the authoritative magazine about high-fidelity

,SOUNDREINFORCEMENTINCONCERT HALLS

Fig. 1-Electrostatic panels, placed on risers behind the National Symphony Orchestra,reinforce sound from the guitar soloist.

Fig. 2-Speakers used during this performance were effective in all three positions noted.However, Position "C" was used to avoid obstructing the view of the conductor.

THE PROBLEM OF AMPLIFYING asound of limited output so it can beheard by a large number of peopleis not a recent problem. Until thedevelopment of electronic amplifiersand supporting equipment, the onlysolution was to design instrumentsand train voices for maximum pro-jection.

The evolvement of our present-day musical instruments can par-tially be traced to the need forreaching specific size audiences.Horns, trumpets, and bugles hadmany uses where sound had to carryover great distances. On the otherhand, strings, woodwinds, and key-board instruments such as the harp-sichord were used to delight nobilityin small salons or, at most, in inti-mate concert halls.

As the halls got larger, the orches-tras grew in size, too. The harpsi-chord became a pianoforte, whichultimately became a concert grand.The technique of singing faced chal-lenges as audiences expanded. Onewonders if the brilliant virtuosos ofthe 18th century could shake therafters of the 3800-seat Metropoli-tan Opera like a Corelli or Nilsson.

Then along came the vacuumtube, the microphone, the loud-speaker. The politician could forgethow to project. Crooners could putdown their megaphones and pick upa microphone. Broadway theatersbecame wired wonderlands withmikes and horns hanging on thewhispered sounds of today's voice-less stars. Yet, despite the prolifera-

Fig. 3-Constitution Hallas viewedfromthe stage.Agreatamountof sound energy must be generated to reach 3,811 listeners inthis one-million cubic-foot auditorium.

Fig. 4-Photographs takenduring the performanceshow laurindoAlmeida, guitarist, accompanied by the National Symphony·Orchestra.Note positionof the condensor microphone.

tion of public address systems, theconcert hall remained aloof until re-cent years.

Now the cost of putting on a con-cert hall performance is so enormousthat room for more concert-goersmust be found, hang the acoustics.So, if a 1500-seat auditorium is con-sidered good size in Europe, 3000is marginal here. In fact, 4000- and5000-seat auditoriums are becomingmore commonplace. To help balancedeficits, our major orchestras aregoing out into the parks to play fortens of thousands.

The jazz groups are climbing outof the speakeasies and into the bigauditoriums. The folksingers areright there with them. The long-playing record has introduced tothousands the beauties of the ba-roque. The harpsichord, the classicguitar have both become "in."

What happens to our hi-fi enthusi-ast when he attends a live concert?Too often he is disappointed. Thesound is faint, indistinct, weak. Acough covers a chord. The live soundis sometimes a "let down" com-pared to the clarity of a home musicsystem!

To meet the challenge of largerareas to be covered by sound, con-cert managers and producers arenow looking at electronic sound re-inforcement as a solution. And toreproduce the original sound sourcewith as little coloration as possible,many have turned to products fa-miliar to high fidelity salons.

The hall already had a fine ampli-fication system at the time this con-

cert was to be given. Though idealfor voice, however, it was notdeemed suitabile for some soloistsscheduled to perform at Constitu-tion Hall, namely a guitarist and aharpsichordist. This is what inducedthe National Symphony Orchestra'sconductor, Howard Mitchell, andmanager, M. Robert Rogers, to em-ploy a different sound system. Afull-range stereo electrostatic loudc

speaker/solid-state amplifier system-the Acoustech Ten-was loanedto the orchestra by a local hi-fidealer for this purpose.

To achieve proper placement, a"dry run" was made with a local,Sophocles Papas, standing in for theactual soloist, Brazilian guitaristLaurindo Almeida.

Sound reinforcementat Constitution Hall

This example of how sound rein-forcement enhanced a concert hallperformance was observed at 'vVash-ington, D. C.'s Constitution Hall nottoo long ago.

