RESEARCH DEPARTMENT

MODIFICATIONS TO TYPE A RmBON MICROPHONE

REPORT NO M.002 Serial No. 1942/3

Work carried out by D.E.L. Shorter

THE BRITISH BROADCASTING CORPORATION

ENGINEERING DIVISION

.. / , -' ..... \,,;, I<.

Hesearch DepclrtT!lent }iEPO}~ L'iO.: J)(l'l \ .. ' :

7th F,Jbruc' ry, Se rj,i:\ :i:-Iu , __ lil :j":i I

Work cfllTieci out by~ D.E.L. Shorter

D:c[ ,?,'ing:; Figs. 1. 9

"

~;lODIFICL'l'IOiiS '1'0 'l'YVC A rnF;OH inCIDPHONE

P i.Rl' L PET&'O RiI.!\,N CL OE' 'i'EJ.~ i3 'I'iJIj';jAFuJ :C:I:JBON Frequency R3i3y;om;f:: 'l'r-dnt:ient !\r;,sponsc: Aural Effects of lli.·bbon Hc:;sou;:.:.nc8 Conclusion

P ),J:'i.T II:' J)ESIGN OF RIBBON }Dti. j" 'ii1;;LO(;1'11 i;iIC.;iQPIJ.O::L;. RLbbon MD.SS

p,j))]Jon Mater:ib.l :Jlo(~es of Reson.~.nco Supply Difficulties'

. Form of Ri;Jbon Conclusion,

APP.E;l~DIX I: liI';i'l'CHING niANSli'ORNI1}\ lU?PEN,DIX :U: iiECHk'HC1!L·· Ii.ESONl·,])JC1; OT:' OU'rBB. CLSIIJG

P Am' I:· PERF'OBltiLNCE OF THE ST}'\NDARD, RIBJ3QN

.~

...

.,..-' J

Fo 1" some time befo re 1939 the pres'mee j :Ul -Ghe i;'. iT. (} ... ;ilt~.!·coni microphone, of ribbon :cesonancGs in the 5O/5()O....:cYcle ret,;icm WP.8 suspened. Since,.howeve)", the only avni1601.e standare. of cornpl.lx'ison for micx-ophoYle measurement was i:n 'N.P .IJ ;..-calib:r:cited. movine coil instrument, the r0f,pOlli3t"

ofwmch at these low freqllencies WoS given' only by L few.cblibrct.:Lon points " the existence of correspondingi:r:t:'e~ul~.ri ties in tho,fl'oCll'3W'::y

. response cU'rvecoulcl:notbe : \fully corlt'inaed., ....... ,.'-'.

In tho courzeof 'U soarch, IfRde' in, Januc 1:; '19~9~ fo;. ~res~ ~;ourcos·of supply £'01.' ribbon materiul, flome oeaten· aluml.nl;Uiil .tol.l ~n -tJ,E' oro.,jr of one-tenth the IiormE.l thickness ',V·S.S obtrd.n8c1 end E. f.:'w e:;Q)(riilh:l'l1:.El ribbons l!lf,de:from it •. " \7i1Ci'i the res:pons'o oi~ r:1ic:r~')phones fitte6.' wit! ..

1IIIiiiilllll 300008342 K

.~ ,

.,.. 2 -

one of these thin ri bbolli~ ':lcl S compaJ.w,)Q on ;; cont:inuoll,s :fre'-~ l.wney rtm with that of c. no:rm~:Jl type A instxWHGnt as st:-mll.L:cd, tll\:J ~J.iffE.!x'lfnce (».I'VG

exhibi t8d irregu~&ri ti8s con3iBtc::llt. ;ii th tiL~ pleS(;nee, in thc) -t;:rp.:! L curve, of the effects o.lrc8.dYf.m,:>p8c"L<jCl. In orO.'_I' to e:l..iminate the pos3ibiJi-ty of tho irregu.lari ties in the Gi.fL: l."'f,:l1l>O C1E''VO being O.uo to i)'le; f,;!::-.p,::,':iJ:lOntcl microphone rother then '~o the :Ti:,.-"E:.' ,rO., further d,iff.'ci'oIlce C'J.I'V0>.: "h: ru taken between the foncler j'licrophonrj [cne: 'hTioUB othC::'~8 1"1f:v.:'ng :dbbGTl;; of 'liffE:J'ent mate.rial and/or tensiOlio The rGsul t oftlli:~ c:;,o8s--check::'ng i;c.;3 to confirm t11&t whilst' thE: frec,w:mc:/ :r:",jS~)On8e curve of tllC experimentiil ribbon :was'subsk,ntially ;::mooth, th""t of tbe type ,h stand8.r6 ,?:8 LreguL.l'.

l!'igs 0 1 to :'1 8hm' 0xamples of difference curves obtc'ineo bctT!een various microphones in the course of this cross-ehoeking PIOC(~;:W. Thi) pair of vertical ClrroWS in ec .. ch figure shows the (jireetion of lncr'Gi ~;iW; response for 82.ch of the tv~{) micr-opli.om,s b8ing compr:,:::~(L The TiLicI'OphonG~; concenle6. Clre designat0ci La follows:

;

i\. Stendaro B.B.G-I~krconi tYPE' A micI'Ophone. Normal li hc;:r(ll

alUJ!lini.unl (durultTI!~in) ribbon. Fundhllt8nt[.1 roson&.nco frcquullcy:; ::-5 cycles.

Ao' ribbon (source 14 cycles.

Evrly (L'Zpcrilnental' type 'A microphone. "H<a-ul! clVllltniuIU offoll not };:noun). " Fun.)Bi'lentnl resom,nee fY3'ilWlh7,

AX_ • B.B.C ... Mbl'eonity;JG A microphorwr~o::;ified. by fitting ribbon of beaten Rl~jinium. foil having Cl mass of .18 mg/sq,.~~m. Ribbon V0YY

slack, &~most sagging. Fundamentnl rc':3onence. fr~~()uency too hee.vily damped to be observ8o. ..-. ' ,.'

I\X" • L

Nominally identicv~ with AJ~l but gre<,:~er ribbon tvnsion.

O. 0 riginal ReA :ei bhon velocit.y aie rophone. R:i boon of oCDtcm alUJ!linil:un foil. The ribbon :ceSOIlcmCG frequency, originRlly 10 cyclCj;3, (Fig.4(a)), 'was temporarily ra.is:.:;Q, by inereE.sing the tenSion, to 23 cycles (Fig.4(b)) and 32 crcle's, (Fig.4(c)).

