Sound-Lab A-1 electrostatic loudspeaker DO measures
Several days after I'd powered down my word processor, Roger West called to inform me that the A-1's electronic interface had been modified to provide increased flexibility in contouring their bass response. Independent shelving of the lower midrange and bass was now possible in steps of 3dB. The response below 100Hz and the range between about 100 and 600Hz could now be adjusted separately for more precise handling of the speaker/room interface. Roger also mentioned a new toy he called "SALLIE"Sonic Attenuation for Lower Levels of Interference Effects (known more mundanely as the BWA-1 Back Wave Attenuator). Yes, yes, of courseI wanted to dally with SALLIE and give the new interface a shot before the review went to print.
Roger delivered and installed the new goodies in March. After I had a chance to spend a full listening session with the new interfaces, I realized to my horror that there was now something very wrong with the sound. The lower treble was recessed to the point of dulling and darkening the timbres of many instrumentsincluding the upper registers of female voice. What had happened? This, then, was a good time to measure the speaker's response (something I had yet to do) and try to correlate my subjective findings with a physically observable problem.
The A-1's published frequency range is 28Hz to ultrasonics. This being somewhat vague and open-ended, I asked Roger specifically what the treble response should be like in the nearfield. "Flat to 22kHz," he told me. Using my Neutrik System 3300 set up for a 1/3-octave frequency sweep with a 5Hz warble modulation to minimize the contribution of room modes, I measured the A-1 in the nearfield. My ACO Pacific mike's calibration curve is flat from 10Hz to 2kHz, after which point it exhibits a 1dB rise to 10kHz and a 2dB rise to about 40kHz. (My response curves should therefore be adjusted above 10kHz.)
Shown in fig.1 is the nearfield response of the A-1 with wings, while fig.2 shows the response with the wings removed. These should be considered typical (for this pair of A-1s, anyway), as there were small response variations with changes in mike placement. The rise in the response below 500Hz is perfectly normal in the nearfield, merely confirming that the distributed bass resonance principle was working as advertised. However, the response above 4kHz was a real eye-opener: The range between 4 and 10kHz was recessed about 4dB. And above 10kHz, there appeared to be a resonance centered at about 13kHz, the response plummeting like a rock above that point. By no stretch of the imagination could this HF behavior be construed as "flat to 22kHz."
Because my listening impressions could be completely explained by these measurements, I felt that I had correctly fingered the physical symptoms. It appeared plausible that there was something wrong with the HF transformers in the new interfaces. I put the matter to Roger West.Dick Olsher
Footnote 1: Using ATI's LMS (Loudspeaker Measurement System), I also measured the A-1's impedance magnitude at several settings of the Brilliance Control. With the BC set at Maximum, my first cut at this produced an impedance minimum of 3 ohms at 20kHz. This seemed a reasonable result, until I realized that I had better correct for the inherent impedance of the long measurement cable run from the speaker back to my computer, which turned out also to be 3 ohms! Allowing LMS to subtract the cable impedance from the speaker impedance curve produced a graph whose minimum was essentially a short circuit above 20kHz.Dick Olsher