measurod under the somo eond,iticns 8S the curves of Fig.l(a). The dotted curve was takon wi th tho cabinot lined wi th IfAsbostoni te" and the effects of inndc'quntely dSI!Ipod roscmmCGS crm bo S0011.

The rolative inofficioncy of sound absorb~nt linings at low fJ:e­quencies is due to the fact thct the dnmpinC rnatorinl is onl;7 a small fraction of a ",ave length from 2 ve loci ty ncde. ExporinlGnts v;ere

. therefore made "'i th sheets of cnrl:X] t fel t stretched hOl"izcntr:lly ccross the eatine t at or. near the points of l11m:imum p{1rticle velo ci ty nssoeiated wi th the more trOUblesome modes. It ,ins f Qund nocessar;y tc sonl the felt around the odges Dnd this ,",as conveniently effected by the introduction of wooden parti tiens having openings 8cross 1I>hich the sheets .. ;ere laid. The full-line curve of Fig. 1 (b) shows the resu1 t ef supplemen tin£; the ItAsbestonite" lining i-iith n single layer of tit feH h:J1f Wf'.y up the cabinet and the effectiveness ef this type Of damping is cle~lrly evident.

The system of dam)ing described nbove mtJY be regl1rded, from 0. .

s lightly different viewpoint, as the di vis ion of the cabinet into sections, each so sm311 that its lOwost mode can be adequately damped by absorbents of the nermal typeo I In such a system, moreover,the sections remote fron the cone unit '"ill receive sonnc1 at 10\\ froqwncies only and

. will require very little acoustic trr'atment.

4.5 At this point, the v,ork y,~s interrupted by the VPri:' rmd resumed, as previously stated, ,~hen a cabinet was required for the \iide ronge Barker cone Units. The suspension of .these 1.111i ts ht'td 8..11 unusuDll~l large compliance and it ,,\08' thus possible to produce a clo a:)d C[1bi110t basically similar to that alroady described. but ~ith on inte:r:no.l voluJ:'le of c:nly 6 cubic feet.

4.6 Although no ne v,' d<wolopmont in design YiaS introduoQct [It, JGhis stage, a ne\"i' method of' t"sthacl to bo devised sinco ilJ18 sound'mo[,surement room, dismantled at the outbroak of' ,\liar, hnd not. boon reoonstructed and weather conditions only occasionall~T permitted mOaSUl'eDonts to bo taken in the 0IX3l1

air. A bridgo circuit was thorefore set up ill '[ihieh ccnstant currentw[s supplied simultanoously to the cono unit in the cabinet, under test, and to a dumI!".y load Simulating the skltio imlXlc1Dnoc of the unit. Th.o vector differonce bet,lioon thG volto[;os ecross these tv,o inr£,od[lt1ces, ',".;hich represents thee.m.f. genor['.ted by the cCJil motion and is pro{lortio1181. to the velocity.

e·

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of the cone, W2S men.sured. By thi s methoa, the effects of air rose-nnnces in the cabinet can be d.irectly observed in the l2-boratory v;i th a 'sufficient degree of accuracy up to about 300 c/s and the internal damping arrangements adjusted accordingly. {For d.eta Us of the circuit used, reference may be made to Research Report M.009 which describes the use of the same tecl;mique for microphone ca.librlltion.)

4. '7 It will be seen that the response sho~;n in Fig. 1 (b) begins to fall off well abo':e the resonance frequency of 45 eis, the presence of which might at fi:i.st sight be expected to maintnin the r(3s,)c,nso. IJ;his etate of affairs is nevertheless. to be eXl"Bcted with any reasol1!~bJ.~r efficient loud.speaker working und.er the usua.l conditions of lov; 801.11'Oe imped.ance. It can readily be Shown that for uniform lor, frequency res;;onse, the cone should move wi th veloci ty inversely proportiona 1 to frequenc;~'" In the neirhbourhood of resonance, however, the. back e.m.f. genernted by ,the motion of the speech coil is noarly eO,unl to the source voltago 9 nssumed con6tant. The cone then moves \\ith constant velocity Dna the pmssure response falls off by o db per octave decr~ase in froqwnc~T. The effect of back e.m.f. may be. regarded GS increas.ing the impedance of the' unit and so pr oelucing an electrical mismatch. This type of loss, "h.ich mus~t?:R/e,1fr to an even greater extent v;ith modern t;y'p',s of cone unH hi:lVing~"'l:t'6'Y:i'HT~1mo;""'i~"~V not, of course, confined to closod. cabinet systems. A The results are, T\ hov~ever, particularly striking incases such as the present one, in vihich the olectro-acoustic ~;;fficiency has ,been somev;hat expensively maint;a::'nod d.o\~n to less than 45 c/s.

