audio research villchur article audio 1954

7/29/2019 Audio Research Villchur Article AUDIO 1954

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Revolutionary Loudspeakerand Enclosure

The author describes a fundamentally new loudspeaker system whose 12-inchwoofer util izes an enclosure volume of only 1.7 cubic feet, but whose bass performance is claimed to be superi or to that of a true infinite baffle installation.

TRE E OUT STANDING PROBLEMS thatstill plague the field of loudspeakerdesign 11Iay be categorized as :1. Ho w to keep harmonic distort ionlow in the frequeue), region below 70 or80 cps, especially at high power.2. Ho w to keep frequcncy responseUlIlJorlll and ex tended at all power levels .3. How to solve the above two prob-

lems without requiring arch itectura l in-sta llations, vcry lar ge cabi nets, and difficult final adju stm ents.The loudspeaker system here describedis the fruit of an investigation that wasprim ar i ly di rected towards solvi ng th efirs t of th ese pro ,lcI115 , tha t is, towardscreating an electro-acoustic transduce r

that made 11 0 compromise with low di stortion bass down to 40 cps. Th e solution to the distortion problem turned outat the same time to be a solution to theproblems of uniform bass frequency response and of cabinet si7.e.Th e g rea test source of dis tortion in atypical high-q ua lity reproduc ing system

is the loudspeaker. Speaker harmonicdistortion in the bass range is toleratedin amounts far g re.lter than would everbe allowed in the a mplifier or pickup-values between 5 and 10 per cent below60 cps a nd at moderate power a re common even in high- quality units. Thegrea test single source of distortion in theloudspeaker itself is the non-linearity ofthe voice-coi l and rim suspensions whichhold the cone and voice-coi l to thespeaker frame. Th e elastic st iffness ofthe suspending members, a propertywhich they must possess in order to per-form their functions properly, docs notremain constant over the excursive pathof the cone: the further the cone movesfro111 its cent ral po sitio n the g reater isthe resist ing" force constant of the suspensions.T he design of the se suspens ions andof the spe;lker 's m:)Ving system has beenrefined but not cha nged radically overthe last tw enty years or so. The s ituationis comparable to that of the acousti cphonograph in the nineteen twenties-there wasn 't much further to go in th edirection of improved performance untildesigners relraced their steps, back to

*Presidcn t, Acollstic RcS'arclz , fI l C. , 23Mt. Auburli St ., Cnlllbridge 38, Mass.

EDGAR M. VILLCHUR*th e basic problems assoc iated with converting needle vibrations to sound, andapplied a new approach, the electricalone. In the present case, instead of at tempting to re-design an a lready re-fin ed mechanical suspens ion sys tem fora linear force displacement relationship.the clas tic stiffness of the mechanicalsuspension system was substantiallyeliminated, and a lin ear , acolfstic elas ticity used instead. Thus, the dominationof voice-coi l Illotion by the non-linearelastic mechanical suspensions was alsosuhstantially eliminated. The phrase"substantia lly eliminated" can meanmany things ; here it is used to denotereduction by a factor hetween 6 and 10.Acoustic Elasticity

Th e acoustic elasticity is provided bythe enclosure's sea led- in air, which mustbe compressed when the cone movesback, and rarefied or stretched when thecone moves forward. In other words theai r of the enc1nsure is used as an elastich i o n , which supplies to the specialspeaker the restoring force that the moving sys tem is by design deficient in, andthat it needs.

Such use of the enclosure's ai r turnsou t to have 1110St fortuna te consequences,and it is possible to reap large ex tradiv id ends over and above the reductionof di stortion. Th e amount of acousticclastic stiffness avai lable is determinedby the cubic volume of the enclosure; th ecubic volume which must be provided(not as a minimum but as an optimumvallie) is of the order of one-fifth thevolume of a conventional tota lly enclosedcabinet for an equivalent speaker mechani slll.The fun ction of an infinite baffie ortotally enclosed cabinet is to provideacoustica l separation between the wavesradiated by the front and back surfacesof the speaker cone, waves which areout- of-phase and would cancel at lowerfrequencies. One may ask theil , why ithas not been possible to simply house aspeaker in any small enclosed box , oreven to close up the back of the speakerframe so that it is airtight, in order toachieve the necessary separation. Th eanswer lies in this same acoust ic elas ticity refer red to, wh ich increasesthe elastic stiffness of the speaker's mov-

