InnerSound Eros Mk.III electrostatic loudspeaker

Larry Greenhill, April, 2003

I was trading e-mails with Roger Sanders, manufacturer of the Eros Mk.III electrostatic (ESL) loudspeakers, when it occurred to me to ask him about his name. I was struck that he had the same last name as Gayle Sanders, president of another American electrostatic speaker company, MartinLogan. Were they related? "No," replied Roger Sanders, "it's simply a coincidence that we have similar names. I've never even met him.

"Do you want to hear an even more amazing coincidence?" he continued. "The president of Sound Lab (the other American maker of ESLs) is Roger West. My first name is Roger. My last name is Sanders, which matches MartinLogan's president's last name. It seems rather amazing that the presidents of all three American ESL companies should share names."

The InnerSound Eros Mk.III
The names of the presidents of the other American electrostatic speaker companies might be one of the few things the InnerSound Eros Mk.III shares with its competition. Take its size and configuration. The Eros is 5' 8" tall, 15" wide, and 18" deep, which is 1" taller, 1" wider, and 10" shallower—and 58 lbs lighter and $2000 cheaper—than its nearest competitor, the MartinLogan Prodigy that I reviewed in July 2001. And unlike the Prodigy's curved radiating screen, the Eros Mk.III's electrostatic panel is flat.

InnerSound deliberately used a flat panel for their electrostatic screen, knowing that the lateral dispersion would be greatly narrowed, and the resulting sweet spot so narrow that only one person could listen. Sanders believes that a speaker that beams its signal rather than disperses it sounds best in the nearfield. (Nearfield listening also reduces the subjective effect of a dipole speaker's backwave.)

The Eros's flat, narrow electrostatic panel is subdivided by a fine metal grids that form the screen's stators. Between the stators, and separated from them by 1/16" spacers, a thin sheet of Mylar serves as a driver. The stators need to be acoustically transparent while being electrically conductive. They also need to be effectively insulated, as the amplifier's output is connected to them through a high-voltage step-up transformer, which increases the audio output to several thousand volts. This high voltage is "felt" by the static charge (hence electrostatic speaker) that is applied to the Eros's Mylar diaphragm by a tiny power supply in the speaker itself (which is why it has to be plugged into the wall). The diaphragm therefore vibrates in response to the audio signal, producing sound.

Because bare Mylar is a good electrical insulator, it's necessary to coat the diaphragm with a material that will conduct the electrical charge and allow it to flow evenly over the diaphragm's surface. The technology associated with these coatings is the key to eliminating the problem of arcing and speaker damage. This is a very difficult and challenging technical problem; manufacturers are secretive about their coatings. The Eros Mk.III uses a rugged, reliable coating that allows it to play at extremely loud levels without arcing. It is also calimed to make it immune to humidity, dust, insects, and dirt. The Eros screens are sold with a five-year warranty.

The cabinet's black lower section curves out behind the screens to form the 10" woofer's transmission-line cabinet. Two pairs of WBT binding posts, to permit biamping are located at the back of the bass module.

There is no passive crossover. A separate Crossover-Bass Amplifier module functions as a control center. The Eros owner uses his own amplifier for the screens, and can drive the woofers with either the InnerSound amplifier or an outboard amp. Frequency settings can be controlled from the crossover's front panel or by the system's remote control. These controls adjust the Eros Mk.III's response to the owner's taste and room acoustics. The overall Level control adjusts the output of the satellite amplifier that drives the electrostatic screens from 360Hz up. The Midrange control adjusts the overall gain of the bass amplifier, raising or lowering the bass/midrange level relative to the ESL screens. Changing the setting of the Midrange control lets the listener blend the woofer output with the ESL output level to get the proper "fullness" in the midrange. The Bass control adjusts the woofer amplifier's output below 100Hz by altering the slope of the frequency response, which pivots around a nominal 100Hz turnover point.

The Eros Mk.III's main power switch is on its rear panel, while a separate switch for the 600Wpc bass amplifier is on the front panel, along with blue LED readouts that show which output level—Level, Midrange, or Bass—is being adjusted by the system's remote.

Looking inside the Crossover-Bass Amplifier, I found top-quality, computer-grade printed circuit boards, excellent components, a minimum of point-to-point wiring, and traces of a spider web. The Eros's electrostatic screens have a very satisfactory fit'n'finish, despite some minor functional problems quickly remedied by a call to the manufacturer. These loudspeakers are quite rugged, and should operate reliably for years.



The Eros system arrived in one of the longest trucks I've ever seen. InnerSound had shipped two 6' loudspeaker cartons and two heavy cartons for the electronics, all secured to a single pallet so large that the truck driver had to separate the four boxes before he could remove them from the van. Even so, the Eros speaker assembly was light enough for my wife and me to carry them into the house and up our narrow stairs to the living room.