; I

Inspection of }'igs. 2, 3 and 4 ShOV/S, thht the i:;::r·:,;gu;l:.I'l "tl': El in the frequency' res'poIlse curves become ':lore seri.'cius: Et: the furld.c:mLmtc.l reson",nce frequencyiB ~iGe9. ~d &re ',,101'30 f{)~e~ '~iJkrdll c:.luminium (6uralwllin) rlblJons, ,;,hich fU'8.springy, thrin 1'02' those.of' bu~tGn foil, which cre soft. In Figs. 2 emd 3; in which ·the;:,' c!e most clec.rly d.efined, they will be soen to occu'r G.t frer:;uGncy intc:;r-vw.s ':rllich stL-rt off ut t"Niee the fundar.lent[~ fTcq"LtencyanJ. increhae above this figu~''O 1~2 the; sCf'.le is [.!.scendeo.. They l'(;PI'i::;::h:m-c the odd. vibn',tionul moc.l:S of [~

str-etehed string, D fact \iLlCh·'i!.~3 confirmed by :stroboscopic inspeet:Lon

,

i i

- 3 -

of .tb3 ribbon ·.fhen·(:~rivc;n. f:,~om ~l tone SCiUY·CU. en :cccrurrc. of the :';Ylrunc;tY'Y of the system} 0V8n ;:lOcies'" Tr,; riot excii:.(:d.. Tb::: fct;" tL.:t iLn h~.g·tleI' ';:0:.·,3

QC) ~10t OCCUT st . ex~'G.t mu1tiplcs of th,:.: :C'LJnd~!1Y18ntcl i:;:·uc~uenc~. mc'.J bo (;118 to the reciuction in th(") mt:ss lou::'.ing imposed ;:;y- the Lir on the :dbbonb.Gll fcijuccmtportiol1s of it [.J:2 moving in (mti-phc..sGyproducing i:!OIilOt.b.iug

<~ppro&.ching an fcccustic short-circuit.

1.'he f111ctu<;;.tions i~' th.e C .... l;'VG of Fig.;. ':.re 'llDO conf.'iuilcc by some cO:ilpc:.rt:tive m8u.$UTement.: k:':3r~ in ·~r.l2 Open ;:.:iTuGt·,1een mie:ro.t'>110nc L allQ A. Neum[;.nn donderwe r velocity T:licrophone. 'Thc-; fre,::uency I'(3BpOll:'3G cm"V8

of the l~,tter is 'unknown, bue if it i;'; l;'SSV.inoQ~:to b(~ c;,iootl: (tl10Ugli not f1(;cessarily fl:::.t) be:l.oVi 500 cycles UK the Ci.<~pLrt:,:'l,~3 of the ci.ifferenec c·o.rve from a mer;.n v.r;:·' p10tteo.., the x'u,JuJ.. ts i;.gr8('; ve:cy cJ.o~;ely 'Vii tj'l t.hs depertUI'flJ of Fig.2 from <:, mean. The t'';iO sets of (icviLtions c,.re plol.tee, in Fig. 5. Fig • .6 Gllmm the co:nJJlete f'l'oouency response:. curve of microphone A buU t irp fromull th(~' iw<cil.v.ble (L.'].tc .•

At i:~bout tlw samE: tir.18 C.8 tL.nS0 ·:::xpc:d.ment.:; Y/err::; being c&rrL'l6 out (Januc.ry·1939), 11 curve of L type, A laicroprlonc suppliud ;jyM"rcon:L!::; was taken in Ge.l1llany by Dr. von Brc:.ulliJuhl and. sent ~o the B.B.C. for corrunent. Fig. 7 i3 replotteci fror.,. tlD.C curve, correction beinG m£;ri.o for the frequency respoilse of the pre-.?.JlJ.plifier used. It v:ill be notrf t}~ki.;

. the fundal1entul resoUlccuce freC1.lenC~; of' t.he microphone te.::;ted b'j' LT. von Braunm~"u-ll Wt.1S· ""bout 65 cycles: .' (it n1:i:3 ;.,ince been i:.,3C8:r'kilW(j. th: ·;~i:,h(; 3peciinen concernod ,-.£ .. s <::~ eo.rly onc;,;&ck priox' to tile iilt:rOG.uctlon of i~he C:.ire.ct rec.ding resonance; irlt~~c,.tor. *) il.r:; ~~ cllGck} ~": ~x;ct)nd. t~'l)(:) A micl''Opnone ';i1~·S tdcen '~'.nc: i tG funci;::.mel1t2~ reSonc.nce ;(~justeQ to (PPTcc:im.:.tely the sLn8 frequency. Tt,is i:iicrop~iOm,i 7:,_.S c;.osiGl1c t<::Q IlQI1. It.s £'r'~cu,;:w':,r . response , tc..kan agLil1.f.<t::1icrophone j~Xl[,;J At':."'JlQ".5~, is silo'.rn in Fig. 8.

Compe.d.:30n of F:ig~;. 7 fill(~. 8 I,Noel;:; condiG.cn. bIe (,ii3Cr8p[:~lcL:;;;:; in the genurd. trend of the curves, D-3poc;ir;.11y in i:'~:"8 hf:ii,..::;ueh gun;1\l discrep.::mciDs, bo·tyjeerl ourmec...surcmeri·~s·c.n(~ those m[C(l(; itl G:;;Yu1:.lW hVIJ.

D.lr~;[tdy beetl.'f;rlCountereo. in test.s i:i~a.e on Nelli:ic'.nn ·Lmic.:.i:::'8ctiou:.l. corn.~cn2(;:r ~icrophone~;, '.:nd will llot bcc.e&lt. with in tho IH".)[;'o)nt 2'8port.N~'v('r't:~u1cS8, the frec;uencle's Qf th0 thirQ"clld .fifth· ribbon modes shm, ~;cti0f; ctory ngrcoJ:lent .f:.lt110:J.gh the n.x.'linr.l, ;:ie\rcnth· node in Fig. 7 LpjJe:. j':; . ·to l!LV,) bH;;Jl suppressed. The oxt-snt or'·,the fluctuntions in. ti:'le CUl'Vl: uf Fig.] ,'..1,,('

some.,hc.t g1'0E:.tertho.1l in Fig .8.. Howevci:, tili.'.) ciopcmis or., tile i1< .. I;,CI'L.l LIl(: thic~cness of the :dbi.)on foil; ~:hich' 1:[,:;3 not clo3eJi :;tLn,.j.::-c,Ll\.J(. fo:;_ tl',;:: ec:rly producti(Jll.modell3 of type A r:licI'Ophone, ~-l1(~ :,0 c:lmbt be) '.;3Sluaec. to bo iCientic:l in th,-". t •. 'Q c<:~scs. In g\)llor::cl, tl'i(; iilt;{~j·u.I·e!'18nts of Dr'. von Braun:auhl give D. gooa. indopendent corroborc.tion of thu conclusLms reLchfjC~

l' D,:)oGribed. in "World R::dio" J 20th iV!c.y, 1938.