Reprinted f r om AUDIO - October 1954

ing system and rai ses its main resonantfrequency . The nature of modern loudspeakers is such that below the resonantfrequency response fa lls off rapidly- atthe rate of 12 db per octave, in terms ofpressure, in an undamped unit.Suppose, fo r example, we have a 12-in. loudspeaker mechanism who se mainresonance occurs at SO cps in free ai r.H we now mount the speaker in a wa llthe resonant frequency will drop due tothe ai r load mass, perhaps to 45 cps, andif the speaker has been welt designed wecan expect good response to ~ o l 1 1 e tbelow 40 cps, with about 6 db of attenuation at 32 cps.

I f we now take this same loudspeakermechanism and mount it instead in aconventional totally enclosed cabinet (asecond choice dictated by the landlord)we will find that the resonant frequencyis raised by th e addi tional aco ustic stiffness of th e enclosed a ir. Probably thebest that we can hope fo r is to keep theresonant frequency at about SO cps, anac hieve men t that will certainly requirea cabinet volume of over 10 cu. ft. Acabinet of 5 cu. ft. will raise the resonant frequency into the 60 cps reg ion.and the system will suffer a corresponding loss of bass response.The problem, then, resolves itself int othe se terms: how provide completeacoustic separation between the fron tand back of the speaker cone, w ith outrai s ing the re so nant fr equency abovewhat we want it to be, and with out awa ll installation or a monster cab inet?Th e answer dovetails with the so lutionfor suspen sion distortion referred to previously. We select the values of massand elasticity for our speaker ~ y s t e m asfor a conventional speaker , on the basisof the resonant frequency we decideupon. V'I.'e then design the speaker mechanism with perhaps only 10 per cent ofthe elastic stiffness that it needs, sothat the resonant frequency for the unmounted speaker mechani sm is subsonic,of the order of 10 cps. Fo r reasons thatwi ll be apparent a litt le later , this speakermechanism is useless as a bass speakerin any conventional mounting- whichwas not des igned for lO-cps resonancebut for 45-cps resonance.Th e final step in the construction ofthe complete speaker system followslog ica ll y. VI/e enclose the back of the


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'Jil l 1 I,= I ", Fig. 2. Experimental enclosure , showing Fiberglas made up in cheesecloth-covered "p illows ,"The enclosed volume of air, rather than mechanical suspensions, supplies clastic restoring forceto the special 12-inch sp eaker.

speaker wi th an acoustica lly scaled volume of ai r wh ich will supply the remain ing 90 per cent of the clastic sti ffness tothe moving system, and whi ch will raisethe resonan t fr equency to 4S cps. T heinterior vo lume of th e experimenta lacoustic suspension speaker, using a 12-inch woofer and des igned according tothis principle, is 1.7 cubic feet. Increasing the cubic volume will not improvethe performance of the system, but willdegrade it.We can now compare the character istic s of th e infinite bame wi th corresponding characteristics of the acoust ic suspension sys tem. Th is is done in Table 1.Speaker Restoring Force

Speaker suspe nsions serve two purposes, that of centering the voice co il inthe magnetic gap so that it does not rub ,and that of providing elastic restoringforce to the moving system. Th e restoring force of a particular speaker cannotbe decreased below an optimum va lue forthat speaker. Too Iowan elastic stiffnesswill result in increased bass distortion,as the vo ice-coil w ill travel out of thepath of linear magnetic fl ux on highamplitude low-f requency signal s, or willactually "bottom" aga inst parts o f th emagnet structure.Th e same principle may be explainedin terms of the main resonant frequencyof the speakcr, whi ch, as we have seen,is determined by the values of elasticityand mass, both mechanical and acousticaL of the suspended sys tem. Otherthings be in g equal it is des irab le to havespeaker reso nance as low in frequcncyas poss ible, but too 10\ \ ' a resonant frequency results in cone excursions toog rea t for the length of the magnetic pathprov ided by the particular speaker.Voice-coil excursion in the bass, forconsta nt rad iated power, must be quad-