My listening room is a lightly damped, rectangular, 5400ft3 area 26' long by 13' wide by 12' high, with a 12' semi-cathedral ceiling. The far end of the room opens into a 25' by 15' kitchen through an 8' by 4' doorway. I placed the speakers 3' from the front wall and 3' from the side walls.

Loudspeaker owner's manuals are some of the least-read publications in existence. Some who buy the Eros Mk.III might be able to skip reading InnerSound's spiral-bound instruction manual, but not I. The Ero loudspeaker system only produces sound if the crossover controls are set to "80" or higher. Both JA and I independently discovered this after much frustration. A quick guide card listing this instruction would prevent similar grief in new owners.

The instruction manual also suggests the optimal speaker setup. The speakers should be placed equidistant from the listener's position. With a carpenter's rule, I carefully measured 10' from each panel front to the bridge of my nose—supposedly equidistant from each ear—and put blue painter's tape on the floor to mark each speaker's exact location. I adjusted each speaker to point it directly at my chair. The manual suggested adjusting the speakers so that my visual reflection was centered in the middles of the Eroses' screens. For final adjustment, I held a small pencil flashlight above my head and made sure that the reflection of the beam was centered in both speakers, and was the same height.

I plugged the interconnects from my Krell KRC preamplifier into the Crossover-Bass Amplifier. The crossover's low-pass filter directed bass signals to its internal bass amplifier, which then drove the Eros's transmission-line woofers. The high-pass crossover outputs were connected to the input jacks of two Bryston 7B-SST monoblock amplifiers, which drove the ESL screens via my PSC ribbon loudspeaker cables. Again following the manual, I set the crossover's readouts for the overall system Level to "85," the Midrange to 85, and the Bass to "5."

I checked the speaker's low-frequency in-room response with 1/3-octave warble tones at -20dB, from Test CD 2 (Stereophile STPH002-2). I switched my RadioShack sound-level meter to its C-weighted, slow ballistics mode, and averaged several readings in a window 4' wide, 3' high, and centered on my listening position and ear height. I adjusted the crossover's overall level so that the 100Hz warble tone registered 0dB on the meter. At that setting, the Eros's bass response was within ±5dB from 200Hz to 50Hz, with a +5dB peak from 50 to 80Hz, which was a characteristic of the room. At 41.5Hz, the output was +4dB; at 31Hz, it was down by -4dB. By 25Hz, the signal had dropped to -10dB, with audible doubling. I repeated this procedure after setting the Crossover-Bass Amplifier's Bass control to "2," but this did not significantly change the response below 100Hz.

The Eros was designed to have a precise, small sweet spot in the nearfield. I substantiated this by playing pink noise and conducting the "sit down, stand up, walk around" test—I heard the Eroses' sweet spot only when I was sitting (ears 38" high), even though the electrostatic panels extend from 26" to 68" above the floor. If I leaned too far forward or too far back in my listening chair, the tonal balance dulled. It dulled even more when I stood up or moved around the room.

During the break-in period, the remote stopped working. Roger Sanders helped me find the culprit: a loose ribbon cable inside the crossover unit. A soft but pesky buzzing sound in the right electrostatic screen was loudest at 100Hz when swept with different frequencies. The buzz was mechanical—I could suppress it by pressing on the stator with my finger. I reduced this rattle somewhat by shoving a piece of paper between the stator and the speaker's wooden frame, and then blasting the screen with compressed air (footnote 1). I was reassured by InnerSound that, should a buyer suffer this type of problem, the company will replace the speaker at no charge.

Using CDs of my favorite vocalists, I varied the crossover settings from my listening chair via remote control. I was surprised that there was no set, standard anechoic-based reference point for these controls, either on the crossover faceplate or in the manual. Could I dial in the correct control settings? I heard the biggest changes in tonal balance when I decreased the Midrange setting from "88" to "78." This shifted female singing voices from smooth and extended to hollow and tunnel-like. For critical listening, I returned the Eros crossover settings to "88" for Level and Midrange, "5" for Bass.

The Eros Mk.IIIs' best quality was their first-rate imaging: They generated a seamless, wall-to-wall soundstage that did not seem to emanate from the speakers themselves. The first track on David Hudson's Didgeridoo Spirit (CD, Indigenous Australia IA2003 D) opens with sounds of a rainforest, complete with a soft rain falling, exotic birds chirping, and wind rustling through the leaves. I heard so much new information over the Eroses that I was startled.