- 4 -

'fr12 exi8tence UL: n.:ture of the t::ipO L :c:i..blJon re Gon:,ncC::"1 l-:avin.r; been 8"'+' blishe-', ·'tte''''y;'' , '.' ..... " .,~ (: e to r('; ","', .L 1-" '.'" 1:r/ T ·r".-. ""F of ("I a·!· 11 o u(' l.\.? 1..- J 1'.:-1-'" ... ,::> ~ ' ... .J.v .'.lGL" _ . ~ , •. --!:1.1...J, \..; .. t ~l..:. t. u_ 11..: ....... I_U J • ,, __ u_

screens J1l0illlt·:)Q clor.e to the ribbon. 'l'hLo) vovieo, 'xrlil(; c:-.pable 01' producing alj'.10;]t ,bony u.est 'c'Gel (E:;gX'S8 of umping f'o:r' t:,18 fUl1.(,c:men·::',1, -; founll to J..eave, tilO r,Li,ghe r r.:Ol..G:3 p:::,~),c'U.cEjlly' illlDfi GC to,), ,', J'tWl!lc ,~1O-C

entirely ·lillC:A-pectec3. in vicwof th8 L.ct, 11r<3l.:!o.7 pointac) Gut, tLb:':.; for thesG ,moclos. tile HiI' louling i3 .largely short-circuit;d. It' ' . .-::".;'! t:~ei:'cfo:::'\~

1 corwluded t:l:, t t:J.G i rre~'lCL,~,j, ti6s which et p resant cLar:J.ctcl:'i';;:e i,the t.\--P~ i, l'i:Lc"ro:9ftone responof: CUt onl:-- boo! l-emoved a.Y"; chrng; in thG· l,lb1.:l0n" i-c,~)81f'.

Finally, '0,-,1'01\') le!:ivini:: the subject of i'TG"LWl1CJ TGSLjOlU8, j.t. PH:;':- be stated that thed.iff'~rence in ribbon rJatcc':iI.:J. :Jc:twc-en l'l:LeJXJpliml, A Dlld Ao on the one h[Lnc~ £Uld AXl and AX2 on 'the othc r proG.11C'J::; riO

nWClsurabl,') C~12.nge in the J:'t'JIQtivt.l rDsponso f..t the UppGt' frf,r:nonc:iJ.:,).

Tz~drisiont Response

It hL;3 urec.6y been Y\:l!llLrkcHJ tk.t. t1j~ :C'rucuoncy }'\.:'SpOll>J cu':v:' of a ribbon ;nicrophonc hecoi:lcS progl'"s.'lively smootli,,,:c iJtL,:: fWlcj)(:':;'.lkl reson".nCG frG(~uGncJ' is lov'~rod. Witi:l t.ho :r.:Lbi)on mo.teri.:J, u",~;,~, in toe PI'CC~11ction mOGcl of tnr.o ·;:';."pc: 1, Elicrop~lono, fj 3~)-c<rcle ft1l1()::'11l1:,;n-c.c 1 \,,".;':; <ll1opted c~3 Stl'<UUCI'C., since mc.n:l Gpecirwms coulu not be mule to go b01o~, this figu:r-e. Int:his c~ se tLe iilc,ximUIfi depr.rturc from .s. 13mootL CUI'V(:

",r;loilllts to only! 2~ db, 0.bOvc 50 c;yclou ,~nd ;,t fir3t sight thi;.,; ni[)lt :not e"ppcP.r V61"JT s,eri-ous 0 It '~1111 b"c !lotec~, 11owc)vur, tila.t t?!.o 8~lop0 o:{' tl!.e fl'equenc~T respons:) GU'rvi) of Fig.6 in the n0ighbollT';:ood of,): c~·:. 11l0(~(]j.s ve,.y ste8p, j.mplying a low CH3gred of Citmpint; /lnll. r.i;nee·befuct.ivo roproQuction of t::"'l.Usients,

In orO.0r to gain some iC~E)a of the ()xtentof t;-u8 tr[.,nf'Jiunt. (~listoJ,tion tlw follOWing test wC.s ;:ppliec.. B~,T meU1~:; of, e. l'lot,o:c'-liri VO[t

comiilUtc,tor the mi.crophone llllc;,er inv::;'3tiGf.tion ,.':::,s COlli'1.Uc)co(~.;;,lt:.;nli"t, .. J.7 to n tone sourco enC. to the input of :.>Yl A ,;mplifio r, t:~t; output c< ,,~Qd!.:;, G (3xaudned on ,< c["thO(i0 ray tub.:). 'rb:.; resulting oseillogl'::1ll im,ic>.:t:}l-. tl,o veJoci t~r of the ribbon uft,:;: r the removc'l of th-:., ciri vinrr fo Tee. :[;-1<; :,holl: oper::;tion ~ns repoE.ted SO!~le ten til:lOS p . .-;:c secom:" ;::~IK4 "t",)(' timd :Y:';:;;~ of i~;'lG tube' wc.s s~vnchronised. D.t this speed •

. . ' :In the c::"su of 1l1icrophoH.Jtl .p:.o' A' ~nc Q 1JEO o3cillogr',:Jn '].'(JV()' 1':'0(;.

low-damped 6scillLtion L,t [:11 sign.:'.l ~::CBcJlOneivs f!'Om. cbout ~iCiCi cyclu~,

QO'VJUw-.:"rd.s, but Gopo0iclly Pl'0110llllCOG. in the rlGighbollrhooC~ ofl:.~_,-, :~no.'l1 rGsonf.i.UCO modes. 'l'hiJ mo.xlI!lUl!i rc:tes of QU;L<" (,~G vL:rious fre'::.11()ncic~,,8l'(;

de·tcnminea roughly by llwp2ctioYl. For the; nonrL~l type j: micJ'op.llOLL' they

V:"'.Y:/ from .1 to .2 sccon':~s P;'T SO ctb OV,T '.Q:t of tile I''.:nf;'; ?JJ .- 500 c:'clc:s.; ,,·""'1 ,. "'r-I . .,..,· -10''''"' ·"··c··'· o·r.- .1-out I! '.- "r""'''' r)"Y' 0"'0 :"', .. L 10'[· "'l'" ," T' v\ lll_l ell v,.:\.. 0;., (. 'Jolt.; L...... .. .. ;y ..... ~ f_ .. r.} (l '";j; • .: ..• :, '~)J 1, _I-'" J ',.: J.. {~. i~ :. i-, .... ~: L.:.) "_·X'_· ~ " ".18

