Moving-Coll Telephone Receivers and Microphones * iy BC. WENTE aad A, Ie THUTRAS oeNG cot ned estes ae ow esenly wd a ‘uality vedicreceiving seta and in talking mocion picture sequputens. ‘The hie rages of the moving eal ever tie moving gesatuce driving mechani ace the absence of a static force, con ancy af fove factor and electrical Snpesiance themsghout a wide Frequemey ange, abd foesdom from ro linear detrtion over wide fame range, Lecauee of these advantages f seems obvious that {he moving coll structure co clan le war paca in head receivers fn mivoqiiones where high auulity sof prime importance. Uae “herefore been ilote! in the inetruments to be decribed, although font o the p-iniples here foesnuaced ean conceivably be apse fe fe instruments wil ymiving armacures, ‘This paper is conceed imei with the general principles of design. ‘The moze practical Phases of the sommercal design and constriction of te microphone fee dicevont in « paper by W. Cy fonee and LW, Giles" "The moving ayatem of x howl revrivar must, ia geaeral, satisfy Uistintly ifesens ceouirements rom that of a eenphone. Ia the letual wo the receiver aemall elena cavity ie Formed between the fe and the digphragm. If tier i ta be oo distortion the pressure CGhveloped within this enclosure per nit of cuereat ia te eeseving ‘oil owid3e independent ot Frequcnes, constancy of npedance of the ceil being ssumed. The pressuce depends not only upon the amplitude n? vbvatka of the dphceem, but sl upon the acral Impedance of Uhe eavicy formed hy the ear abe te receives Thia fmpesaie i sc, if she cavity i enol enclose, chat at Faw fe= fquenvion be pressures will be very eariy propnstional to the die Hlucagat ofthe diaphragm. At higher lequencos it eof uncertain Walue and varies trom ear to ear, I it appears, from unpublished ‘our Ae See Aer, Jay 195 at ay DF ncting of Ba Stan De data obtained by L. J. Sivan om a large nurater of ears chat ennstant amplitude ef motion ef the ciapkragm per unit euent cought the Frequency range ic on the avecege the best condita Uo sive for fn the design of a high qualiy recever. We shall uLeiefore mssume ‘that st any froqueney the amglitude of motion of che diaplragun per ‘init curent ie # corect memire of the response of the receiver. It ‘ll ees sn that heim ofthe eavity ie without eect fn the digsoement of the diaphragm. For the rervivers rm be con- Siece hia assumption iaueduees ut Kile eevee, aluougl the eek Js cot neghigiein gener. ‘Tae vellage generated by a moving oil ia a mageeti Feld pro portional to the velocity; theelore, the dioheagm of & xllormly eunilive miecuphone with rigily tached coil should have, xe 3 Trequenci, the same valocity per unit of presnare in the actuating sound wave. Euresed is sauther wey, if dhe diplrwsus tas oactant effetive ares, the mechanical impedance force per unit Sekcity) of a microphase diana should be the save at all ee. ‘Quencie, wbceas that ofthe ceever sold Be versely pro tial to the frequency. ‘The receiver and the microphone to be described are quite similar in desiga and conscruction, but thir dynamical eon ‘stant diler #0 as to spproech chose conditions of impedasce. Ie zceciver or mirophone ie constructed wich a diaphraga baviag a single degree of feedom, the operatinz cancsione of che diaphragr: an be reprecented by the cteut diagram shown in Fig, 1, 280 te fs thy oirs've maths: che stitfaess, ro the mechanical resisrance of dang, an #2" the aerating force een wn the dia raga The abarkee vale nf te bey a Tae shea Hen by fax ( nf ( ule ample Sy a where A= 2 eset ‘the velocses and amai- oc = (R= 2 x ros Segaens tudes ors consi foal for tro diferent values of 4, ealntaeed froin these expremonm, we grapsica-y tepzesented in Vig 1. Meth the amplitude anc velisity eurves how mide varietons in ceponse ‘with ireqeeney, "Tiny inlicate shat fo sal aviation ia aaplicade The setonant ireqvency mut be sear Ue upper mt bf the fee fo be vasamied ane Tor all vecetions