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Oliver Acoustic Research AR-94 specifications
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Introduction

I and Kari have been testing new ESL contruction methods. One test was to use copper plated glassfibre laminate board as a stator. First test were made for miniature element (4x20cm). That sized panel could be used as a tweeter instead of dynamic one. That experiment got Kari inspired to create electrostatic headphones. Harri had made a pair few years earlier.

Electrostatic headphones

Kari modified hearing protectors so that they could be used with panels. Stators were drilled with automatic drill to fit exactly to the modified frame. Outside of the stator was insulated with polyurethan. Diaphgram is 6 micron Mylar and spacers 1 mm thick. Polarization voltage used has varied from 0.8 to 2kV.

Electrostatic headphones

Headphones were originally tested with separate transformers and polarization supplies that were designed for hybrid panels. The result was 3D-wiring that seemed too hasardous for use. Kari designed a pair of new polarization supplies (this time ladder types) and integrated them with audio transformers to an instrument case. Speaker level terminals are behind the box. Front plate has terminals for two ESL panels and selector for appropriate polarization voltage.

Nobody has got any electric shocks from the panels so far, even they are sometimes handled rather carelessly. So even without cover they seem to be safe to use. Stators are in same potential without music signal and inner plates were always kept in same potential by connecting them together. Touching inner and outer stators at same time causes potential risk when music signal is on.

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Listening tests

Several persons has listened headphones so far and opinions have been very positive. Reference headphones used are Sennheiser HD570 and HD656. Sound quality was close to our reference headphones and differences were quite difficult to find. Most listeners including myself thought that soundfield was quite open. Sound didn't pack inside listeners head as much as in many headphones.

Some of the listeners heard problems in bass region and some in treble region. Reason for the problems in the bass region became very clear after measurements. Leak in the enclosure greatly attenuates lowest frequencies and resonance peak typical to ESL panel is boosted because of the leak.

In my opinion something was missing from highest frequencies but few listeners thought almost the opposite, so differences weren't so big. Problems in treble area are most likely due to cabinet reflections and resonances. That problem can be reduced with suitable paddings.

Measurements

Several persons has listened headphones so far and opinions have been very positive. Reference headphones used are Sennheiser HD570 and HD656. Sound quality was close to our reference headphones and differences were quite difficult to find. Most listeners including myself thought that soundfield was quite open. Sound didn't pack inside listeners head as much as in many headphones.

Some of the listeners heard problems in bass region and some in treble region. Reason for the problems in the bass region became very clear after measurements. Leak in the enclosure greatly attenuates lowest frequencies and resonance peak typical to ESL panel is boosted because of the leak.

In my opinion something was missing from highest frequencies but few listeners thought almost the opposite, so differences weren't so big. Problems in treble area are most likely due to cabinet reflections and resonances. That problem can be reduced with suitable paddings.