Constitution Hall is one of thelarger concert halls in the country(over one million cubic feet ofspace) with seating for 3,811 lis-teners. The National SymphonyOrchestra of Washington, D. C.,Howard Mitchell, music director,performs here in an acoustically-liveenvironment (reverberation time:1.9 seconds). The hall is designedwithout overhanging balconies; thebalcony is a horseshoe surroundingthe orchestra seats which are in thearena below, the stage being at the

open end of the horseshoe.To preserve the illusion that the

sound was coming only from thesoloist, the speaker panels wereplaced first in Position A and thenin Position B (Fig. 2). As electro-static panels radiate as much soundto the rear as they do to the front,the orchestral musicians had an op-portunity to hear the soloist, notusually possible with a guitar soloist.

Acoustic feedback was minimalfor a number of reasons. The Tensystem, for example, incorporates abass contour which permits the lowfrequencies to be tailored for opti·mum bass response with minimumfeedback. And placement of an Alteccondenser mike directly in front ofthe guitar made use of the instru-ment's shielding effect to further re-duce feedback possibilities.

The next day's orchestral rehear-sal with Almeida introduced severalproblems. The orchestra sitting be-hind the panels heard the guitar soclearly that they misjudged the vol-ume of sound they needed to pro-duce. As a result, the orchestrastarted by playing too loudly. LloydGeisler, the orchestra's associateconductor and conductor of this con-cert, effectively remedied that prob-lem. The second difficulty requiredmore drastic action. To facilitatemoving the panels off and on stage,they had been placed on dollies. Asthey stand six feet high to start with,this raised them to a height wherethey interfered with the sight linesfrom musicians to conductor.

Ironically, the radiators used in

Fig. 5-Washington, D, C. guitarist, Sophocles Papas, participated in testing the soundenhancement system on stage at Constitution Hall before arrival of featured soloist,Laurindo Almeida.

the panels are only 9" by 12" andcan be mounted in any configura-tion. The panels could be low,square, curved - any way desired.Unfortunately, in the time available,the standard home model had to beused.

The only solution was to move thepanels behind the orchestra andabout 30 inches in front of a newacoustic screen along the back of thestage. They were placed about 25feet apart and about 30 feet behindthe soloist. (Position C in Fig. 2.)The sound from the back of thespeakers hit the screen and was re-flected out to the audience. The onedisadvantage of this position was thepossibility that those sitting close to

Inside anElectrostatic

Speaker

Electrostatic speakers, sometimescalled condenser speakers, have cap-tured the imagination of audio buffsfrom time to time. Today there areat least three full-range electrostaticspeaker systems, not to mention anumber of electrostatic tweeters, onthe market.

the stage would notice sound corn-ing from the back of the stage apartfrom that coming from the soloist.

The concert proved these fearsgroundless. Charles Crowder, musiccritic of the Washington Post, re-ported as follows:

"... An extraordinary innovationin guitar playing was displayedlast night at Constitution Hallwhen the National Symphonybrought Laurindo Almeida to playthe Villa-Lobos Concerto for thatinstrument.

"Guitarists are the rage nowadays,but until last night a guitar recitalin Constitution Hall with its 3811seats was unthinkable. During the

Fig. 1-Power pack used with Acous-tech-Ten full-range speaker.

Low mass and having the abilityto be driven uniformly over theentire diaphragm doubtlessly con-tributes to its appeal. Sales deter-rents for the masses (as opposed tothe cognoscenti), however, includeShoji-screen size (for full-rangeunits), more stringent amplifier

first two movements of the Con-certo, my ears picked up the in-timate, resonant sounds of theinstrument from my seat, ratherclose to the stage. There is nothingunusual about that, for I was sit-ting close enough to claim the van-tage point of a rather large livingroom."For the third movement, I movedto the extreme rear and the soundremained the same. _Guitarists,gear your sights on the" bighalls .... "The audience reaction was equally

enthusiastic. Almeida was calledupon to perform five encores, a re-flection not only of his virtuosity butalso on the ability of the listeners tohear distinctly what he was playing.