Q-"--lC' ~'T · .... .-:·+£'~\S '!-" •..• ,r.l Ct'l'V"ll '['0;' 61') r-;l). ~"O " .:', +r) f; p-; -I-,"+.: ;.~ .. !"OYnI')~ -;"'J" ;"Oil -;"':.·.i -;-.:1"1. '.,I .•. ! ... :,:; le:; JC''; .Le __ 'Jv' • ' •. ~ L> <. _ _ ,. ,.,' ;:; <d.. ~. ..' .v""·"c V,·, U" .J 'a .'" .~ ...... ~- '.

rcvcrbo:r'Lction time of [; :"00:'1. . :,;iCTO;)hon.js P~~1"'S<2 [;no~, 0, on h'::lld, v;",ro fOl,U1d on this test "':'0 bu [;ubsk·.ntic..llj r-. p:;l"io(~icJ i

vihicll is 110t Gl11priGi11g iYl vi~..:;rl o~I .. ·tli.t:) rl;~;.t~;.r(; of ·lJ~t~ .. : .foi1 U.30C:_o

'Gh,..; () t1~.J r ",suIt

CompaI'utive s.~Je8cL :t0~:;t" \,(:;(0 mb.(~e on -tl,,(o microphon.::;3 i211 i~o' ·A cnd Q. Tbi) (.',.cA Iilicrop~lono 0 ,T';'~ cv~cluc~cd fro;:i' ;)',(; to::Jt;:; so t:~,t 'Gl!c anI;! c:.i£'i·6J~()nC0 betr;('cn 'Li10 :3prjci,llOJl3 lr;.;;· in thl; llEtGI'i;1,chicl:.!h;.S8 ~n\~L tonsion of tho ri bton3 •

. A::.~ the ret1ul t· of t8Sti-j tt.c L}.ierolJl1011CD ~('\jrb ~)l~ ... cu( in. tJ-L:. ~-:'-' ,.-.t"

of i:lcri t' giV0~l L.bovc:. The ,.u~,'''·i. ,:,ifftjl\;EC(} in jY:J.'fo:rm ll(;,") I: J c.d.: ::.:y in the .cletcruldss of rOj)l'O'J.uctiun, p,,,rtic'_<LL1'ly in till) 1or:,;!' I:1L,cl8 :cC[;-i.,-,t,,1'·, clthough in the C~LSC of 'r:1icrophol1o Q ,. g,.m,":,:;.,l "j:Ld:t of .;.!;.,j·'2J:i!lC;"i3

['ppear.~d to extond up -to 1 or 2 KC. SUb;3;Y:U8ut ,)Y.p,;iiunc:,. ,,J th convort8c lnicropl1onE;s of t}lC i~~:{ t:lPE) hf~.;J ('.Cjllrirmc:(~! f~_.:;~='i!LL t\:: SUT)f;--L'j_oT.'it~ti-

OVGI' the tyPO A on sp,-,och. '1.'ho if'lp:ccJV'..:!lilnnt on music i::. le ~3 il1n-j.::..d bE 'c·

i8 lloticenble in cC:J'tc.il1 pi<.ss:.~ge3~

·Furtil~?r ~;.f.1ee911 t,(~St3' B:i--l~Y:-:J(~, Lto HligL.t tJt: E;)~i)c:ct(.;(i., tl'l·~,t -!-,}.iU

c.bsence, in the J~.n:ic~pjjOl1C., ~)f 'v!.lC lww .. l b[:.3f:-~ E·~;SOfE:nC:j m.Q.k~f3 it unneCGSS[l!Y to J:l0Qliy 't,!W ba-ss reSpOn3(~ of thE:: P :'mplifi.,:r ':i thin thE; uscfpl frequcmcyrL.nge. Suff::'ci(;nt 0xpcn-ienGs Lu~ not ;,;" t ·oe.:Jl g'.,incd to ,Q'ecJ.·Q'e '~.·1-v~·~l'l"'r· aY' no+ ~t "O,,'l n b-' 'r..,,~C.C''''' ,,',7 "'0 r··.t·, :"'1 "'.}'" "'·"·7·.·· ,··t·-of'i' _ ., _ ... \,.; lJ '-" ..... v -'- ' . t.A...!- _:. . \. .. .1.J,.t..;--' ..... ~"J....,J'_._.'!., Lt . ~~ Ul ... J.,....L l: ...... I;,; ... \.-,.-' .... _.1) t..~ _

below 40 cycles to c.voi.6 thu .. ff"'J:i.:.::~ of i:j()ci;c~,nicC:.l ViiJT~;.-t,io{t rw.'~ 0,:~~Ut;ht8,

,9ol1clu8ion

Since, from. the foregoing, i:':~ ,XlS [:,PI)c,:c'<.mt tJ;:.t :'. cP,:·.ngu :;::·ro:[1. the p:cesunt type of 6.1j:,r:lurnin 'ribbo~l "woul(~ be: LJ::V'!lt:.gO(nG~ t!~c; 1;{Lo~Lt) C,uostiOll . of 'ribbon c~c~;rigl1 \,'LB Tev:L:"\"i(:ti '>l tL th0 obj Q:::tof In: .kiEc'. ;:JUc~, improvC;!l.ent8 ::-,s might be; pos.-:;ibL; ·.,ith ·the Gxisting W::.t;1l8t ·~;yst~jm.