Although the averDgo slopo of the dottod c1-U'ved in Fig. l(b) approaches the theoroticnl fit.,'Uro of 6 do per octave in the neighbourhood of the resonance jroqucmc;y of 45 .0/ s, the full-line curve truces a different form 7.i th [;\ sharper cut-off. Thu cause of this is not yet understood.

4.8 Comparing Pigs, 1(0) Dnd 1(b) it ,ill be seon that tho closed cabinet shmvs los::.; gonora.l falling off in responsE! towards the bnss than the labyrinth but thG ultimate 10\\ frcquon::lY cut-off is higher. In view of tho re)ativo volumes ef tbot\,o enclOSures and the fr{ct thot th@' suspension of the COll0 unit had to bo modifiod for use in the clo sed cabinet, the l:3tter arrnngeme11t is 0vidontly loss efficient for its si ze.

5. VENTED CA3IN;i;T

5.1 The principle of the vented or phase-inverter c2binet, like that of th8 l~~byrinth, is v,ell ltxJo\m. Sound le r1l101;,ed to escape from the cabinet at low frequencies through [l vent \',~.cich is commonly of the same order of. area ns the con(;, and of such [lcoust ic inductance as .to resonate v,ith the acoustic c8}?3city ef the cabinot at Dbout the lov.:est frequency of interest. It ct:m bE'; shOim that over f! ro;:;tricted frequ.onc~' band the

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radiation from the vent is approximately in phase wi th that from the front of the cone. As in the case of the labyrinth, acoustic absorbent material has to be provided to reduce the vent output at higher frequenc.ies, at which destructive interference would othen"iise take place.

5.2 In the region of the vent resonance the acoustic impedance presented to the cone is high. The motionnl impedance at the terminals is then lm'V, so tha t the electriool mismatch referred t 0 in the last section is avoided; but for tho same reason, the vent resonance cannot be danped by the electrical circuit. The extension in t:\i) low frequency range b:rought about b~T the vent is thus ObtD ined [It th8 price of bad transient re sponse. K:perienoe 'With n number of comrrvrcial

. vented cabinets suggested that if audible colouration'Were to be avoided, the frequency 0 f vent res,onanco should not be grenter thun 60 c/s and preferably much less.

5.3 It has already been remarked that the' area of the vent is frequently made equal to that of the cone. Some reduction in the capacity of the cabinet required for a gi VGn frequency of vent resommce can be effected by increasing the acoustic inductnnce of the vent but DS the ncoustic output from that apJrture will then be reduced the optimum condi tion is a cOIppromise to bo experimentally determined for each case. Tho acoustic inductance of the vent may be varied eithel' by changing the area of the a~erture or by adding a short tube inside the cnbinet. Provided that the space occupied by the tube aoes not [) k)J?I'ociably reduce the effective volume of the cabinet, the choice betvveen ~;lle tv.o forms of construction is largely a mntter of convenience, since H CDn be shov.n that the resistance component of the vent radiation :imped[lTICe remnins the same in either case.

5.4 The first eJrperimental vented C8binet for nn R.K. unit hEld a _ volumo of 5.2 cubic feet and 1:1 vent I.lD.ssage capnble of being varied in ,.,

, both length- and area. The vont resonance frequency v,as set as a, compromiso at GlJproximately 50 c/s by observing the electrical im,pedance of the system "hile adjus ~ing the acoustic inductance. He free air measuroments y,ere p088lb~,e at the time but from frequency reSponse curves tnken in tho sound n,en8uremont room the: rosl'cnso at thcfroquency of vont re sonance V;2S seen tc be well belo\', the goner[~l levol, fa llirlg very sharply at lo\'\er frequ'3ncies. It \,as evident that the acoustic inductonce of :11e vent "ould have to be nmch reduced but at the same time a decrease in the frequency of resommce V';T~S des~r[1ble 6 'rhus 9 a con-siderobIc increaSE) in c!:!binet volume \~as unavoidable.