An interesting observation wasmade by Crowder in his review con-cerning the first two movements ofthe concerto. At this point he wassitting about 50 feet from the soloistand thought that what he heard mustbe coming from the performer, notthe speakers. However, in rehear-sal, the sound had been checked at apoint not far from his seat. With thereinforcement turned off, the guitarwas inaudible!

Before the concerto began, Rogersspoke to the audience about thenovel experiment they were going towitness. He concluded his remarkswith the observation that the soundsystem was to operate like a well-designed brassiere. "It should en-hance, not replace." .IE

requirements than for dynamicspeakers, and relatively high cost.Nonetheless, sound quality fromelectrostatics has been lauded bymany people. So it is particularlyinteresting to observe its use in theconcert hall, and to examine theinnards of the type used.

An Acoustech Ten electrostaticspeaker panel (the type utilized atthe Washington, D. C., concert de-scribed in this issue) incorporates13 electrostatic radiators. One is forhigh frequencies, the other twelvecover the middle and low fre-quencies. The size, resonance, andchoice of diaphragm material differ,but the construction and design of

(Continued on page 34)

the two types have broad similari-ties. (Whereas electrostatic tweetersare used in conjunction with dy-namic woofers in many contem-porary systems, the electrostaticwoofer and midrange is unusual.)

When thinking of an electrostaticradiator, think of a sandwich. Twoplastic flat molds on the outside rep-resent the slices of bread. Inside,instead of ham (or corned beef) is athin plastic diaphragm. A fixed d.c.bias is applied to the diaphragm.The audio signal is carried on thetwo outside forms, causing the dia-phragm to move forward and back-ward in accordance with the signal.The movement is very small; it isthe large surface that provides suffi-cient air movement to reproduce lowfrequencies.

While it sounds simple in theory,building a full-range electrostatic isanything but simple. Much researchand experimentation has gone intoperfecting the chemical compoundsused to coat the elements. Empiri-cal data based on years of use hasbeen significant.

A plastic molded frame (one ofthe slices of bread) is 9" by 12" insize. The size and number of holeshas a direct bearing on performance.Actually, every part of the assemblyis critical. One side is silk-screenedwith a conduct.ive compound, theprincipal ingredient being carbon.TWhen dry, <l protectiv~ insulatedcoat is screen.ed over this. Thenvarious terminals and mountingcontacts (silver conductive paint isused) are mounted. On the inside ofthe frame, a ribbon of conductivepaint is carefully applied.

The frame is then placed on acustom-built machine which lookssuspiciously like a Rube Goldberginvention (Fig. 2). Contact cementis placed around the edges of themold; the plastic diaphragm isstretched across the frame andtightened. At the proper tension, theIrtachine stops and firms the dia-phragm down ~n the frame. A de-viation of a few cycles on thediaphragm resonance can seriouslyaffect results. The exposed surfaceof the diaphragm is then coatedwi th another specially preparedcompound to make the surface con-ductive but with a high degree ofresistance. Not only is the formulaimportant, but the thickness of thecoat is equally so.

Fig. 2-Custom-built machine stretchesand mounts diaphragm material tomolded frame.

The radiator is now in the formof an open-faced sandwich. Tocomplete the assembly another silk-screened frame (but without a dia-phragm) is attached (Fig. 3). Nylonrods hold the entire device together.

How, you may ask, does the d.c.bias (over 6000 Volts) reach thepiastic diaphragm inside the sand-wich? On' the outside of the mold isa silver contact point.. The contactpoint connects to the ribbon of con-ductive paint on the inside of theframe. The ribbon, in turn, is indirect contact with the surface ofthe diaphragm, thus carrying thebias voltage to its destination.

Though construction involvesgreat precision and care, the fin-ished radiators are astoundinglyrugged and trouble-free.