PAro' II: . DBSIGN OF RIBWN IiUn .;-, V5.:J . ..oCI:'j'Y ItJiICBJPHU;j:r;;

Ribbon Mu.ss

It 1:, ~Jell ;:':no1".'n 'Ghat the c:i;:c' 10udili8 on "tl1G moving ..::i!:"liLnt of a velocity aie rophcno l~3 .';rl ::..lmo:Jt pur.:.; rfltd3S rt.3~ .. ct(:·11~G~. Fo.:' (! . .' :"irnum

- 6

efficic:ncy tile ribbon El;,,"~J ShOlllCi ecn:wl the 8C",liv:,10mt ;~il' !i):'ss. Th~l.::;

If.ttel' qunntity is ;,lrc;';.I:L~· iixeu in t.he VL'C;-;:c!t!t inGt.:~n.c(': by i:.Jw :Ji",:', :11<),

shECpe of tho ;'Lgnet utructure. It,f; Vi ,1L1l~ c;tXl ~s d8clLlcc(~ £',,'om mecsu:rements of open circv.lt e.m.f. per nni-(j ~J01m6 i'ielci. nit,;i ti,"U (r: DO~':j

I'l' bbon'" of a"]' .l'''{''''J."~''l'lt "tP S'" (t}'Ollt')'J:'l no+ Y,'el"n c ""!" 'r'l' 'j "," 0'[" t,l}F> C"';I'l," ,"" ,I "1': "'I)' '-' . __ ~.~. ;.. 0 .... '; v ... "._ u\..,~ _,_ ~J .. ..L _, \..1(. ..... -' IH .. ~ ..... \~ .1,..1 ...• _ lit

The,geno!':ctl~;G. c.m.f. iG p~fJor~iom_~ to the,ri'?l)On vcl~ci[-;::',(,n6,.it~;, Y.'CC1PTO CcJ,. lo:) thus propo~I'"(':Lon:,:"L to 'c118 COrnnL71eG, Iw)ch,:·,n;l.(~!:.l lt1lpCCfIC(; oj rib'Don Qno. :dr lOLd •. ]i'ig. 9 J!.lOY1S l/(g(mcc:ctoct e.m.f. pe:c unit i.;OUUL

pJ.'88SUr(~) plott8d o.W'.in;:;t "ibbon :1[,8.8 pl"Or unit ;:''C'Ui for L-. number of ",vpe ~':L--n(',}ltr 1 .,...; bb071" of' ';'c)'I" QUY"'" W'll' "1 "~n° Q" V" J"" (-'1' '" t 1'11' c"'::n "<~F" C" o'J.D, ""'~ .. ~...,. ... c~ _..L..J.. _ .•. :'1 _ u _u~, _.l- . .L. ... J.. C. ' •. _I_~' -4.1.]..,J _. t ... ~ .JU,\-) • .

[,lWIlinium. The 'bc:;t ;5'tl'D.ight lin,,, \vbich c;.n bo (_~l'~:.i\in t.~lr0tlgh -tilGSe pO:7nts gives, b:r 8xh':tiJoJ.r..tion, ' figurt) of' 0.6 mg/s~,"cm. fori~hc f)ctuivalent of [ir 10,;,(.i. y;J-lir.;h i:, rlso thd optimum ribbon m."_.s,;~ •.

'1.'h8 ,3010ctioIl of ribbon j,1:.t",rLl :L'l gov0m()d~):\T tCe; I"<.c,ily delllorwtrabL'; r;;ct trw.t for :J. given d.bbon mcloss -Chi; rtl:,.xil:n;:'jl ontiHJ.t volt~~gu obt[,in:~blc [,Cl'O::l3 i given 10ta is proporlion.:.1 to liV' l''()sifJtlvit;, x ,_1.cms:i.t;·f. Helr;tivc w~luo~1 0:: this ouanti ty Gxp:c;:~ssecl in Gb. (:,1"'", giV~,l b,;lm;for L.

nULl ~)O l' of !TW. to riD.ls.

MD..te:cie..l -----Li.thiml Aluniniurll Durc.l-u.;.-;in Copper Silvs::' Gold

HeIr. ti ve mexi.rn:um .output oote.ill! bIe

dh. +2·5

o -0·5 -2·5 -3·4 -7.7

The figure for ).ithium is gj.V8ll L.S & LL .. tter of interest ;:d.nc0 :i..t Ll thE,

only 110tiJ.I givLlg [< grecteJ:' d'ficir:,;ncy th~n :.lluminim'!. Du:,::'clumin hi :3

b~;en incluu8cl i3incG it i;3 :lre:'dy ill U3l", out othor ;,lJ.O~113 h::.v(; not lx:;:.o:J:1 considoroci,:lince their l\;sistivitic;s urG, in gene:':'~.l, gJ'8,-,t0:L~ t~lL.n tbo;'),} of the pure metLls.

F:::'OI:l the '~b9V(: consicl~~c, tion:3 it ·,;i1.li:>; 38C-Yl. th: t m;'':''Ciillwn

efficioncy YiiIl h .. ~ obt;::.inecL. wi th [~tlJTliniLlIr;, in ;ihich CL.!'c~e the opt.:i.num m,'- 8;3 of 0.6 i:1g!sq.cm. corresponds to v thicY .. l1,;SS of 0.00009!!.

lliocks of PcOCOnEtIlCE.)

(u) Norm[.l IVloo.os.

-- 7 -

of nOlU;l.+ rtiOQ28 o?1.'Gsoni~'J.cu (L,). th08,~ iri.vo.lvlnt; I:lOtioll LOITn:.l to tho plDneo of the ribbon) 8D1.1 bs -rechieca. to rH::gligibltO iH'Oportiono pTOyic.,::D tht.:t the ribbon is of soft ~n'-cteri:::.l cn,i res :'. Vi .r-v loc, i\mdilT:WLtr.l· I'GsOrlmcc frequency. l'helut.ter eonC,i tion neS;;!s'3,ol:;:-':L,ly 'invol',(t;c) tb.'" u;:;(: of suffic:L;utly t.l:lin me.t8ri~:.1 c:nd it ;';us fm.-me:. . tit! .t this rc;c~u.iremcnt (:oulci b . ·D· .. , .. r lUlr"n"l' 'n f' t' ""1 +',' Cl"'-e~'" ·V·; It' " }. "I. S ~.1.' -. e 1-:1e1:. . t:_ ll,'.L L.. O. fie op1:.lf.1U;, .,11..L"-l.1",,>:> 1.,rrl ',,;0 ::~t. ' 1 't;rl(, ,;",'u UL:l>lOIl.

