As with the Bang & Olufsen BeoLab 90, reviewed elsewhere in this issue, it made more sense for me to measure MartinLogan's Masterpiece Renaissance ESL 15A at the reviewer's home than having them shipped to my Brooklyn lab. (I also wanted to take a listen to the speakers.) Also as with the BeoLab 90, I used a test setup different from my usual MLSSA software/hardware system, which is not readily transportable. (As MLSSA needs to be installed in a 1997-vintage desktop computer running MS-DOS, it isn't something I can take on a plane as carry-on baggage.)

So when I visited Jon Iverson in central California immediately after last October's Rocky Mountain Audio Fest, I had with me my MacBook Pro running SMUGSoftware's Fuzzmeasure 3 app, along with calibrated DPA 4006 and Earthworks QTC-40 microphones, an Earthworks microphone preamplifier, and a bunch of cables. (The TSA inspector, puzzled by the mikes, asked me if they were electronic cigarettes.) The FireWire-connected Metric Halo ULN-2 audio interface I'd used for the BeoLab measurements had subsequently failed, so to measure the MartinLogan I relied on the laptop's analog output and line input, the latter sampling at 96kHz. Subsequent measuring with this setup of my reference BBC LS3/5a speaker following my return to Brooklyn confirmed that use of the laptop hadn't affected the accuracy of the response measurements.

Because of the combination of uncalibrated mike preamp and A/D converter, I couldn't measure the MartinLogan's absolute sensitivity. However, when I used the Studio Six Sound Pressure Level app with my iPhone 6's internal mike (C weighting, Slow Response) held level with the electrostatic panel's midpoint, the Renaissance ESL 15A's output measured 4.5dB higher than the LS3/5a's at the same distance and drive voltage. This suggests that the ML's voltage sensitivity is about 87dB/2.83V/m, significantly lower than the specified 92dB/2.83V/m.

The impedance plug-in for Fuzzmeasure 3 doesn't produce graphs of high enough quality to publish. However, using a 0.1%-tolerance 10 ohm resistor as a reference with the Fuzzmeasure plug-in, it looks as if MartinLogan's specifications of 4 ohms for the ESL 15A's impedance and 0.52 ohm at 20kHz for its minimum magnitude are correct. This speaker will need a good 4 ohmrated amplifier to perform at its best.

Turning to the acoustic measurements: Jon had a small table on which we could place one of the ESL 15As so that the midpoint of its panel, which is usually 42" above the floor, was exactly midway between the floor and ceiling of his large, 11'-high listening room. By aiming the speaker across the diagonal of his room, all of the reflections of the speaker's sound from the room's boundaries would occur sufficiently later in time to allow me to place the DPA microphone at my usual distance of 50".

The speaker's farfield response, averaged across a 30° horizontal window centered on the mid-panel axis, is shown in fig.1 as the blue trace above 300Hz. Below 300Hz, the blue trace shows the panel's nearfield output. While the MartinLogan's treble is broken up by small peaks and dips, the overall trend is flat and even up to 16kHz, above which the response starts to fall off. The midrange is balanced slightly higher than the treble, but the rise in output between 200 and 300Hz is an artifact of the nearfield measurement technique. The powered woofer section, measured in the nearfield with Anthem Room Correction disabled, is shown in fig.1 (red trace). It covers the decade from 20 to 200Hz, above which it smoothly rolls off. This is a full-range speaker!

Fig.1 MartinLogan Renaissance ESL 15A, anechoic response on mid-panel axis at 50", averaged across 30° horizontal window and corrected for microphone response, with nearfield responses of panel (blue) and woofers (red), respectively plotted below 300Hz and 2kHz.

At home, I usually examine a speaker's off-axis behavior by mounting it on a computer-controlled Outline turntable and taking a quasi-anechoic response measurement every 5°. At JI's we kept the speaker stationary and, with one end of a 50"-long string tied to a nail driven into the floor at the speaker's center point, marked out with blue masking tape an arc that covered the quadrant of 0° to 90°, in 5° increments. The result is shown in fig.2, with the on-axis response indicated by the black trace, and the responses in 15° increments plotted in traces of orange, green, blue, magenta, purple, and turquoise. (I did take a measurement every 5°, but the resulting Fuzzmeasure plot was too complicated to publish at a size that would fit on this page.) The Renaissance ESL 15A maintains its response up to 15° to its sides, but then the dipole cancellations begin to take effect in the treble, and by 45° off axis (blue trace), the top octave is depressed by 10dB. Below 1kHz the panel is not as directional as I'd expected, presumably due to the curved shape of MartinLogan's panel diaphragm.

