INNERSOUND EROS Peter Moncrieff, reviewer

International Audio Review, Spring 1998, Peter Moncrieff, reviewer
Excerpts from the InnerSound EROS review based on an audition at the '98 Consumer Electronics Show . . .

The Eros is a hybrid electrostatic system, selling for less than $5000, which includes a full electronic crossover and a 200 watt-per-channel bass amp to drive the woofers.  It is modest in size (15" wide by 68" tall, similar to the Martin-Logan SL3 hybrid), so it should suit any room size.  This is the first speaker system to succeed at sonically marrying a dynamic woofer with an electrostatic for mid and upper frequencies, and it is the first electrostatic system of any kind (either full-range or hybrid) to furnish truly excellent quality bass.  Furthermore, the electrostatic unit itself is more tonally neutral than most of the better electrostatic systems available today.

Full-range electrostatics put out bass that is too weak in quantity, and also peculiar in quality, having a puffy, wimpy nature that utterly fails to convey the impact of (say) a kick drum.  Other hybrid electrostatics put out very poor quality bass, which is boomy, lumpy, colored, and poor in definition  just the opposite of the sonic qualities an electrostatic lover values most.  Their fault has not been in the use of a dynamic woofer per se, but rather in the poor design of their woofer enclosures.

The press have, over the years, speculated endlessly about dynamic woofers having too slow a risetime to mate with fast risetime electrostatics.  All this speculation has been total bullshit, betraying the technical naiveté of all those writers.  The technical truth is that bass frequencies inherently have a slow risetime, and thus do not need a fast, light, or rigid woofer to be successfully reproduced.  Most any dynamic woofer driver easily meets the required risetime spec, to mate to an electrostatic midrange and tweeter.

However, most woofer enclosures impart a very, very slow fall-time to their enclosed drivers, creating a very large, very long-lasting amount of overshoot and ringing.  As discussed and measured in IAR over the years (starting in Journal 3), this bass overshoot and ringing lasts many times longer than even the slow fall-time of the original bass note as recorded.  The ear/brain perceives this long-lasting bass overshoot and ringing as bass boom, an excessive bass heaviness that makes the bass sound slow and poorly defined, and which also obscures the rest of the music (including higher frequencies) that immediately follow the bass note, thus degrading the clarity of the whole speaker system.

Thus, the apparent slowness of many dynamic woofer systems is due to their fall-time, not their risetime, and is specifically due to the large and long-lasting overshoot and ringing imparted by poor speaker enclosure design.  As discussed many time in IAR, bass enclosures which try for maximally-flat, maximally-extended bass response are usually the worst offenders, with the worst overshoot and ringing.  Thus, the modern trend toward obtaining the most extended flat bass response out of the smallest possible box tends to produce bass with the worst musical qualities.  This includes the compact dynamic woofer enclosures furnished with other hybrid electrostatic speaker systems.

The InnerSound Eros takes a different approach.  Recognizing, correctly for a change, that it is the woofer enclosure design that need to be addressed, InnerSound's engineer, Roger Sanders, designed a transmission line bass enclosure for their hybrid electrostatic system.  Transmission line enclosures, when correctly designed, have the ability to produce tight, well-defined, accurate bass, utilizing a dynamic woofer.  He also chose to drive his dynamic woofer directly with a supplied solid state amplifier, fed by his own electronic crossover.  Feeding a woofer directly from a solid state amp (without intervening passive crossover parts) allows better control of the dynamic driver.  Furthermore, feeding that power amp from a custom electronic crossover allows tailoring of the signal to secure the most accurate bass possible from the particular woofer driver and enclosure with which it will be partnered.

The bass of the Eros is impressively deep in its extension and powerful in its impact.  And the quality of this speaker's bass is superb, being very tight, dry, and well-defined.  The bass quality of this system perfectly matches the clarity of its electrostatic panels.  This is a major breakthrough for electrostatic lovers, who will greet this system with hosannas.  At long last, you can have your cake and eat it too.

The electrostatic panels of the Eros also merit special praise.  Other electrostatic systems, including the Martin-Logans, the old Acoustats, and most of the Sound Labs models, have a tonally recessed midrange.  Thus, their tonal balance is warm and perhaps sparkling in the treble, but it lacks enough midrange presence and bite for the music to sound immediate and real.  The Eros electrostatic panels are designed by Roger Sanders, who has been a dean and guru of electrostatic speaker design for many years.  They are the most tonally neutral electrostatic panels we have yet heard.  Through them, music has excellent presence and immediacy, and sounds exquisitely balanced and natural throughout the spectrum, with no part of the spectrum being unduly favored or neglected.

This is no easy trick.  Most writers would have you believe that electrostatics are inherently the most perfect drivers possible, being nearly massless and uniformly driven in push-pull, and therefore that their frequency response and tonal balance must be intrinsically linear.  Nothing could be further from the truth.  In point of fact, the technical engineering factors of electrostatics guarantee that their frequency response is intrinsically anything but flat.  For example, the panel itself is a capacitive load, whose impedance changes continually with frequency.  Then, the transformers needed to drive an electrostatic themselves are inherently non-flat in frequency response, and bring their own set of non-flat design tradeoffs to the speaker system.  This explains why most electrostatic speaker systems actually have very non-flat frequency response and non-neutral tonal balance.  Roger Sanders, with his years of design experience, has evidently successfully engineered a masterful balancing among the many design tradeoffs, and produced an electrostatic panel that finally brings you music with accurate tonal neutrality.

Electrostatic panels have other engineering problems which must also be conquered.  The diaphragm may be light and uniformly-driven over its center regions, but it must still be clamped at its edges, and it is still composed of a physically real material.  These factors combine to produce unwanted resonances in an electrostatic diaphragm, similar to the problems encountered by any conventional dynamic driver diaphragm breaking up, which similarly act to add unwanted colorations to the music.  For example, and electrostatic diaphragm still has a bass resonance, where the diaphragm as a whole most easily flaps back and forth.  And the material of which an electrostatic diaphragm is made still has a lateral propagation speed for vibrations, which creates upper frequency coloration resonances that sonically betray the material of which the diaphragm is made (giving that typical plastic, credit-card snapping sound one hears from most electrostatics).  These colorations have also been well-addressed in the design engineering of the Eros electrostatic panels.  We hear very little if any diaphragm coloration from the Eros' electrostatic panels.  The Eros give you more of the music, more neutrally than any other electrostatic, and with less of itself intruding.

The Eros also sounds impressively dynamic, especially for an electrostatic.  Much of the credit for this goes to its immediacy and presence in the midrange, where music has much of its dynamic impact and bite (and where by contrast, other electrostatics are tonally recessed)  and to the system's strong, impactive, well-defined bass.  In addition, the Eros can play very loudly, so there is good dynamic headroom.

The Eros does not have a beaming bright hot-spot on-axis, as do many other electrostatics, so it is amenable to casual placement and casual listening positions for general music listening.  For critical listening, however, we would suggest carefully aligning the speakers to a prime listening seat and listening from that seat.  

We found that there is a subtle lobing of various frequencies by the panels, which when listening in stereo, produced a slight comb-filtering effect at various frequencies as one's head moves from side to side.  This filtering is easily solved by pointing both panels precisely at the prime listening seat so that you can sit in the prime lobe for both panels simultaneously (as opposed to being in the prime lobe of one speaker but in a notch at some frequency from the other speaker).  

The Eros is a major step forward for electrostatics and is the clear first-choice for electrostatic devotees.

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