(b) Trc.nsvtJTse Mocies. In Lddi-tion i:{) t.he !lOJ;1Uc.l 1:10UJG of rN~onLnC(,", th,)T':'; 21'8 other noa8s, D.8soci,·:.ted rrith ;5, siCi(;-to-;:l1.(~0mot,ion • .A-ny s,light tTIist ill ti:.e ribbon h'J.s been founa to intmduc'.:;co-uplirlg bct\',ecm thG norrrc,l.1::.l1o. trnnSv0 ,'SQ ,:10tions, v:ith the rosulttllLttho po::foJ.'fanneo of t!lB.nicr'Opr1one 1.s 'l'po.ii'i0C by tbe J:'es~n.<J .. nc.'J ··of· tho l·tL r, The; stiffness of the riboon in tho tr:.nsvO:'Se ..... L:\;c:t:Lon id TBlt-.tivel:; l1ie1i cmi CL'10S't incioponcicn-t 6f :r;i.bbofl tension,' GO thLct: :;VGn ;':11611 tLl.; offG(;ts or nO:1:1TIi:l rC:;ion(";.nce mod.:-;;:> hr;v,-, r..,c;<.:,n cliI~inc:;.t::~d by tt< .... T!'li.:'I.;.td

. .#lroc..d.;y in·~;ic[;.tecL,:-thosc;· of -Ch;') 'tnnsy,:;:cse E10!lCS :11h;Y "Jti:i};bc; [,ppr\ei: L·lL:.

GGnBl'o.l:0xpc;TI.Once Sh01;"18 that cC;:lplGte i'rc:erior.1· irom t'Ui:Jt Cr.·rlno+ 'oeo (T" q....:.Yo t,,· ·,.f' '\'.' 'th OUi' <:!t<>n('1'" "'0". It ""g.L }.. of' .,...; bbon' V')'; 'thou{;·--nlI··' o'·:~ 'Il!"" ... _ v ou..C .... J .• ....u..1 vC\,..I. ~ + .l.... ._ . "'" l;..4. '-"..L ...... ..1..1. lI_! ...... :".J~ •••• _ ,\;.I. ' ....... • .l.. .",C :..-;

exci"ssiv::: t,,;,nsion.' Inp;1.rticulE.r, with alWllinil.lJll ribboris of-Opt:Lmum th:;Lckne.Gs, :const~"'!lct,--,Q aYJ.Q mc.nmtod ,Jith cOYlsici6rc.ble c::..:r.-o, the .d.'fGGts of

. trfJlSYOrSE; res·Ql:l~;J1Ca,·· wl:ili~ not ~.pp0nring on the fr8C:UGncy r:pspor!8(; (;U~:"i'':;, h[',ve bt:icm llisclosecL by"Uie t'I'nnsient t0St cln3Ldy (;.8sc:dbed. " I't:. ;7[:8~ ~. thoreforJdecid.ed to, insurf, agc.insttroubleof this ~;:iji:;. by r;::d.tictj,qpoi' ribbon thickness (.'::.rid hCl~ceof t.rp..JlSV8Tt{0 stiffness) to t:.'c polnt.'i:;t vJ'h;i.-c!'. such offi~cts should. be negligible. As:. result of '':irperim,nt., it ' uppCC.r-8C, th[,t &J.urninllm f(jil of r.li:.lf tho optimum t~"ickn()S8 (i. u. 1,cii;;'.ing 0.3 fug/sel.cm.) W01.LL.C, give [; ::.s:tisi'cctoJ.--y pCH·fonn1:."n.ce' :'.-ith . 1-058 il!-out.put of 011ly ~ db. .

.. ~ Supply Difficult-ias

'.~ .

The benten .c:.luminiumfoil with.i;,·hich the ["bove .,:!:x.pU:.:~i!JK:fl.ti3 -,{ere carried out. coulo_to' o btL..ine a. only in two gr[;.a.es, 0.18 cnd -0.2::'; mg/ sc> CJ:i •.

(the latter·, so~ewhD.t v[~:c:i.ablein tJ:-...:iclr..ness). Fb:cexpurimc;;nt£lpu:cpDt~C;S, ribbons ·of gr8t:t·2r mess wem obtninociby hi-,ving seven::_l 1~:.y8r:s S(!Ud3~;Jll togother in cpress),.trJ.s 0pOH.i.tion buing6.one for us; by the mrJ{(irs) Geo.M. Whiley J.Jtd., .54 Villi tfield Str.:;ot,-V:.L After iilSl'loction off;~<:;,;:;8 composite ribbons, ,it.YiE<.s. felt"thQ.t,~:whiist the Gt)1.10si':m· net\'retm,l:::Y,C::I's ViaS .suffiei·ontfb'r, ~he purpose of . the pJ:'t3Bont r:lSGLrch, :Lt .WGUl.(l lJe'

., inadvisable to Uf,o. such an arrangement in service. In -:-::di tion .. Gi10' m.akei·s wore uno.ble to duplic[;.te the.23 .]gj~c;~ cm. foil,lr,iu:y surj pli,::C1' and the only relir~ble snppl:y ll~c:"sth[.t oi'.the .18mgj'sq.·cm. foil. . 'Jltifl is nbout one. trllrd oi'the Optirllum' noss,' but }>Y011 H() tll<~ output if> ',·.:Lthiil ~ db of thnt, obtuined with thE:: stand:;.lyl QUI'rJ_ULlin foil, thu l!,·t.tc.'T having

8 -,

2~' time3the optimum lJ.['.Sd ',nd. :~ slightly gi'ce~~':'J: rc~;int.ivi ty th'~n Yju::,'c !~umini1[,ll. The .18 mg/sq.cm. it> nO~i ::]J.C1,'!l cs the] "X" f'o:iJ. cnQ type microphones 'iihich hLve boen £'i tted i:'iith it D.r-e cie;;iguD.tE.!C: 11 t::rpe JJ(H. The tensioning ofAX tYJ)C;;;1 .ribbons .:ioes not 2.ppe::.J' to IX; critie:.l rn(:~ It:· is thought th~).t the v.~.ritLtion· in performanee ;'iho\ifl in Fig.~ iD .;omO'.·.·:l~:t grec.ter thc:rl thc,t lilcely to be cmcounten.!Ct in p:t:'::.ctiCf;.

F 0 lID of' 1QJ?.QQ.g,

.1;;'; ", The mo.teric], Wld tbickness of the x'ibbon heving bGY~:ii?:tubli:!hi:!' ti·iere. re!na.ins L choice of three main fonns - (2.) completely \mco:r'I'ugr~te';, (b). transversely corrug1),tod Ul1C~ (c) 10ngitudinLlly corrugr:,tul. '.