rupled for each lower octave, and theattenuation of response below resonanceprotects the speaker aga inst over-largeexcursions.Thus when a speaker is designed withthe correct resonant frequency, voicecoil excurs ion is always kept within thelimits of linear flux for a ll s ignals, rega rdless of frequency, up to rated power.Small speakers which can onl y a llowshort voice-co il travel relative to theirpower rating, and which provide relatively poor coup ling to the ai r ar e properly assign ed hi gh resonant frequenc ies,while speakers which allow g reater excur sion, or can radiate the same powerwith less excursion due to some spec ialmeans for matching them to the ai r,(s uch as a horn , for example) can begiven lower resonant freque ncies.\ ith the unders tanding, then, that thenon-linearity of the speaker's elastic resto ring force cannot be cured by re movin g or reducing the restoring force itself, the necessity fo r substituting anacoustic restoring force for the dec imated mechanical one becomes apparent.Boyle's Jaw tells us that the restoringforce will be symmetrical-that it will bethe same coming and go ing. Acousticpressure is a function of volume, and itmakes no difference that the va riationsin pressure occur above and below normal atmospheric pressure as a referenceleve l. When the cone moves back the en":closure press ure on the back of the coneis greater than the atmospheric pressureon the front su rface; when the conemoves forwa rd the atmospheric pressureon the front of the cone is g reater thanthe press ure of the ra refied enclosed ai ron the back surface.Fo r twenty-five years the a ir inspeaker enclosures has been consideredan unavoidable evil. I t has been unavo idable because of the necessi ty for provid-

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Fig. 1.axis, of

/ f-"INPUT: 1 WATT AT 300


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little unreal to hear the fundamental sof organ pedal notes, which could befelt as. well as heard, issuing from thi slittle box.Later measurements of frequency response and harmonic distorti on indicatedthe reasons for the bass sounding as itdid. Figure 1 shows th e bass frequencyresponse of the experimental mode l,take n under open fie ld condi tions. Bassresponse uniform within 10 db, asindicated in Fig. 1, wou ld be ordinaryfor an amplifier, but is quite unusualfor a loudspeaker sys telll . This uniformity of response partly results from thefact that the resto rin g force is appliedsmooth ly to the whole of the cone surfaces, rather than to the apex an d rimof th e cone by 1 l l e h a l 1 i c a ~ suspensions,and partly fro 111 the optimum dampingof the resonant peak.The practical resul t of such uniformresponse is the ab sence of boominess.Speech program materia l, which norl1Ia lly contains 110 energy below 100 cps,gives no hint of the fact that the wooferrea ches down into the low bass. Organpedal notes, bowed o r plucked doublebasses, ctc., ar e reproduced true in pitch; l l ld without ringing.It must be emphasized that the reproduced response curve is for a completesys tem rather than fo r a loudspeakermechanism alone, mounted as the tes tinglaboratory sees fit. As an illustr ation ofthe necessity for care in in terpretingresponse curves for loudspeakers alone,it has been demons trated that va riationsin mounting the same speaker in different commercial cabinets can change theeffective bass cut-off frequency by anoctave, and the amplitude of the bassresonant peak by more than 10 db.I t must also be emphasiz ed that theresonant frequency of 45 cps is for thecomplete sys tem rather than [or an unmount ed speaker mechan ism, or for aspeaker mechanism mounted by the testing laboratory in an infinite baffie. Theva lue of 45 cps was chosen to g ive fu llrC3pollse down to slightl y low er than40 cps ; thi s low-frequency limit wasdetermined to be as low as practicallyrequired. Although the above determi nation was made on the bas is of directexperiment with various types of prograll1111aterial, it is supportcd by au thorities in the field, such as 01so l1. 1T he harmonic di storti on of the experimental model speaker was reduced, fromthat of the control model in the infinitebame, by a factor of about three. Th eharll10nic distortion of a later modelwas measured by an olltside testinglaboratory and found to reach 1.4 perccnt at 46 cps, 10 watts l 1 p u t . ~ It wi lltherefore be see n that thi s speaker sys-

I Harry F. OI SOIl, "Ehments oj AeoflsticalEngineeriug," D. Van Nostrand Co., 2ndcd., 1949, p. 4i7. Dr. 0 150 11 lists 40 cps asthe low-frequcncy limit required for the reproduction of orchcst ral mu sic with perfectrldeli ty.: These figures wc re takcn with th e electrical input as reference lcvel, and arc therefore fav ored by the low efficiency of thesystem.