5.5 At this stage in the investigation, an urgent need nrcse fer a rep12 cement for the uns,," tisfr:ctc,ry cabinet supplied v; i th the P:rmeko coaxinl units thon under test. As described in Research Report ~ff.008/Z9 an existing Parmeko cabinet ~,3S utilised in the first instance by closing the original vent, cutting a v.ir.dcv; in the bottom and adding a holle'\, vented plinth. T":e totnl volume "as no:» lO~ cu'b~C feet and the vent

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reSonance was adjusted to appro:dma.tely 45 c/s. The ratio of acoustic capacity to acoustic inductance Vias thus about 3t times that of the first model so that the vent resonance v.as much less sharp.

The he ight of the cabinet '-;2..f; such that 8. top ... to-bottom air column resonance took place at about 120 c/s, and this \HlS damped in the manner described in Section 4.4 by the i;ltroduction of a fel t partition between the upper and lower compartments.

\

As indicated in Sect ion 4.4, very 1i tUe t:bsorbent material is required in the 10,"e1' compartment of this cnti not, and the only treatment used there consi sts of carpet felt held awn;;; 1'1' an the iiiood by 11" spacers~ In the upper compnrtmol1t, the original treatl:10llt provided by Parmeko, consistitg of thin felt spaced fra:l the wood by oorrugated cardboard, \'ias found to be barely adequate to suppress resonance in the 450 c/s region, and the effoctiveness of the domping v.as found to vary frem one cabinet to another. In later specir:Jens, thoro foro , the uppor conpartrJont v;as lined "Hh about It" of l{apok held in place by carpet fel t ~ The photograph of Fig. 2 shows the general appearance, of the asse!:1bly. It will be noted that the vent is ,for convenience divi.a.ed into t\,O parts, opCining on oi ther stde of the nr:1plifior housing.

5~6 . The full lino curve c,fFig. l{c) ShOYiS the axial fr~quency response of the f insl a1'rat:lgement taken under free 2.il' cendi tionsa • It will be noted that the response falls sharply below 50 c/s although, from simple theory, the cut-off should not occur above the vent resonance fre-quenoy of 45 c/s. T>B cause of the discropnilcy has not yet been trnced. Interrupted tone tests .on the system shq\. the expected "ring" at 45 c/s, but at this 10' .• freque'noy, ne objoctionnb1e effects bave .been noticed en prograr.une. It will 81so bo seen from Fig. l{c) th2t the cllpacity of the cabinet and the acousti'c incluotr::.nce of the vont are no,:; a'bout the optimum for response dC\,,'n to 50 c/ s, s meo neither [\ rise iri the cut-off frequenoy nor a fall in the 1ow-freq IDncy rospQ!1se coul d be to1ernted.

Fig. l(c) also sho\,;s the offect of the f(,lt betv,een the u-pper and 101,,'61' portions or the cabinet. The chain dotted curve shoviis the . responso without the feH ancl the clotted curve the rusult of pIecing an e·xcessi ve thickness of naterin1 8.t this point.

5.7 .At this stago, the design inforx:1at icn 'has llL'nde'd over to Designs

*It will be noted that the curve of Fig. 1(c) is much smoother , than that givel~ in Research Report M. 008/2 for the same .

. cabinet.· The discrepnncy is due to rasiclu21 reflections \in the 1agf,"Gd 1'0(:[1 in "hich the latter curve \\8S taken.

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Departnent for use in the n8Y~ cnbinet LB/S. T- 0 finnl vorsion, <les-cribec. in Desifns Department Report No. 3.5, is in principlo the scne ns that of Fig~ 2 and has the snoo accustic ccnst2nts but the vGnt cl1.cnnel is formed by the space bet\,een the bottOD of the c8b1net, the supporting battens and the floor.

5.8 Fpr lack of tine, the question ef the lptinun voluDG of D v()utecl cabine·t for use Vii th an R.K. unit has net been pursued, but the required figure v;i 11 certainly not o~=cco(l the 1013· cub ic feet U;3{j(; 1;', ith the I'nrDeko unit, the cone area ef ;-,hich is t'bcut 113 tiuos that ef the R.K. unit. Oomparing Figs. l(b) ano. l(c) and. bearing in 8in(1 thct [1 closo·u cabinet for an untnodified R.K. unit would require over 12 cubic feet (see 4.3) it is' clear that other things .. Du ing equE'-l, the vented cabinet is [joro efficient for its size tha.n the closod c~l1:Jil1ot, though less so thnn the labyrinth.