I had the same stunned reaction when I played Fleetwood Mac's The Dance (CD, Reprise 46702-2). Sure, the mids and highs were fast, transparent, and rich, with a sheen and harmonic structure that were stunningly new and different. Sure, the speakers "disappeared" and engulfed me in sound. But there was more. Never had I heard such a large soundstage, such a distinctive layering of instruments and vocalists, or such realistic crowd noise. I could easily tell that lead singer Stevie Nicks was moving around, alternating between singing close to the microphone and then turning away. As she sang "Silver Springs," I wrote in my notes, "Nicks has never sounded as realistic, as emotionally moving, or as powerful. It's as if I've never heard this album before."

Footnote 1: According to Roger Sanders after he had examined the speaker, the cause of the buzzing was a grain of sand that had found its way into the electrostatic panel. "Fixing the problem was simple," he wrote. "Using a blow-gun, we shot some compressed air into the electrostatic panel. This simply blew the grain of sand away. A customer probably could probably remove the grain of sand using a vacuum cleaner with a soft brush. But the panel is very rugged. Blasting it with compressed air will not harm it."—Larry Greenhill



These qualities also benefited studio recordings. Suzanne Vega's startling a cappella vocal in "Tom's Diner," from Solitude Standing (CD, A&M CD 5136), materialized in the room between the speakers. And I was transfixed by the ethereal, translucent a cappella choral blend on "Calling My Children Home," from Emmylou Harris' Spyboy (CD, Eminent EM 25001-2).

The Eros Mk.III took first place in transparency and imaging. This was surprising, given its highly variable in-room frequency response. Somehow, simply tuning by ear, I had found this speaker's most felicitous—if perhaps not its most accurate—response. This was possible only because, using the remote control, I could make adjustments in real time from the sweet spot.

Male vocalists also benefited. José Carreras' clear tenor was clear and pure as never before during the Kyrie of Ariel Ramirez's Misa Criolla (CD, Philips 420 955-2). Harry Connick, Jr.'s "Don't Get around Much Anymore" (CD, When Harry Met Sally..., Columbia CK 45319) was natural, with no sign of muddiness, darkness, or over-richness. Willie Nelson's voice on "Getting Over You" and "Don't Give Up," from Across the Borderline (CD, Columbia CK 52752), was smooth, detailed, and three-dimensional.

The combination of Eros Mk.III and Bryston 7B-SST preserved the sweet, sad, delicate harmonies of Richard and Linda Thompson singing "Dimming of the Day," from the Divine Secrets of the Ya-Ya Sisterhood soundtrack (CD, DMZ/Columbia CK 86534). A distinct sadness emanated from this duet, perhaps the result of this duo's well-publicized marital problems.

Instrumental timbres were rich and involving. The distinct tonalities of saxophone and guitar emerged clearly from the title track of the L.A. Four's Going Home (CD, Ai Music 3 2JD 10043). When I listened to Eiji Oue conduct Stravinsky's The Rite of Spring (CD, Reference RR-70CD), the Eros Mk.III conveyed the raw, hot, passionate trumpet "brassiness" that this orchestral piece demands. During the opening of Alison Krauss's "Sitting in the Window of My Room," from Ya-Ya Sisterhood, I could easily pick out the delicate hammer dulcimer, the lap-steel guitar, and the tack piano.

The Eros Mk.III's treble range was effortless, grain-free, smooth, and extended. There was no extra brightness, steeliness, or metallic edge. Chimes heard through this speaker were transparent and shimmering. Paul Simon's vocal sibilants on "Trailways Bus," from Songs from The Capeman (CD, Warner Bros. 46814-2), were subtle, not irritating. The opening cymbal ride with brushes on the Jerome Harris Quintet's recording of "The Mooche," from Rendezvous (CD, Stereophile STPH013-2), had the right amount of sizzle.

The bass whacks in Stravinsky's Firebird Suite (track 7) and Rite of Spring (tracks 21-24) on the Eiji Oue recording shook the room but it bottomed-out the Eroses' 10" woofers. When I cut back the crossover's Bass control from 5 to 2, the Eroses continued to deliver solid bass above 30Hz in my room, with none of the clicking heard at higher volumes.

Other recordings exposed the subtle rattle from the right speaker's electrostatic screen, that I mentioned above. It was triggered, for example, by Michael Anopol's plucked standup bass on Patricia Barber's "Use Me," from Companion (CD, Premonition/Blue Note 5 22963 2). "Gnomus," from Jean Guillou's organ transcription of Mussorgsky's Pictures at an Exhibition (CD, Dorian DOR-90117) delivered good room lock but also made the right ESL rattle.