TABLE ICompororiYe characterist ics of on infinite baffle, a 12-cu . ft . totally enclosed con y?ntiona I cabinet, ..nd the ..caustic system, all usin g a 12-in. spe .ker mechanism

Distortion Raising ofdue to resonantsuspensions fr equencyabove de-sired valueInf inite Am ount normal Ver;baffle and to speake rs of Slightinstallation current desig n

ll-cu . ft . totally Slightly les s S ligh tenclosed conven_ than abovetion al cabinet

Acoustic Practically Nonesuspensio n non-exis tentsystem

tem ha s not been designed as a compromise /Isma il unit," and it was not intended that a handicap we ighting of itsperformance be a llotted to it because ofthe small size of its enclosur e. It is theauthor 's op inion that the bass performance of a speaker with given magneticand electrical design will be optimum,a t the present sta te of the art, when amov ing system with the acoustical suspens ion is utilized; the small enclosureno t on ly entails no penalties but contributes a t remendous advantage from thepoint of v iew of performance quali ty.It is anticipated that the acoustical suspension principle will become increasin g ly un iversal in th e ind ustry, and willhave ge nera l application to speaker systems of all s izes. One obvious appl ica -

Acoustic Resistive Introductionseparatien damping of acousticbetween on cono resonancesfront andb


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resonant impedance peak is broadened.The Q can be contro lled at will ; toolittle F ibe rglas yields an uutput peak inthe bass, while too much ovcrdamps thesystem and attenuates the bass response.Since the enclosure is actua lly an intcgral part of the speaker it cannot bebuilt like conventiona l cabinets. Anacousti cal seal must be provided, a ndthe unrelieved pressures that are builtup arc so great that an unusually largenumbcr of ribs a nd braces are required,together with ~ walls. Figure 3is a photograph of the exper im entalmodel of the loudspeaker, its outsidedimensions measuring 19 x 19 x 11 in.l t uses a twelve-inch woofer and an outside tweeter, the latter not shown. Theproduction model of the acoustical suspension loudspeaker that will be exhibited at Th e Audio Fair wi ll not besquare, for greater convenience in useon shelves or bookcases, and will utilizea twel ve-inch woofer with 52 -oz. Alnico5 magnet, plus a high-frequcncy sectionmounted within the enclosure. Eitherthe speaker mechanism or the enclosureis useless by itself. The spcakcr mechanism a lone is only half a speaker ,and mounting it in any convcntionalcabi nct would be no more feas ible thanmounting a conventional twelve-inchspeakcr in the totally enclosed cabinetof approximately 11 cubic ft. interiorvolu l1lc.

Figlfre 4 illustrates by el ectrical analogy the substitution of acoustical formechanical stiffness in the mov ing system. T he total compliance, that is,C llpefl!.c r Cull'C 8l lcaker X C (l h .'

I '::; not changed by the new design fromits opt imum value in terms of the movingmass and length of linear magnetic path.An examina tion of the negat ive sideof the ledger will reveal the fact thatthe acoustic suspens ion speakcr systemis relatively ine fficient. The efficiencyra ting does not, however, fall outsidethe range of values for conventionaldirec t radiators and a lO-watt amplifieris sufficicnt to provide ample volume fora room of 3,000 cubic fee t. Some of therea sons for the relative inefficiency are:1. Thcre are no acoustical resonatorsemploy cd in coupling the cone to the air.

2. The moving system has purposelybcen g iv en a low QJ to damp the bassresonant peak, and has a relatively highmass reactance in the commercial model.3. The voice-coil gap cannot be madevery narrow due to the nature of thecentering spider, although the gap widthdoes !lot fa ll out side the range of valuesfor convent iona l units. The acousticalsuspension speaker system can, of course,be used as the dr iver unit for a hornwhere hi gh efficiency is req uircd.A patent application for the speakersystcm described ill thi s articl e has beenfiled by th e auth or.

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J SP(AKtll [NON -LINEAR_ VERY LOW STIFFNESS REACTANCET- (ALMOST SHOR T)_ _ - - - - i C. 11 rUN EAR '\.I:.' LHIGH STIFFNESS REACTANCE

MASS OF SPEAKERMOVING SYSTEMAND AIR LOAD

IS}Fig. 4. (a) Elec trical-m ec hanical anal ogy of the mo ving system of a conventianal speaker in atotally enclased cabinet. Since C ' l r is made as large as poss ible, the non - linear C . l.nl

audio research villchur article audio 1954

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