6. OONCLUSIONS

Al thcugh the oxperimonts doscribecl in the forGGoil:lV sections are far from baine; a cooplote invostif.,'ctic.n ef tt....e cn-tlinot dosil;n prpbleI:1 the fellowing conclusions cen bo drawn:-

(1) In respect ef economy of SlX1C8 for a 10,", frocluClllCY cut-off in the region ef 50 cl s, tho order ef oori t fer the three types c,f onclcstu'O/3 investigated is: - labyrinth, vented c!'?,binet, cle,sed ~8binet.

Pr} The frequency res~x,nsc of, the labyrinth is nnturolly irre§;ular, a defect which ca.n only be removed, if at all, by c[1reful acoustic ~reatment. It nay not be v,ithout significance that nlthough the first v,ork on the 2COUStiC

12byrinth ;-;as :pu"olishecl as enrly es 1936 and is still que·ted in oertoin text bool{s, no cor:morcial enoodinent of this device is Imot";n.

(3) ~h.e p.otentiali ties c·f n. clo rod cabinot cannet be fully . uti11sec1 .."i th [1 nearly constrmt voltage source such cs is necessarily provid.eel by normnl lcuc1spec]{er 8!1pli:fiers.

(4) The vented cabinet, v;h11o efficient in utilisinG QVnil8bla Space, should ne t bo used. unless the vent rusona roe is 60 c/s or 10\\;01'.

(5) Onbinets large enough tc have air l'esonances in the 10,,01' c11ddle frequency 1'011£8 can be effectively dar.ped by subd.ivicUng thorJ into secticns by p8rtitions of aPIJropriate acoustic resistance.

MJB

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(6 J For a high quality loudspeaker system, the response of which is required to extend down to 50 eis, the vented cabinet is the mO$t economic design.

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£!,,21:'ENDIX

Materials For OonstructionOf Loudspeaker Cabinets

1.1 The experimental loudspeaker cabinets referred. to in this repol't were constructed of plyv,ooc\ varying from fJl to l~~" in thickness with in

. plywood for the internal parti tiOllS forming the labyrinth.

1.2 The questions 0 f the most suitable material for the construction of loud'speaker cabinets and of the minimu'l1 th.:clmese required to keep the effects of structural resonance iT, ithin tolerohle limits have not yet been systematically investigated. It is doubtful ,",hether simple nnswere 'could in any case be given, since much depend.s on the maximum unbraced area of material, but experience with the cnbinets referred to suggests that plywood is st ill the mo st generally EEtisfnctory mnteriol and thl:!t -£-'1 should be regnrded as ,the minimum satisfactory thicl::l1oSS o~· wood for cabinets of more than 3 cubic feot in volume.

2.1 Some consideration has been Liven to possible nltornatives to plyv,ood, "i th particular reference to the sf!ving of \voiCht& A cabinet of the type shown in Fig. 2 TIns required for portfible use in experimental work. After consideration of nvailnble mnterials it " ... as decided to use, instea.d of wood, a commerCial product consisting of Il -i" thick core of light spongy material, knov; n as "Onazote", sa ndwiched bot\;,oen i" plJ,,-,ood facings. At the edges of t~e composite material, the SpOllg'Y "0nazote" core, which carnot be Erltisfootor11y glued or SJrev;ed, is exposed, and the normal technique of cabinet making has oonsequently to be modified and a Wooden frame added. It y~as thereforo decidf:Jd to have n first morel made up by Fle:lw-Plywood Industr ies Ltd., the manufncturors of the material. A compar8tive \\'eight analysis of this model and of the ply\'iood prototype is given belo;;;, treating the uppor section and plinth as one.

Weight of ompty oabinet

Weight of Loudspeaker , Cross-over unit and, acoustic troatmGnt

Weight of amplifior

Total weight of system excluding amplifier

Tota.l \-;e1ght of system including amplifier

1" Pl~Tiood

186 lb.

50 lb.

\ 32 lb.

242 lb.

274 lb.

1" 'OnDzote" paving in \-.eig41 'lvith "Onazoto"

Makors' l GO lb. - 5;;0;, estim[lte

First morbI 99 lb.

56 lb.

32 lb.

n~'5" ,.24-~ lb. 361c

1ca--1 . <r2-?4: l1Je 32%

e.