The Eros Mk.III's bass response was airier-sounding than that of other top speaker systems. The 32Hz double-bass notes at the beginning of Richard Strauss's Also sprach Zarathustra, excerpted in "Ascent" on Telarc's Time Warp CD (Telarc CD-80106), produced a dull, heavy presence more than any distinct bass note. I also heard this in the final organ chords from Elgar's The Dream of Gerontius, Part 1 (Test CD 2, Stereophile STPH004-2), and in the sullen, repetitive bass-drum beat on "Cosmos Old Friend," from the Sneakers soundtrack (CD, Columbia CK 53146). The crossover point chosen allowed the woofer's sonic characteristics to mesh well with the electrostatic panels. On the other hand, the huge bass drum and synthesizer in "Silk Road," from I Ching's Of the Marsh and Moon (CD, Chesky WO144), was convincingly deep, resonant, and tuneful. The Eroses' transmission-line woofers captured the tight, fast, punchy tom-toms on Richard Thompson's "I Misunderstood," from his Rumor and Sigh (CD, Capitol CDP 7 95713 2).

I easily enjoyed dynamics at high volume levels, as heard during the over-the-top drum solo in "The Maker," from Emmylou Harris' Spyboy. Rim shots, tom-tom beats, and kick-drum notes exploded high above the muttered conversations of the crowd.

Few loudspeakers I've auditioned have pleased and vexed me as did the InnerSound Eros Mk.III. Pleasure came with this speaker's excellent blend of dynamic transmission-line woofer and electrostatic screen, its transparency, wide dynamic range, freedom from overload, competitive pricing ($1000 less than the Quad ESL-989, $2000 less than the MartinLogan Prodigy), outboard tuneable electronic crossover, and astounding imaging. I'd never heard such a wide, deep soundstage, even from speakers costing six to ten times as much. The inclusion of the remote control and the external Crossover-Bass Amplifier for such a relatively low price further increase the Eros Mk.III's value.

At the same time, I had to deal with a loose internal ribbon cable, a rattling electrostatic screen, woofers that overloaded and bottomed out, a bass response that didn't reach the lowest octave, the need for ultra-precise speaker placement, and a sweet spot that seemed only millimeters in diameter.

Still, I recommend the InnerSound Eros Mk.III. The pair of them produced clear, transparent sound with superb imaging that always involved me in the music. These ESL hybrids played with low distortion and less listener fatigue, making longer listening sessions possible. Unlike the Quad ESL-989, the Eros Mk.III needs no protection system, and so tolerated high-powered solid-state amplifiers, played louder, and had a more extended top end. While the MartinLogan Prodigy's Bass Forward woofer produces tighter, deeper bass, and its sweet spot is much wider, the Eros created the broadest, deepest, and most three-dimensional soundstage of these three ESLs.

Despite the commonality of their designers' names, the sounds of these three electrostatic loudspeakers differ in fundamental ways. You'll just have to audition all of them before deciding what to buy. It should be fun.



Sidebar 1: Specifications

Description: Hybrid electrostatic/moving-coil loudspeaker system with remote-control active crossover/bass amplifier module. Drive-units: 41" by 11.5" flat electrostatic midrange/HF transducer; transmission line-loaded, 10" fiber-cone woofer. Crossover frequency: 360Hz. Crossover filters: fourth-order (24dB/octave, Linkwitz-Reilly). Frequency response: 24Hz-27kHz, ±2dB. Sensitivity: 98dB/2.83V/m. Electrostatic impedance: 112 ohms at 500Hz, falling to 2 ohms at 20kHz. Woofer impedance: 4 ohms. Recommended panel amplifier power: 80-300W. Amplifier/crossover controls: Level, 100 1dB steps; Midrange, 100 1dB steps; Bass, 12 1dB steps below 100Hz. Amplifier output power: 600Wpc into 4 ohms (24.8dBW).
Dimensions: Speaker: 68" H by 15" W by 18" D. Weight: 75 lbs (34kg). Crossover-Bass Amplifier: 17" W by 5.5" H by 14" D. Weight: 42 lbs (19kg).
Finishes: Wood trim on sides of electrostatic screens—cherry, maple, red oak, walnut, mahogany—and black aluminum anodized trim.
Serial numbers of units reviewed: 001648, 001649, speakers; CGX20967, crossover.
Price: $7995/pair. Approximate number of dealers: 30.
Manufacturer: InnerSound, Inc., 7161 Valtec Court, Unit D, Boulder, CO 80301. Tel: (303) 440-3561. Fax: (303) 440-3562. Web:



