MartinLogan Aerius loudspeaker Measurements

Sidebar 3: Measurements

As can be seen from its impedance plot (fig.1), the Aerius is considerably easier to drive than its larger sibling, the Quest Z (reviewed by Dick Olsher elsewhere in this issue). Yes, it does drop to 1.7 ohms above the audio band—the cursor position—but the phase angle in this region is close to 0 degrees; if you don't use an MC cartridge with a vicious tip-mass resonance or you only listen to CD, the amplifier won't be asked to do any work in this region. And lower in frequency, where the bulk of musical energy lies, the Aerius's impedance magnitude stays above 5 ohms all the way to 4kHz. The sealed-box woofer tuning is revealed by the 25 ohm peak at 46Hz. The wrinkles in the solid magnitude plot at 230Hz (major) and 300Hz (minor) indicate some sort of cabinet resonant problems at these frequencies.

Fig.1 MartinLogan Aerius, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

The Aerius is reasonably sensitive for a panel speaker, at an estimated 85dB/W/m (B-weighted), though this is below specification. However, this is the same sensitivity as I measured for the Quest Z, despite the more expensive speaker's significantly larger panel area: 668 in2 vs 281 in2.

The Aerius's impulse response on an axis 36" from the ground, measured with a B&K 4006 microphone calibrated to be flat on-axis and the DRA Labs MLSSA system, is shown in fig.2. Its shape implies good time coherence, as confirmed by the step response (fig.3), but the tail of the impulse is disturbed by high-frequency ringing. The large hump centered at 4.8ms in the step response is due to the panel's lower-frequency ringing being reinforced by the woofer output, though this is lower in level. Both drive-units are connected with the same acoustic polarity; reversing the woofer's electrical polarity eliminates the hump in fig.3, implying that the outputs of the two drive-units cancel in the crossover region when connected in this manner. (Inverting the woofer polarity does indeed result in a large measured suckout covering the entire 200Hz-1kHz region, showing that MartinLogan has correctly implemented the Aerius's crossover.)

Fig.2 MartinLogan Aerius, on-axis impulse response at 45" (5ms time window, 30kHz bandwidth).

Fig.3 MartinLogan Aerius, on-axis step response at 45" (5ms time window, 30kHz bandwidth).

Turning to the frequency domain, fig.4 shows the individual responses of the woofer and panel, the trace for each a composite made up from the upper-frequency quasi-anechoic response at 45" spliced to the lower-frequency nearfield response. The woofer can be seen to roll off smoothly above 500Hz, the 12dB/octave rollout slope being broken up by a couple of cone modes. These are well down in level, however. The woofer's output is flat down to 55Hz, rolling off to -6dB at 37Hz. The gentle 12dB/octave slope, the result of the sealed-box alignment, means that the Aerius should give good bass extension in a typical room.

Fig.4 MartinLogan Aerius, acoustic crossover on tweeter axis at 45", corrected for microphone response, with the nearfield woofer response plotted below 300Hz.

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