(D.) Uncorrugoted Ribbon. 'rhi,::i type of :i'ibbon 1'.!r •• S beon used in commercial iilicrophonos' (;1::01' GX:::,iTIplc, the E.M.I. ribbon velocity mi':~1'0I"i10E";) c.ne] tried out by us ezporimentnlly·ith vG'rious t:.rpes of foil. un,LG.::';:h:. is, m::'.d.c ofunui3ueJ.ly stiff materi<.'l, it 1;; T:1ec}·1I.lnied.ly UILV.bL'l <,,/L. will flop oval' suddenly from on2 Gl1ll of .H,o tl'f;Yel tr) the O'c'~l~n' :~,; " TO;Hll t of :c slight dra.ught. Unless the foil is emite' fJ.'fo8 from ',',:,',;: '')()SJ t.11.':) flexure of, the :;:ibbon it] L discontinuous proCGSE; ',nd c tdLught [fLY' C,"..1,fjCJ

crinIcling soundstQ be emitted. A,ttempts have been iilGeO to u0ethE> X type foil in the type A micl'Ophol1.e uncorl'ugated, but there V,LS .1.,

tendency on the pc.rt of the ribbon to curl up "",t t~:8 edges into ,~ U~,s,,:,ctiO;';, the who~e ribbon becO!'lling almost rigid.. 'rhe genel'r,l condusion Ci.r:,:Till

from the VI.l.rlous cXIJ<:3rifncm,ts WC.f: tllC.t forsatlsfr.ctoIY rasul ts L'n

uncorrugated ribbon should either. be not moro then IY in length, Hj in the E.iv1.I. microphoJ.1e, 1.:>1' els8 tlwt tile Ciffec.:tiv0 gc.p L~~pth, i:hiGh lirnHs the pennissible excursion of the ribboJi, zb.ould be: et, lsud::, twieo th::t, provided. by the stnndL.rd. B.i3.C. pole piec,:;s.

(b) TrLnsvo:r3cly Corrug[ctc0 Ribbon. This is thE, coramOll-S'iJt t7pl: of ribbon r.:.nd. the one us6d jll the ty'P() A microphone 0 It po,snesses -eLL c"dVt:ntugG ti.l~"t the foil is ci vi(!(~d up into SITLD.ll ;~ections, th(;; in.(',i v:i.d.Uf I resonsnces of which do not normc.lly 'giv0tr:::ruble, 8V811 ,:.it:1 "hr'reil! foji. M\;;chc.nicu.. stability is· good. The ~,~d.itiom:,l longth of ;.'l~ te:n.:l t.:'l~(:ri nr) P:f the corrugations does· not norm[~ll:i, .exceed. 15% [ne( in PlC; ci,seoi'thin foils of soft tletc,l !D.[.y 0e luuch loss. 'rho GOl'I'Ilg:-.tions in DeB.C. microphoilostypcs A and· B have D. pitch of,cbout }L~, to the incl .... w:LtL L

roundvd profile, but tho siZE:- nnd shapeof thE.! co j'TIlgc.t:;,ons do,:s 1l0t:;,ppe[;T to h':'.ve £myeffect on the perfonncnco ,,:xcept tll'.".t v:ith ve'!.;r t,hick :.llt~,

springy foil fu"l ungu]J.l.j::- profile produces :~. tendency to intc'rlla.1rl;soncn'::e of the COI'I'Ug<.:ctiohS. In the c:::.se of th.::) X foil the co nug~ t.ion tl0Q,d 1:;0 no deeper tbcn is re qui rod. to prevent the riboon f'romeu:d.ing up s:l.c~cV,:.:;[,l;

deeper corrugl.'ttior1cl may pull .. :~t with USG [.(16 lenvG thG ribbon Si:,gging, ,.

::...: 9 -

The perfcnnc.tlco (d.' '. ;:'ihbc)ll;itlich 1,) ,pl<.:.in eXc2pt fcH" [ few tr::.llsvcrse corrug,l't,iom., ,-,t ei'Gl.'l(:U~ ellc., il'~;, :ix:, g()ue:c;:..l, ':-JimilL:':' to ouo :Ln ... h tl ~.. 1-' '1 " ~'I ."! ~ (~ •. :.'\ . r:,·-r ........ ..L ,-. +"; J.. -, • win. c_ 110 co rrugt:. l~J.Oll:;e)::vell(). 1:.J1(:) hilO .• e .... ·_llC; v, .c, " . ..'.Ct., 1" v I 0 r ... +~,.'(,J:1J ... enC:J"";J

to crinkle or. cU~'l up ~.s' .ili CES8. (~.) •

(c)' ·I,ongitn6.irwJ,ly Corrup.;; •. t"",d Ribbon. 'rhis typo of'·;or.1.bbon .Lv

rendered rigiC., foT. the greater p,_,rt of :~ t8 L:ngtl~by onc; a:I" 1'101'0

" (W'I:T.L~ '~l"Ctl'11" 1, c.o"rr~'i'"· 1"; (') ..... s ,. .r .... ,.,? + ·r'-·'.rl"'V':·' )''"''' corY'I'J:::t', t't' ,..", "1P,' b0l" n' ,:- ,.., 'ro"'j c:' <'0 ..L. ... _;) V <!.\.., c... ~.JC. '.J..-...,.l.:." ._~ ...... _ v ..... -" ,--' ~.,;:.)'-" ; J..l."oc. ..... VJ.·~ J i;J .z.,'- 'I._--'.C_

t; t Ga.ch eucl to gi Vr.) the: lleCG8 G"x'/ comp1ience, Cm r..ccoullt of tho rid;: of t8bI'S in the foil b.t the junct:Loa .01't11e 10neitudinc.l [fie: tn':.:t1sVU)'>.'::

cqrrJ.gntions this for"l of cons'(,o:uction CGn 'only 1);3 C,,::cci8G out wjth tho thioker gro.cl';;3 of foil - four Ol~ more times "vLe opti!aum ;.1,":~S, : ",0 tlwt :'.ts ff · . ., ~ .J... - .1 4 t' 't f' '1 i. , ·"t 1'" ...... "' e lGlency mcy Os 80W;;'Wl:1£;.".Lovicr ('hLn !:.L' Ol.· l; le C'tJ:~.l1G.L'.l'Ci. 7{)(; O· -"li)!JO!l.

Its :J.dV.lli1togo lies in the fccct tJ:"cct th·~ fDnQW.'1entf.:~L, }.' )~;on<"nc:(; fJ:'!>:~'c:.l;ncy ffi[l.Y bo H![~de c,uito !~igh; fay' e-xnmple, 200 cyclos, withoutinL::o(~ucing thJ highel" I!lOdof'W}d.ch [,T:J fundmnent:':.ll.y inSttPhJ.'Ctolc iron ,;~rr'( contimiou:;:J:y

. flexiblo G~lUml.;nt·.'J'hii3font~lro ~1,-;3' bOem ~tili,sod in ::3pe~iilL C[S(;3 .(fo:,' t=;xL!.Hiple, in'lic!'ophones to be 'l',ry,;cl. :L."1 the r)rE'G,~nCo of wi~,J) ...-fh·::::..,; [;. Cj'L~:LJ mochwicLl lx~'3S cut 'V{[;.s c.cosir:ed.. In ;;UC\l Q(;[:)igns, d&mping' ()fthc flIDdcJ£lentr::l I'Gson,:::nce i;:; f:ffectJl by' t:1,l 'J.)e of' clOt;'l 3~re.J'.18.pl,. CC(:~ (;2.0;:;0 to the !':i.bbon. :/loGrw.n.tc:.,l stcbility 1.;;goo( :':~lld t'hm,.jv·Gi;s~faO\r(;inent is ll£gligible.