in velocigy t1e damping ‘conatant mul li ig Bue igetcumentsdesizned un tris bos wold be restively Srwastve even if such conditions could be met realy ir thie coneteusian Trthe ieiaa of electieal networks for the tnsmiising a wide feesuerty nl ond 3 attained ay che combination of ture Usan tne reecnant circ, We ean advanlagemsly rosin to a sae Cxpodiene a nwrhauial ersten by che tse of teuelare mine omplicied Tat rte Laing «single destee of freedom. The dix og my 36 oped Ho ier usezaaurea vs seoustca! vaswork {EE she proper type nt 4a give us the desired uniformity nf resyumen The crcl stags of ne seh mesvanial netwolf fo ig 2 where 4 io the stitines, m, che mast, uml, une rate: of ie flemenss of she coupled nenverl, The conetrution af "eheviad srscem tspreeented oy this eizg-em ie brouzh: out iv desl in the Alscussin ot the mechanical design of the insruments which In Tolloe. he actual vavaee of the constants ner ro he a choses, if poesble, thae the meckanical impedance, = oF the whale welseork ix oustan® wich Arequezey in the ees of che micrhine snl inversely Dpropatioaal to te trequeney for the rrecine, The alunite value Of this impedance, cis vr =f where > 0 ‘Tar Rrnersormssone Reever the mechanical syetem of che receiver can be epeseeated by the circuit digcem shown in Tig. 2, chen, asthe amplitude per uit force Ie riesute of the rorciverreaponer sem may ealeulane He prod 0 Trewern gol al yok epolenaeiey character iio fae any same seo hues of the wma, Such chacaster Tacs are rapbiealy shown in Tig. 3 for several acc of values. Curves of identical character Int wf ifegent evel woud of or tine if she rune ade a each ef che corresponding imperauc ee Irene were rhongel in Ihe wane poopocion, Te ie ree (rom Ti ‘eves that, thenceielly at las, tHe posse ln iia a unifors fesounie aves ide Rrquency ranze. Curve 6, fr ean showa Triton of em shan for teguendies up ta 0,000 ep. Ak tnigit a espacted, the wider the frequency rage of monn sponse Ive lower the sersitiv'y. In ats it cam be sw oan equations 1 that if the sale of frequencies se dianyed by 2 factor, &, he vals ofthe ardinaes sil ie anchanzed provid a fy see mien hy fy ards ds, by &, bot char the amplitude per Tin Zrge st gorsolng, pete on che exeve we changed By & Fach equal to 1/84 A reeves tcanamicting up € 11.000 cp. il ‘thus bw 12 lh ese efcene thon oe -eansinieig lly well up to Daly SIN ep, the sme sen see. diaphragm Imig assume oteetin nf i Recter he geval construction fw veniver emzadving the ubuve pein- ciplee in soma sa bige 2, Th censral poi n° Hoe Mapluagen 5 fossa in the fon of serial dome to news os vigidity. The tecelving cil is for ceitaspacting bien type, che em wction Ol sich has born dered presse? This igi gv ar}ua to the Thaw we the van) poction of hw avphcsgan, The sedi anager Feld je'vel fon. a germane: magact Them asol tae diary tl thet ef the oo rman 20.) im F, 2, the sree of he Cophongat te ay and he wehancal reste be sanel solute ef air oomplecely ewrlesed tween teslipeegnt sil pe plees save fo- nacre at st 0. The acoustic esr © “al al yu seetance of lit of tia haractee fs equal to swlere pie the ven fa, the rah Hg. tae width, he a Tala lenech fest aad the dene ty ele. ho ririn the chamrbe reve sturmyrenle a mei resistance at nevlance won be Timea om the diaphram ‘iy vise of che air low Ubon the +, Thin seageeive values weld De eqs tn the acoustic esis ance and Muh ttseaemcana hy 52. sm iLL SYSTESE PrOANICLL sOUREAL the comet mushance of dle SiC multiplied by die square othe Motive area af the dlaphragm. ‘There quancicis ace reprerested by ryan jmsain Fig. 2 the ali, 9, were dled tae stillness i 10" posed by thei es athe diaphragm would be equal ta 7401", where diz she into aren of che diaphragm, V be volume of aris Tus enceence and 7 the ratio of speuibe hosts of ait, ‘This e the a fess repromrel by 28 Fig. 2 Tn aciveire Fim wilh othe ai to the desired value its cesistarce eae mesural execuutally, Hoe tis