The radiators are mounted on awooden frame, terminals connectedtogether, &nd the asselYJbled panelappears as in Fig. 4. The panel isplaced in ,:"calibrated reverberation

Fig. 3-Twelve woofer-midrange andone tweeter radiators are mounted ona wdoden frame to create a completefull-range electrostatic speaker system.

room. A series of tests, using pinknOIse primarily, are performed.Final panels are so similar thatmatching for stereo systems is un-necessary.

Separate coupling transformersare provided in this system for thetweeter and the woofer. Each trans-former is directly coupled to its ownsolid-state power amplifier. Theseparation of high and low fre-quencies is accomplished electroni-cally ahead of the amplifiers.

\.T""/\

.~

. ':I 0! •

, \;, i~}~;, : _.7 ,.-.

:-""

- ~ - . 1 ",-. . :;: .

:;,' _ /.f.f _ .. - .' . : - 6

Fig. 4-The electrostatic radiator re-sembles a sandwich. The stretcheddiaphragm, coated with conductivematerial, is situated between the twoplastic frames. The black, silk-screenedarea in the center of ,the assembledradiator (foreground) accepts audiosignals, causing the diaphragm to moveforward and back.

Theoret;cally, the amplifier Ispeaker system can operate withsignal from any preamplifier ortuner. However, as' the system isseparated from the preamplifier bya distance of 10 to 200 feet, it isdesirable for the output impedanceof the preamplifier to be quite low(200 Ohms is a good figure).

Customizing a system for profes-sional application is simple. Thesmall radiators can be mounted inmany ways, in many locations, inU1;1imited combinations, and by add·ing more tweeters or less woofers asdesired. It is even possible to mountthem in the wall (allowing extraunits to compensate for the loss ofthe back radiation).

Where maximum sound level withminimum power input is desired,the electrostatic is not the answer.Next to an electrostatic, all speakersare efficient. For this reason, over500 Watts of transient power is pro-vided in the four amplifiers builtinto the Ten system. The forte ofthe system, says its manufacturer, isminimum coloration of sound. JE

Potted woofer step-up trans-former and bios supply isextra size for optimum repro-duction of low frequenc ies.

Oversize power tra:1sformerprovides power to give front-row symphony hall intensi-ties.

Plug-in gloss·epoxy printedc i r cui t board 5 - one forwoofer section, the other for

tweeter section.

Silicon output transistorsused for both woofer andtweeter sections for out-stCJnding reliability and per-

formance.

Enccsed ~eloy turns systemon end off when Ac.oL'stechVI control center is turnedon and off.

frequency co~tour cl-leviotes the sometirr.es un-

plecsant cmphGsis of low-frequency bonds c~used byroom resonances.

What the Experts say about the Acoustech X

"In factt the system is so clean that we found

ourselves runningit at levels perhaps 3 to 8 db.

higher than we normally use for prolonged lis-

tening sessions, and these levels were toler-

ablet even enjoyable."f--------------.------ ..-.----.-.-.---r- .._.._ _ _-._ , ..- """.

"On the other hand when the volume was low " ... the Model X accommodates itself to lower

the quality still remained crisp and clear "- no listening levelst tOOt and with no dropout of

blanketing of strings or brasst for instancet and musical material. Instrumentst the voicet small

no obliteration of transients." ensembles qnd the formidable arrays of grand

opera compan i es all soun ded natural and in

proper acoustic focus. "

Percy Wi I son in

THE GRAMOPHONESeptember 1965.

" ... Wonderfully good at all volume levelst

whether from voices or instruments."

SOUNDQUALITY

LOUDLEVELS

"Even when the volume was high I could detect

no listener fatigue. Nor did my wife, which is

much more significantt for she always declares

that I play too loudly!"

