MartinLogan Aerius loudspeaker Measurements 1998
As coincidence would have it, Stereophile Guide to Home Theater received a pair of MartinLogan Aerius i's for review in late 1998. The planned review never actually appeared in print, but I did perform a full set of measurements at that time.
Fig.1 shows the later version's plot of impedance magnitude and electrical phase angle against frequency. The broad-brush picture is similar to the earlier speaker's plot. The impedance drops to a very low value above the audioband and there is a single, sealed-box peak in the bass, but the details are different. The lowest value is now 1.5 ohms around 25kHz instead of 1.73 ohms, and the woofer tuning is a couple of hertz lower in frequency. But instead of the original's single impedance peak at 1kHz, due to the crossover, the <>i has a more complicated appearance in this region, perhaps due to some equalization. There is also a dip to below 4 ohms in the midrange, which will, as ST surmises, prove a little more demanding on the partnering amplifier.
Fig.1 MartinLogan Aerius, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)
The original Aerius had a vicious cabinet resonance at 230Hz, which was associated with a wrinkle in the impedance trace at the same frequency. The i still had a slight wrinkle at a slightly lower frequency—200Hz—and investigating the cabinet's vibrational behavior with a tape accelerometer revealed that this mode was reduced in amplitude.
In the time domain, the 1998 Aerius i's step response (fig.2) is virtually identical to that of the 1993 speaker's, as was the waterfall plot (not shown). A point that has become apparent in the seven years since I first measured the original Aerius is that the hashy-looking appearance of panel speaker waterfall plots is not only due to the chaotic nature of the diaphragm, but also to the presence of early reflections. A cumulative spectral-decay plot cannot easily distinguish between the effect of the multiple arrivals and delayed energy due to resonances.
Fig.2 MartinLogan Aerius, on-axis step response at 50" (5ms time window, 30kHz bandwidth).
In the frequency domain, the i's amplitude response (fig.3) is broadly similar to the original's, with the top three octaves of the panel's output shelved-down. But the new speaker has less presence-region apparent, which may be the result of the revised crossover design. That the speaker doesn't sound recessed is probably due to the fact that this frequency region is not suppressed off-axis to the sides of the panel (fig.4), which in smaller rooms will tend to balance the lack of on-axis energy in the same region.—John Atkinson
Fig.3 MartinLogan Aerius, anechoic response on-axis at 50", averaged across 30 degrees horizontal window and corrected for microphone response, with the nearfield woofer response plotted below 300Hz.
Fig.4 MartinLogan Aerius, lateral response family at 50", normalized to response on tweeter axis, from back to front: differences in response 90 degrees-5 degrees off-axis, reference response, differences in response 5 degrees-90 degrees off-axis.