Sidebar 2: Associated Equipment

Analog source: Linn Sondek LP12/Lingo turntable, Linn Ittok tonearm, Spectral moving-coil cartridge.
Digital sources: Krell KRC-28 CD transport, Sony SCD-C555ES multichannel SACD player.
FM tuners: Day-Sequerra FM Reference Classic, Rotel RH-10, Fanfare FT-1A, Magnum Dynalab MD-102, with Model 205 Sleuth RF amplifier.
Preamplification: Krell KCT, Sony TA-P9000ES, Mark Levinson ML-7A with L-2 phono section, Conrad-Johnson Premier 18LS, Margulis phono preamplifier, Duntech MX 10 moving-coil amplifier.
Power amplifiers: Mark Levinson No.334, Krell FPB 600C, Bryston 14B-SST monoblocks.
Loudspeakers: Quad ESL-63 (on Arcici Stands), MartinLogan Prodigy, Velodyne HGS 18 subwoofer.
Cables: 75 ohm digital coax: Silver Starlight, Ultralink 75. Interconnect, balanced: Krell CAST and Cogelco Yellow, Bryston Balanced, PSC Pristine R-30 silver alloy. Interconnect, single-ended: Randall Research, Mark Levinson HFC (with Camacs), Totem Acoustic Sinew, Coincident CST Interface, Ultralink Performance Audio. Speaker: Mark Levinson HFC 10, PSC Pristine R50 biwire double ribbons, Ultralink Excelsior 6N OFHC, Coincident Speaker Technology CST 1.—Larry Greenhill



Sidebar 3: Measurements

I looked first at the InnerSound Eros Mk.III's electronic crossover, but was initially stymied by my inability to get any signal from its low-pass (woofer) outputs. I consulted the manual, which was no help, and it was only several days later, with the loan of a second sample from Brooklyn-based photographer Wes Bender, that I realized what I was doing wrong. Other than the Level control, the crossover's controls—marked Bass and Midrange—both affect only the woofer. As I had the Midrange control set to "0," the woofer was effectively turned off.

Fig.1 shows the crossover's woofer output with the Midrange control operated in steps of 10 from an indicated "60" to "99." The shape of the curve doesn't change, only the absolute level; thus, the Midrange control actually acts as the woofer-level control, the number displayed representing the gain in dB above a nominal reference. The unity-gain setting for high-pass (at 1kHz) and low-pass (at 100Hz, Midrange set to "90," Bass set to "12") line outputs appeared to be "90" and "89," respectively. To minimize the possibility of clipping the outboard power amplifier, the Eros crossover's settings should only be increased above this with care. The low-pass power amplifier offered 27.4dB of voltage gain at 100Hz into 8 ohms at this setting, which will match the sensitivity of many third-party amplifiers.

Fig.1 InnerSound Eros Mk.III crossover, Bass = "6," low-pass frequency response with Midrange = "60" (bottom) to "90" (top). (5dB/vertical div.)

Fig.2 shows the high-pass output (right), revealing some shaped boost in the drive signal to the panel between 450Hz and 8kHz, with a steep, 24dB/octave rolloff below that region. To the left of fig.2 is shown the effect of the Bass control with the Midrange set to "90." The lowest trace is with the Bass control set to "0"; each successively higher trace was taken with an increase of "3" in the numeric reading, with the top trace equivalent to "12." The control can be seen to affect the level of the entire bass region below 300Hz. In combination with the Midrange control, the Bass control should allow an optimal match to be obtained between the panel and woofer outputs and the room acoustics.

Fig.2 InnerSound Eros Mk.III crossover, Level = "90," Midrange = "90," Bass = "6," high-pass frequency response (right) and low-pass frequency response (left) with Bass = "0" (bottom) to "12" (top). (5dB/vertical div.)

The crossover's input impedance at 1kHz was a usefully high 49k ohms unbalanced or 85k ohms balanced. The line-output source impedance depended on frequency, filter section, and whether it was balanced or unbalanced. The unbalanced high-pass (panel) output impedance was a moderate 480 ohms over most of the audioband, rising slightly to 716 ohms at 200Hz. The balanced figures were just under twice the unbalanced, but as long as an amplifier is used with an input impedance of at least 10k ohms, there will be no matching problems. The low-pass line-level output features quite a high source impedance, at 1280 ohms unbalanced and 1580 ohms balanced at 100Hz, rising to 7k ohms at 20Hz. In the unlikely event that the user wants to use a separate woofer amplifier with the Eros, it should have an input impedance approaching 100k ohms. The Eros crossover's amplifier impedance was a healthily low 0.12 ohm.

The high-pass output features low levels of noise and distortion. The spectrum shown in fig.3 was taken with the crossover's unbalanced output driving 1kHz at 1V into 8k ohms. The second harmonic is the highest in level, but at -102dB (0.0008%), this won't upset anyone. With its gain set to "90," the high-pass output could deliver enough output to drive the partnering amplifier into clipping (fig.4). Peculiarly, though the maximum output voltage was curtailed into 600 ohms, as expected, this load actually resulted in lower levels of noise and distortion than the more benign 100k ohm load. The low-pass amplifier also gave low levels of noise and distortion, up to its clipping point (fig.5). The amplifier met its 600Wpc specification to 4 ohms (24.8dBW), with 380W available into 8 ohms (25.8dBW), both figures at 1% THD.