Fig.2 MartinLogan Renaissance ESL 15A, lateral response family at 50", responses on mid-panel axis (black) and at: 15° (orange), 30° (green), 45° (blue), 60° (magenta), 75° (purple), 90° (turquoise) off axis.

In the vertical plane, the Fuzzmeasure plot (fig.3) is difficult to interpret, but basically, the panel maintains its treble response over a wide (±10°) window compared with the response on the panel's midpoint (black trace).

Fig.3 MartinLogan Renaissance ESL 15A, vertical response family at 50": responses at 15° (purple), 10° (orange), 5° (blue) above mid-panel axis, response on mid-panel axis, responses 5° (magenta), 10° (turquoise), 15° (blue) below mid-panel axis.

With the speakers in the positions that JI had determined were optimal for his auditioning, I examined their spatially averaged response. Using the Earthworks microphone, I average 20 1/6-octavesmoothed spectra, individually taken for the left and right speakers in a rectangular grid 36" wide by 18" high and centered on the positions of the listener's ears. This mostly eliminates the room acoustic's effects, and, I have found, correlates well with a speaker's perceived tonal balance.

Fig.4 shows what I found. (The Bass Level was set to 0dB for all the measurements.) The red trace is the spatially averaged response without Anthem Room Correction applied to the woofers, the blue trace with ARC. You can easily see the effects of ARC; in fact, with ARC applied, this is one of the flattest spatially averaged in-room responses I have measured in 27 years of performing this test. Yes, this excellent behavior was partly due to JI's superbly proportioned roombut if you disregard the narrow peak just above 100Hz and the slight lack of energy between 150 and 250Hz, the response falls within ±2dB limits from 20Hz to 10kHz. Wow! (What I haven't shown is how closely the left and right speakers matched at the listening position: within 1dB from 100Hz to 20kHz!)

Fig.4 MartinLogan Renaissance ESL 15A, spatially averaged, 1/6-octave response in JI's listening room without (red) and with (blue) Anthem Room Correction.

Well, not quite "Wow!"with an in-room measurement such as this, the target response should gently slope down in the mid- and high treble. The traces in fig.4 suggest that the Renaissance ESL 15As should sound slightly bright, which was what I heard when I listened to them in JI's room; still, the speakers' extended, powerful lows and extraordinarily transparent, uncolored mids and highs took my breath away.

I exported the impulse-response data that I took with Fuzzmeasure on the mid-panel axis from a distance of 50", and used MLSSA to calculate the step response from those data (fig.5). This suggests that the electrostatic panel is connected in positive acoustic polarity, the powered woofers in negative polarity, and that the decay of the panel's step smoothly blends with the start of the woofers' step, implying optimal crossover design. Note, by the way, that the panel's step begins about 500µs later than would be expected at the 50" mike distance; this is a result of the latency of the laptop's A/D converter.

Fig.5 MartinLogan Renaissance ESL 15A, step response on mid-panel axis at 50" (5ms time window, 48kHz bandwidth).

Finally, I used MLSSA's processing power to calculate a cumulative spectral-decay plot from the Fuzzmeasure impulse-response data (fig.6). This doesn't look anywhere near as good as I'd anticipated from listening to the ESL 15As. As I've written in the past, panel speakers never perform as well on this test as do point-source speakers, and their impulse responses also look hashy. However, what I actually examine in my reviews is not a speaker's response to an impulse, but its impulse response as calculated by cross-correlating an MLS signal (MLSSA) or chirp (Fuzzmeasure) signal, based on the speaker being a perfectly linear system. Perhaps, then, the problem with measuring a panel speaker is that its calculated impulse response does not accurately reflect the behavior of its panel in the same way a conventional speaker's impulse response does. Though the panel is evenly driven over its entire area, and the average motion of the diaphragm reacts in a linear manner to the drive signal, I suspect that the panel actually behaves in a Chaotic manner (footnote 1). In effect, the panel "shimmies" as it movesand that, together with local interference from multiple sources arriving at the microphone, gives rise to the messy-looking waterfall plot.

Fig.6 MartinLogan Renaissance ESL 15A, cumulative spectral-decay plot on mid-panel axis at 50".

That aside, I keep returning to the Renaissance ESL 15A's extraordinary in-room response, which is even better than the BeoLab 90's. As I said: "Wow!"John Atkinson

Footnote 1:in the formal mathematical sense: "the inherent unpredictability in the behavior of a complex natural system."