SOFTLEVELS

HIGH FIDELITY MAGAZINEOctober 1965

" ... a top quality speakert with a strongt

cleant open and well defined response ... its

definitiont transient responset and freedom from

boxiness or artifi cial resonance's is readi Iy ap-

parent and something that a perfectionist con

appreci ate •.. "

SPECIFICATIONSMaximum permissible output impedance of Standard Frame Finish: Oiled walnut (othercontrol center used with this system: 200 ohms. finishes available on special order).AC power requirements: 20-700 watts, 50-60· . .cycle, 110-125 V.(220 volt models available at G~~" cloth: Neutral boucle grill, easily replace-no extra charge on special order). AC power is a e.applied when, and only when, a 6 volt DC signal Price: Stereo amplifier/speaker system compris-is received from the Acoustech control center. ing two full·range electrostatic panels, two solidDimensions: Panels stand 6' nigh (including state dual amplifiers, and two fifty foot lengthsbase), 263,4'1 wide, and are 4'1 deep. Base con· of special three-conductor cable connectingtaining amplifiers is 11/2' deep (from front of control center to the system: $1,690.00panel to rear of amplifier), 263/4'1 wide, and 6112" ($1,699.00 west of Rockies). Additional 50 foothigh. cable extensions - $7.50 each. Control centerShipping weight: 225 Ibs. extra.

Acoustech XFull Range, Stereo Electrostatic LoudspeakerSolid-State Amplifier System*

"The finest stereo reproduction thatit has ever, and anywhere, been mygood fortune to witness ... the newsystem is some five years ahead."

Percy Wilson,The GramoPDone, Sept. 1965

"The Acoustech X," wrote High Fidelity Maga-zine (October, 1965), "represents an aII-outform of component integration involving ad-vanced amplifier and speaker techniques on anelaborate scale not hitherto encountered inequipment offered for home use. It is, essen-tially, a reproducing system, embodying ...basic or power amplifiers and speaker systems- all of which are designed to be used witheach other. The speakers are full-range elec-trostatics that are driven by separate poweramplifiers which fit into the bottom of eachpanel and form a base for supporting the sys-tem. Each of these two amplifiers is itself a'bi-amplifier' with separate circuitry and suit-able dividing networks for handling frequenciesbelow and above 1,300 cps ... it is recom-mended that for best sound a preamp with anoutput impedance of 200 ohms or less be usedahead of it. The Acoustech VI is such a preamp.In addition, the Acoustech VI supplies a uniquelow-voltage DC triggering signal through thespecial cable supplied; this signal by remotecontrol turns the basic amplifier/electrostatic

panel combinations on and off. As far as weknow, no other preamp includes this fea-ture ... "

Design and manufacturing refinements pro-vide several dramatic improvements over otherelectrostatics on the market today - morethan an extra half octave of fundamental bassis available, and the sound pressures attain-able approa'ch those of a high quality dynamicloudspeaker which exceed the acoustic pres-sures measured in the front rows of a concerthall.

The Gramophone review concluded " ...both amplifiers and speakers have been builtto the best known standards, without compro-mise. No wonder that the sounds they produceare without compromise, too. So, alas, is theprice, inevitably. In America the list price ofthe control unit [Acoustech VI], amplifiers andspeakers adds up to $1939. When I think ofthat there comes into my mind a Latin tagwhich I learnt at school: Non cuivis hominicontingit adire Corinthum (It is not every man'sfortune to go to Corinth) ... "

include all koss productsin your stereo systemKoss Electronics Inc., makers of KossStereophones, Rek-O-Kut tu rntables, andAcoustech amplifiers, tuners, and electrostaticspeaker systems offers you the very finest inquality stereo equipment. Painstakingly designedto the very limits of the state of the art andcarefully manufactured to provide years oftrouble-free service, Koss, Rek-O-Kut, andAcoustech products are matched for performance.Each contains the "know-how" developed overthe years of experience gained by Koss technicians.

The Koss Electronics label is your assurance ofadvanced design, precise manufacture,guaranteed performance. Display it proudly as asymbol of your appreciation of the finest.

DKOSS2227 N. 31st Street, Milwaukee, Wisconsin 53208

Export Cable: STEREOFONEKoss-Impetus / 2 Via Berna / Lugano, Switzerland

~ ubiorama ~nrg.5990 SAINT-HUBERT

MONTREAL 10 273-0747