purean a steady ream of fis Toy elect as pasted in series “hrgh the it val» eapary tule. ‘The presse drep ebrough the tulle aed Gat themgl Te Fesisaive rode then roeneteed ith a mesuimetee. The eatin af these “Clues under chie condition equal she rain Uh: ease of che ihe in that of the slit. The mvilaney of ie tube laid previously Ten termined a fore af uhe presaure difecence becieca ic fun ends yehen sir we pei! Hanugh Zt at & known steady rate, The apparatus egret wanes in Big 5 heraqnane “ieuency cic-acevstcc? the recsiva: was dotermized Cexveriverilly. For these nemareigaca 2 wee placed over a eal brated condenver nieceshone so ast form a 18 ec, eaclomare betwee™ Hncswrsiver ant the uceoophone diaghvagave. This epace wee led wit hye to vil acous’e ezonauce at tae hieber Foauercies. While uzentéram a vecum tube sister wor yr lev the eciver col, t3e woltaze senecyzed boy the neon, ax ell the veoniver eusent, was manures. Trom these vase, Uke ablation curve ofthe mierephone urd the yur of the wicloaue, Ce ampli- tude of che cecsiver dlaphrasm per url= cere is reallly delernied, Values so chtined, exp:eseed in db, are plowed in Tig. lu Ue ECRIVRES AND AECRAPSONES some fgure are given values of the cespoase as determined by com ulation ofthe mechanical impedance ‘rom te enmaats of che fecever, “The wlaates were 20 adfuetel sebitcely as to bring the Computed snl chservel values into coicitente at ie Tower frequen- fice, There fs general gwen hetweaa the computed and Bl i =f | Bo ig 6am of secved curves, yt the variations ave Tergr than can be acconstel for fn Ue basi of experimental eure. [Lis prabable thet che quantities hed in the ealeulaions wre nol striely constant up to the higher Feyuences, where she diameter f the daplisgan ocosnes emparale inthe wavelength ef sound, However. cept for a depresion in the neighborhosd of 4000 eps, He measured is etter than Vie sme pated charserere ih rove'ves of this geen chacseee~ nas anpplied for the Mester Kolenewee Spates for Telephone Traverisio © in Europe ard in [Americy where ita been in service snee 925 ‘Taw Movise Con. Bhiczornoxe te has been pointed ont tat in an eleccredynamie micropme of high quality the diragn with a sgidly attached coi shoul ave the same velocity per unit of forge thronghnt the Fequency range ihe dypaaical system of the micnmhione represented By the rmerdeaieleicuit of fag. 2, thie oto reauios that che conatants ff che various elements ofthis evnit beso ebecan that te magritatle fof the impedance ze the sme #1 all roqueacies. Ie‘s evident that Nese values will difer materlly roan those of the high owsry In Fi 7 the fmpelamce expec inh as determined by equatims Ap isshown as efonclion ol beguency tr eovern ate of values of Ce staat of the ingedaace elements. Vey alow low, by the proper ‘lovee of these values, 9 unierm reponse may be cbtained over a ne SEE ee el ti ot DRI SYSTEM TRCTRICAL SOURBAL wide frequency range: Carve C, for inetance, shows variation uf Tee thaw 1-3 dl fram 200 to 1,000 eps. TE may be shown from otis {2 thai the scale of Frequeacies is changed by 2 factor & Ue Far of the response enrve will remain unchanged, provided 7 land reare multiplied by 8, andy and sw by #8; but the alolnie vale ff the velacity per unit force wil be changed by the factor 1/8 at all mute ome curve. Thus, under thes conditions, if the last vale Of the absccse in Vig. 71s deegnaced ze SOW instead of 10000 cpus, : me . maniily then 8 ~ O.Sand theearveswill enn lunged in foam bub the or ‘ares ill he raisad 623, The form of any ofthe carves of Hig. T will of enurse, not be changed “a! the eoeresponding conscants ace vue proportionally, although the absolute value of the veleity per unit of force wil vary imsura’'y wet the cmuilule of these ‘At axa frequency the velcity of she diaphragm per unit of force is ceecarily sero, In pasing (the loner frequen pint shee fare Enally reached where the reponse decrenen appweviably. Thin point depends