-ll-

The difference betv;oen tha malmrs' estimate a.nd the actual .,eight of the enpty cabinet is attributed to the additional wood~n fraroing alroady roferrod. to. In considering the above figures, it shculd be borne in mind that whcre8s the 1" plywood. could in many cases be reduced to tt, it is d Qubtful vihether n. corrospondinc reduction could be made

. in the layer of "Onazote ll ... ~hile rot.s\ining the nocessary mechanical strength unlo ss a str ong frmnoworlr were addocT.

2.3 The cost of the ply-facecl "Onazcto" is nbout 2.8 t.imes that of 1" ply,,~ood tmd. 4 times that of *" ply>~ood. 2.4 The perforrrP.noe of the "0n~;zote" cnbj.net iHJS in no way inferior to t1bat of its ply>,Qod counterport. There v,as in faot some slight

. evidence of improved tnternn 1 damping but further y,crl{ is required to confirm that the change c,f material is responsible fer this result.

2.5 It "~ill be ·seen fror.l the foregoing that an appreciable but not spectacul ar reduction in the ,;eight of loudspeaker cnbinets can be effected at increased cost by the use of specinl r.J.aterinls. It is . thougr.t that future progross in this direction v,i2.l be limited as much by the \.eight of the associe:tec1 equiprJent and by the requirements of mechanical strength as by the 8COUStiC properties of such materials.

I

ISSUE I FREQUENCY IN CYC~ES PER SECOND.

00000 00 0 00 0 0 0 8 0 -t1tl191'Ql It) It) 0 0 0

9 - 12 - 49 .... I 0 r--~-..-r-..-r-.---T--_,-_---',Nr----TC\I-"'+---T"--ilt)"---'ioo

. ' I I

. I I

!

BBC DS/1/CX; .

db

FREQUENCY IN CYCLES PER SECOND. 00000 0 0 0 0 ~ 0 o . ~ ~IOID""CO 0 '" 0 11) o · 0

+lor----..-r-..-r-~--_,--~Nr_~N_+-~ .. -~ItI~

db

,~ / ' OJ---rrtrD/~~\I'--~~~=+~~~_71~-j , I ' "";:_ , , /

'~ I' f " \ , \ .. '"

VI ' : /~ I

\~I

FIG.I (a.) R.K. UNIT IN EXPERIMENTAL ACOUSTIC LABYRINTH.

6 Cu. FT.

AXIAL FREQUENCY RESPONSE .

(OPEN AIR)

F1G.1 (b) R.K. UNIT IN EXPERIMENTAL CLOSED CABINET, 8 Cu. FT.

AXIAL FREQUENCY RESPONSE .

(OPEN AIR)

- 10 / ( 1) - - - -- I ~O FELT SEiWEEN TOP~ BOTTOM SECTIONS OF" CABINET

(Z)--4 F"ELT BETWEEN "TOP & BOTTOM SECTIONS OF CABINET.

db

.- /-~~ o r.,-----r~~~~~~-+~~+_~~

't' \( ~ \. ~/ I '

. I

_ FIG. I CC) PARMEKO UNIT IN EXPERIMENTAL VENTED

I CABINET] lO-z Cu. FT.

/ I ' -10 r-----1I~/I +-+-H_-++i1~--+--+_t_-+___+___l

II! I ~

AXIA L ~REQ.UENCY RESPONS E.

(OPEN AIR)

I il , -20~--~~~-~~~~~~-~~

(t)_·_-- NO FELT BETWEEN TOP & BOiTOM SECTIONS OF" CABINE.T III

(2) 3 LAYERS 4 FELT BETwEEN TOP a BOTTOM SECTIONS OF CAB1NET

(3)----- A,5 F'OR 2} sui 2. LAYERS OF CANVAS-BACKED FELT ADDEO .

THE DESIGN OF A CABINET FOR USE WITH MONITORING

LOUDS PEAKERS.

RESEARCH DR'N c .w J REPORT

CH,D ~ . I M. 0 I 2 APDil&h..1 2 SHEETS N9/

OEPT

ISSUE 1

9 -12-49

FIG. 2 PROTOTYPE CABINET FOR PARMEKO LOUDSPEAKER

DESIGN OF A CABINET~~=~=:;:--~o:...:.; USE WITH MONITORING ~rtr=1

LOUDSPEAKERS.