Fig.3 InnerSound Eros Mk.III, unbalanced high-pass output, spectrum of 1kHz sinewave, DC-10kHz, at 1V into 8k ohms (linear frequency scale).

Fig.4 InnerSound Eros Mk.III crossover, high-pass balanced output, distortion (%) vs 1kHz output voltage into 100k ohms (top) and 600 ohms (bottom).

Fig.5 InnerSound Eros Mk.III crossover, low-pass amplifier, distortion (%) vs 100Hz output power into 4 ohms (top) and 8 ohms (bottom).

I then turned to the speaker itself. With its crossover level control set to the unity-gain setting of "90," its B-weighted sensitivity was around 88dB(B)/2.83V/m, which is above-average for an electrostatic design. However, as can be seen from the panel's impedance plot (fig.6, upper trace), it draws very little current from the amplifier, hence almost no power. The panel's impedance stays above 50 ohms between 90Hz and 1.8kHz, reaching a maximum value of 137 ohms at 490Hz. (Note also the small spike at 86Hz, which is probably due to the panel's fundamental "drumskin" tuning resonance.) However, the panel's impedance drops rapidly above 2kHz, and is compromised by a severely capacitive phase angle over much of the treble region.

The impedance actually drops to a minimum of 0.6 ohm at 35kHz, which is probably just the series resistance of the connecting wire. Fortunately, the phase angle at this frequency is 0 degrees! Possibly the worst-case condition is at 18kHz, where the magnitude is still low at 3.2 ohms but the phase angle is -79 degrees. Musical energy is unlikely to be very high in this region, but some amplifiers will not like driving this kind of load—where the current is at its greatest when the voltage approaches zero—at any level.

Fig.6 also shows the woofer impedance (lower trace). It drops to 4.7 ohms at 300Hz, but is otherwise an easy load. The port—or, more correctly, the vent terminating the transmission line—appears from this graph to be tuned to 30Hz. However, there are a number of small peaks between 100Hz and 400Hz, which suggest the presence of acoustic resonances, either in the line or in the woofer enclosure panels. Using an accelerometer, I found some fairly strong panel resonant modes in this region (fig.7). Because the woofer is driven by a low-pass-filtered signal, the higher-frequency modes will be less likely to be fully excited, but, all things considered, the Eros' bass cabinet is not as dead as I would have liked.

Fig.6 InnerSound Eros Mk.III, electrical impedance (solid) and phase (dashed) of electrostatic panel (top at 1kHz) and woofer. (5 ohms/vertical div.)

Fig.7 InnerSound Eros Mk.III, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the center of the woofer cabinet's side panel. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)

Fig.8 shows the farfield responses of the electrostatic panel (blue trace) and woofer (red above 350Hz), along with the nearfield woofer (red below 350Hz) and port (green) responses. (The crossover was in-circuit for all four measurements, with the Bass set to "6" and the Midrange to "90," which sounded most natural to my ears.)

Fig.8 InnerSound Eros Mk.III, Midrange = "90," Bass = "6," anechoic response at 50" on mid-panel axis of electrostatic panel (blue trace) and woofer (red above 350Hz), both corrected for microphone response, with the nearfield responses of the woofer (red below 350Hz) and port (green).



The panel's output appears to tilt down throughout its passband. As I've explained before, this will be due, at least in part, to the "proximity effect": with a speaker having a physically large radiating area, it is difficult to get the microphone far enough away that it will be in the actual farfield. As a result, a distance-dependent amount of low-frequency boost will affect the measured response. Apart from that, the panel's response is smooth, with just a couple of notches apparent at the bottom of its passband, and a small peak evident at 20kHz.

The high-pass rolloff features a steep 30dB/octave slope, this arising from the fourth-order crossover slope being increased by 6dB/octave by the usual dipole action. (At the frequency drops, there is an increasing amount of cancellation of the front wave by the antiphase backwave.) This is just as well; without the crossover, the panel has a significant peak at 86Hz—which, as I noted above, is the frequency of its "drumskin" resonance. This is so strong that you can hear it coloring your voice when you speak close to the Eros. It's fair to point out that the panel will not be fed an electrical signal that will excite this resonance. However, the panel is in close proximity to the woofer, which puts out high levels of energy in this region, and it will be excited. All things being equal, I would have thought it audible with music. Certainly I could hear it with pink noise as a slight "hummy" quality.