This tYl'Je of :cibbon has tWciTI&in disa.clvunu·ges~' . In t~,lQ. find; place, only &lirn.:I:(,eci.'aIlI.Olmtofcoll1plic.n~G C,,;11 t'i obt[!.ined· froiri"ihd: few anti. corrugc. tions sot-het it is difficult v;i t.hth8 thick foil u8u1.l11y E:JIlplo./~';(;.

to bring the fundamentd. r(,;sonance below cboutfu cyclcG. " In· tl1.E;::~:"'Gonc; p16ce the end corrug;'.tiol1.[] ha.ve'tlK'liro\"irl n0.turnl r,:;solv'.ht L'lbc"G'S;oi' 'ddGL :the f1..1ndmllsnt::l is frocTl'!.entlyof the. orLor of l~OGO cycl;js. 'rj-~c 1,t-(;v:;c . ':ffect may bo "litigc.bd by i.,.rn~ngL"1gi:,o ht::.ve tilo' bf~f0nc~ing t:m~, PQrtionn of tilt) :d bbon' o1:J.tsiQC . t]:K~ mEgnetiG fLld und 'by ~c~m<uc:L."1g -(.cl(ji:r i:!loL'oct

"offoc'tJ on the totel IiJ.,;chunic~.l·_iillped.''Oi.llCe by.cxtn:. ~ie!.;vy :"-:'f~lilping,of the tYPOLII0,,"Q:fc.e;3c:c:i.1x~(;L .Even so, 'che: ~[\',sult,ing 'quc.lity hi iiot>~B gooo. ELS Uic.tobt&inGd with the trcnsvers81y cOrrtlgLtOQ X foil (i .. ltho1.ig.r:~··i t ur.y be mcntiolJuCi; in·p,~.ssing· -Gh, .. ttholongi tu~intlly COI'JUgct.ed: ~:ibbbli$ ;.'101'0

used fOl' t1':O ,mu ":. l:~~lf yeu!"J in tht!. l):£:::p8rim'3rlteJ. eOIllillontLtoX"~ hi.~nc~ lllit~phone3 wi thoLlt &1iy cotuplu.int being mL:[~0 of tll.:; ;:ffE';cts of 'ond corrugation Y'<~sonance) •

. F~ .. Y)nl th.f .. :;·.£!.bci:v(~ I'ev:~'o~j it, 1iii:l.l· 'bc:· soen tJj.,' '.I t.l-:e ol~idj~n.~~l t~~~l)~r .. l)1-'

corrugct:;.OIl u:3ed:Lnthu' ty'p(; L ri;Jbon i8' the I'lost ;3uit~'~'L,10 :f0~" it.:. pur'P0~x.,

... ~.'T: ... "j

,

- 10 -

the Quralumin ribbons at preselJ.t u;iod in 3.1. C. typo A micropl10nCJ i:;hould ultinu: .. te1.y be replacec~ by 13irni1.:;r1y :~OI':cugc.tE.:6 ribbon8 of ()8c.tien ,J.l,::.minium foil weighing approxiw .. tel~r 0.3 mg/s::·.cm. Itl the ·meuntime r ..... ~~(.tlpfc..ctOlJr substi tUt0 would be the X i'oil.3lrecLCty mentioni,,! weighing .18 mg/ dC. cm. 1.',i th which up to th,,,, pres,:mt ;.'..bout 20 roicrophon0;:;. havo 1)Gon 8XpGr1.m(mtcll~c fitted. .

i'

;·APpritnn.' i: ill.ATCHrdG TPJ,N3FOH4IER ,::::

The Gh;;.nge f'.'om t::pe A to type ;,:;. ribbon invol VG 13 c. ciiffe:rent nmtchingtx.'o.nsformer to· bring t~c microphone impodhnce to :ilo olll11iJ. Fo)"' convenience i11 mounting; t:w o;dsting size of COl'a (36 T N[u~wtcl) iU:.y l\e retc~inecicdt,hough it is notacivisc.ble to 1.UJ(~ the· ·old flt8.mpinJ8 [gdn. The new wincing consists of

.l~rilnary:· '22/1,/20 et ~ 'Vo'S ,., 2 viil~HG in pn.~""E~J .. el Secondary; 5(;0/2/36' c1.~'fDS •

. other details rem[1_in GS before.

APPENDIX. 11: ~ffiCF.JiNICdJ RiSONJIi'1CE OF OUTEli' CASING

J;t ,has been noticed tt""t the' eusing .... ,l.ti(~h covors the trcnl.)fonner OIl the type A mic;rophone gives 2. pronoUIlCed ring lvhen te.pped., c~i:ld th:i.s characteristic soUnd c~:n sometines be idontifi0(-l etS an accornpcni.J:lljrit to the .reproduction of speech. It was then.:;fore C:.CCiCl€>ci:, w:r.~en fi tt:ing X type ribbons to these microphonos, to td:'3 the opportunity of ~'-pplyi.:.1g SOl1W dv.mpingmateri<-.J. to the c;lsing. The most sct:Lsf,:·.ctory in::" to~::i.~:l fo?:' this purpose W2.S fOlmd to 00 Gomme:::-cici grade.') plt~8tic:irie j 8prOD.(~ c~v·.mly

. over the inSide. of thotn4'1sformcr cov(-:r 8.n(~ press.,;:Q firmly into eorlt,i:(~t

with the surfDce; &n( it is reCiJJl'Hll';mcieo. thDt:this 't:t-eatment ·:)loulo. ultitn''.i.teJ~J-· be nppl:i. f3a.to all type A :nicrophon,.,s. About 4 cz;~. of plasticine b.re requiT.:~d for_~':ch :mic:copb)ne D.

· ... - ",

~.x2~~~~~tf~~~~~f~4~~.~~~~~~ii"~ ==~====~----------------~==~~-------===

" 0 0 .. ~

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