MartinLogan SL3 loudspeaker Measurements
I hate measuring panel speakers. Why? Because the usual assumptions you make when you measure a loudspeaker—that the microphone distance is large compared with the physical size of the speaker and that the mike is therefore in the speaker's farfield—are no longer true. As a result, the interaction between the microphone and the speaker is much more complex than is normally the case. In addition, the measured performance of panel speakers is enigmatic. Compared with a typical moving-coil speaker, the measurements can look worse—yet listeners like the sound more. So why measure? Because I believe even a small bit of knowledge is better than a large amount of ignorance.
Having got that off my chest, how did the SL3 look in the test lab?
The Martin-Logan's B-weighted sensitivity weighed in at an estimated 85dB/2.83V/m. This is somewhat lower than the specification, but in a room the line-source behavior of the panel will mean that the SL3 will sound louder than a similarly specified point-source speaker. Its impedance (fig.1), taken with the woofer switch set to "Flat," reveals it to be a hard load for the partnering amplifier, which might partially explain why WP found it very revealing of amplifier quality. Though the impedance drops to 1.5 ohms at 20kHz, there isn't much musical energy present up there, meaning that the amplifier will not be taxed too much. In addition, as is typical, the lowest impedance magnitude coincides with a phase angle of 0 degrees.
Fig.1 Martin-Logan SL3, electrical impedance (solid) and phase (dashed) with woofer switch in "Flat" position (2 ohms/vertical div.).
However, there are three problem areas at lower frequencies, both where the music has a lot of energy and where the SL3 features a combination of low impedance and high phase angle, something that asks a lot of an amplifier. At 50Hz, 4.5 ohms impedance is coupled with a capacitive phase angle of -47 degrees; at 1kHz, a magnitude of 5 ohms coincides with a phase angle of -42 degrees; while at 8.2kHz, a magnitude of 3.8 ohms combines with a phase angle of -57 degrees. In each case, the amplifier's output stage will be maximally stressed.
Switching in the 3dB woofer control usefully increases the impedance in the bass, and reduces the woofer output by between 3dB and 4dB, depending on frequency. The tuning of the moving-coil woofer is revealed by the peak in fig.1 at 36Hz. The impedance plot was free from resonance-induced wrinkles—I found the enclosure to be acoustically dead.
Fig.2 Martin-Logan SL3, anechoic response on mid-panel axis at 50", averaged across 30-degree horizontal window and corrected for microphone response, with complex sum of nearfield woofer and panel responses plotted below 350Hz.