Returning to fig.8, the woofer (red trace) rolls off with an approximate 18dB/octave slope above 400Hz. (With the crossover's Midrange control set to "90," as it was for this measurement, the woofer's level appears to be too high in absolute level, but 3dB of this will be due to the nearfield measurement technique.) Its output is fairly smooth within its passband, but there is a notch apparent at 120Hz. This is also the frequency of a small peak in the port output, which, like the bumps in the impedance graph, suggests a resonant problem in the transmission line. The port's higher-frequency response is disturbed by some peaks. Lower in frequency, it offers only a small degree of bass extension compared with a reflex design.

How all these responses sum in the nominal farfield is shown in fig.9. Again, not too much should be made of the apparent downward slope above 1kHz, which will, to some extent, be due to the proximity effect. However, the response trend is overall very smooth.

Fig.9 InnerSound Eros Mk.III, Midrange = "90," Bass = "6," anechoic response on mid-panel axis at 50", averaged across 30 degrees horizontal window and corrected for microphone response, with the complex sum of the nearfield responses plotted below 300Hz.

Regarding the phasing of the drive-units: When I connected them as the manual recommends, I got a sharp cancellation notch at 490Hz. To get the much smoother measured farfield integration between the panel and the woofer shown in fig.9, I had to invert the woofer polarity. As the mostly excellent manual states, however, Eros owners should experiment with this in their rooms and choose the woofer phase that sounds best to them.

The traditional problem with an electrostatic speaker that uses a flat panel is that its radiation pattern beams severely in the frequency region where the wavelength is smaller than the speaker's size. This is why MartinLogan and SoundLab use curved panels, and why Quad uses a multiple time delayed-ring approach. Fig.10 reveals, however, that the Eros is capable of delivering a full measure of high frequencies in the horizontal plane only if the listener sits exactly on-axis. This explains Larry Greenhill's comment that the sweet spot "seemed only millimeters in diameter," and it is fair to note that this restriction of the speaker's HF dispersion is deliberate. In the vertical plane, the panel's 41" dimension is large enough to result in some line-source behavior, which means there should be only moderate changes in balance over quite a wide range of listener ear heights (fig.11). Nevertheless, LG found that not to be the case.

Fig.10 InnerSound Eros Mk.III, lateral response family at 50", normalized to response on mid-panel axis, from back to front: differences in response 90 degrees-5 degrees off-axis, reference response, differences in response 5 degrees-90 degrees off-axis.

Fig.11 InnerSound Eros Mk.III, vertical response family at 50", normalized to response on mid-panel axis, from back to front: differences in response 15 degrees-5 degrees above axis, reference response, differences in response 5 degrees-15 degrees below axis.

In the time domain, the step responses of the panel (fig.12, red trace) and woofer (blue) are both positive-going, though the woofer's output arrives a while later and is opposed by the panel's output, which is negative-going by that time. This is why I got better measured integration between the two disparate drive-units by reversing the woofer polarity. Note the excellent right-triangle shape of the panel's arrival, which suggests a time-coherent presentation, at least over its passband. The cumulative spectral-decay plot (fig.13) features a much cleaner treble region than is usually the case with panel speakers. There appears to be some delayed energy apparent in the midrange, however, though the resolution in this graph is not good enough to reveal what is going on.

Fig.12 InnerSound Eros Mk.III, step response on mid-panel axis at 50" of electrostatic panel (red trace) and woofer (blue). (5ms time window, 30kHz bandwidth.)

Fig.13 InnerSound Eros Mk.III, cumulative spectral-decay plot at 50" (0.15ms risetime).

One I'd figured out how to use it, the InnerSound crossover-amplifier got a clean bill of health. The Eros Mk.III itself, however, is an enigma. Its clean treble decay and smooth response suggest that a listener sitting in the speakers' tiny sweet spot will experience excellent sonic clarity and a superb ease to the music's presentation, at least in the midrange and above, that could prove addictive. But the woofer's use of transmission-line loading is nothing I could be enthusiastic about; it introduces the possibility of upper-bass coloration and as LG found, it does not significantly extend the speaker's low-frequency limit.—John Atkinson



John Atkinson followed up his measurements in May 2003 (Vol.26 No.5):

My plan to thoroughly audition the InnerSound Eros Mk.III was thwarted when one of the pair had to be returned to InnerSound to investigate why it was buzzing on some bass notes (see April 2003, p.127). However, I did perform a full set of measurements on the survivor, including the same spatially averaged room response as I had done on the Quad ESL-989. (As each speaker is measured separately, I could use the same Eros sample for both left and right positions.)

The Eros's electrostatic panel was driven by a Lamm M2.1 monoblock, the woofer by the dedicated amplifier. The crossover's Midrange control (which adjusts the woofer level) was set to "90" and the Bass control to "6," which is its midway setting. Despite the feelings I expressed in last month's review about the possible lack of integration between the woofer and panel when driven with the same electrical polarity, this was how I ended up driving the speaker in my room.