primary upon the aifaem, sy ofthe diaphragm. fhethval for sverciming ts ls Sa oesatiity at low Frequencies wil Te deumsed Taer, Consrucion ofthe Microphone A mictoptone was constructed vesy similar in design co that of the sectives just desetbed, bus with che exp omitted im over fo eepene the diapiragm to the action ef sand waves, The dimensions of the ‘various elements were changed so thatthe impedarieof the diphagen Ith iteaeociated retwork shonld have stberantaly enastant value Throughout a we freqency ninge The reponse ae caput is shoven in Ti. "The moving miceophenewaacalibratedesperimentallyy compar sem with calbrated coideasee misophone. For thiscomparizan exch Trsontier yas mounced with fart outwatd ia an opening ia the end wall ofa eynd-iei runt 30 em in diametae and Fem. deep. “The {eo openings were spaced 280" with respect ta the axis of the drum fd on fad of Fe. Cranks between the esecophores and the ‘val wese carefully seated. ‘The wall thus forme a baile of the same feeral character foreach miecophone, The dust was mounte am a att passing vis asis, abou whic iL was rotated at a speed Poa eT | paul as es m7 : ves Fe ese te of 100 rpm. Slip rings were provided Zor malking eeetrical cone festions co the microphones. The dram six parc ia @ sound Sed fet up iy = anoving eo loud spealeer sped with cxrvent from a Vacuum lube veilaton, ‘Te sweege generated by each anrophone twa then messured with :4w arnpifer sued Cherme-galvanometer With this arrangement eset wicrophane passed through prectieally he tame sour fehl.” virae of the eyrmetoal hare af Ce ti iis 1oution hoe very lire ilvenen om say lading ave pattern ia the room. "A check on the reliably of the measurements thas the face thet te tio of sh oud epesker was camged very Tete citfecence was olie-ve! iv he ratio of the volagee generated fevetl az the higher frecuences Likewise, no change wee observe) lwhen the elertredswanis ieopone was moved a small distance silly in ite mousting. ‘The condeases mieropnone used jn these tests had bees calibrated by means uf thermepines, bub a costection teas sade for the resonance die to the cavity wwe the face of the Giapnrage, which is mot snessured ix Lie Uesnophone calibration Te response tle tee a detratined i thie way i showy the curve din Fi & ‘The disgreement between dhe olnerved and computed values at the higher freetses is Teeved to be due to sesonance oscillations tvithin the airsdioner beneath the diaphragm. In order ¢0 reduce ‘he maguial of dase osillations the chamber was comnested Cough f nario sit 7, (Fig: 8) t0 a emall cavity forme) within the cential st ann svete TRC! polepiece. With this change in construction, the microphone was fata calibrated. The reaults olan inthis ease atthe higher fre ‘quencis are chown by curve B of ig. 8 Tes scen shat the response ef the microphone is quite uniform over a wide resaeey ranee, but thac it decreares a the lores frequencies, ‘This decree ex be avoided by a reduc expedient has the pracies|dicadvantage that ic makes the teas rere elicate sox) increnes ts susceptibility to mechanical ilar, The reson a hee Frajces sim Be inereaged more profitably by a staple modiceation which iareaaes the force an the Siagragioueder te action of ound waves, Ifthe airspace enlowed hy the magnet on the sear of the diaparazm is connected with the ‘outside ai dough a tube, then, anéer the action of saund, a presure Will be developed within this epace through she cube, diffsiag in Thagaituse and phace from that of the eouad outside, This prewure fete on he rear of the diaphragm. Lover crim creumatances the total force on the diaphragm will Ue increwed Uy virtue of this ‘The riiomphone shun fn Fig. provided with a cul for pe forming ths lanction. The amatie anpedaace of tube any be caeulated fromm the formula 1 1p 2 en % eae in which & = \jajps, UUs the length and r the cahon of the tube, a the viscosity and p the density of air. At Tow Trequencies, Z may be