InnerSound Eros Mk.III electrostatic loudspeaker John Atkinson Measures May 2003

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

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