The result is the blue trace in fig.14. While the Eros's bass and midrange were superbly flat—remember, this is the in-room response—the high frequencies rolled off, apparently critically. However, as Larry Greenhill explained in his review, this is a result of InnerSound's Roger Sanders deciding to go in the opposite direction from other designers of electrostatic loudspeakers and do nothing to ameliorate the inherent beaminess of a large, flat diaphragm. While this drastically reduces the high-frequency content of the room's reverberant field, it can be overcome to some extent by listening relatively close, in the speaker's nearfield.

Fig.14 InnerSound Eros Mk.III, spatially averaged, 1/3-octave, freefield response in JA's listening room (blue); on-axis response at listening position (red).

The red trace in fig.14 demonstrates that listening on-axis will reveal an astonishingly flat response from the low bass through to the top octave. But, as LG noted, the tolerance of the listener's head position that gives this extraordinary performance is measured in millimeters. Turn your head slightly, and the Eros's high treble simply disappears.

LG commented in his review on the InnerSound's superbly stable stereo imaging. As I had only one speaker, I had to take his word on this. However, while I was doing some auditioning in mono—far more revealing of loudspeaker coloration than stereo listening—sound sources were eerily present in the room, with no sense of the sound emanating from a physical source (as long as I was sitting exactly on-axis, of course, with my head in a virtual vise).

Only in the bass did I feel that the Eros failed to achieve the level of achievement offered by the electrostatic panel. But if you're content with what the InnerSound speaker offers in that region and can cope with having to sit with your head clamped in the sweet spot, the Eros will offer a lot of magic for not a lot of money.—John Atkinson



Manufacturer's Comment

Editor: While there will always be strong opinions about certain approaches to audio design, the laws of physics cannot be circumvented. Accordingly, only a speaker that minimizes room interaction can produce accurate transient response, three dimensional holographic imaging, and flat frequency response when you consider it within the context of its own interaction with the room. We are pleased that LG recognized the Eros Mk.III's unique ability to offer these outstanding qualities and are confident that serious audiophiles will recognize them too.

On the subject of bass response, our other design mandate was to create an electrostatic speaker that unlike a full-range electrostatic, had virtually no limitation in achieving realistic output levels and deep bass response. After considering many design approaches Roger and his design team determined that a hybrid approach was the only practical way of meeting these objectives in a reasonably sized package.

We were then left with the perennial challenge of seamlessly integrating these two disparate technologies. We found that the solution required transmission line loading, a proprietary, low-mass SEAS woofer with a patented magnetic damping system, direct coupling of the woofer to a high-power high-damping factor amplifier, and the use of a sophisticated electronic crossover.

For those readers interested in achieving the ultimate in low bass response, look for our soon to be released "Nemesis Subwoofer System" incorporating a 24" woofer in a 12' transmission line. Larry and John, we promise not to ask you guys to carry this beast up the stairs!

We are pleased that JA's measurements of the Eros Mk.III validate our design goals: a true reference speaker offering flat frequency response, low distortion, and wide dynamic range. While its "sweet spot" is admittedly smaller than those of some speakers, Roger Sanders set out to design a speaker that had no equal. Although Roger holds the distinction of inventing the curved electrostatic speaker, he discovered after years of research that a curved panel could never provide the level of refinement and accuracy that he had set out to achieve. The Eros Mk.III is the culmination of Roger's lifetime of study regarding electrostatic speaker technology and design. Again, the choice to limit dispersion was a conscious one. Science will tell us that designing speakers is a series of tradeoffs. Accordingly, for its intended purpose (that of a true reference for serious listeners), there is no way to produce such a speaker by making the listening room a significant part of the equation.

We also wish to clarify a few items mentioned in the review: The buzzing sound observed by Larry Greenhill was later found to be a grain of sand—its origin unknown! There was never a problem with the actual speaker. We also wish to point out that, properly set up, the Eros' high-frequency response extends to 27kHz and beyond. In support of that objective claim, we offer the following subjective statement from an unsolicited listener, which recently appeared on Audio Asylum: "The Eros disperse treble just fine. Off axis they still sound better than most speakers. In the sweet spot they are in a world of their own."

We believe that the combination of Innersound's "Ultrastat" panel technology (patent pending), transmission-line woofer system, and electronic crossover amplifier offers our customers previously unobtainable performance at a very reasonable price.

In closing, we wish to refer to a comment Stereophile made about the Quad ESL-989, also Followed-Up on in the May issue: "Something like the Quads must be heard first, ideally in your home." As far as we know, InnerSound is one of the few manufacturers that offers its customers a "totally risk-free" 30-day home trial. We invite you to listen for yourself (preferably in stereo).

We thank Larry Greenhill, John Atkinson, and Stereophile for an excellent review.—Gary Leeds, CEO, Innersound LLC