MartinLogan Aerius loudspeaker Measurements part 3
Fig.8 MartinLogan Aerius, cumulative spectral-decay plot at 45" (0.15ms risetime).
As I explain in the Quest Z review, this kind of waterfall behavior is typical of panel speakers in that a large, uniformly driven, low-mass diaphragm appears to exhibit chaotic behavior. While the diaphragm's average position responds uniformly to the driving force, there are small areas which move more than the average and others that move less. I believe that such behavior is often mistaken by audiophiles for "clarity," "fast transients," and "transparency." I also believe that it tends to accentuate/exaggerate detail by surrounding each transient edge of the music with a little halo of hash. However, whether its presence outweighs the virtues of any particular panel speaker is an individual decision. In the case of the Aerius, I felt that the sound developed glare or hardness only at very high playback levels. At more practical levels, I reveled in the wealth of unforced detail presented. I also failed to hear any significant distortion or overload problems in the bottom two octaves of the panel's passband, unless the playback level was ridiculously loud.
Harking back to the Aerius's impedance plot (fig.1), a wrinkle at approximately 230Hz implied some sort of cabinet problem. Using a simple PVDF accelerometer (footnote 7), I investigated the speaker's structural resonances. The woofer bin does indeed have a strong resonant mode near this frequency, as can be seen from the waterfall plot calculated from the impulse response of the accelerometer fastened to the center of the back panel (fig.9). This mode could also be found on the side panels and, to a lesser degree, on the curved surface of the front stator. The latter also featured a mild mode at 305Hz, again correlating with the mild wrinkle in the impedance trace. Given the high level of the 227Hz mode, I'm surprised that I didn't hear much lower-midrange congestion. Turning the speakers around so that their backs faced the listening seat made this problem very audible. But with the speakers set up correctly, only occasionally was I reminded that there was something untoward going on in this region.—John Atkinson
Fig.9 MartinLogan Aerius, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the woofer cabinet back panel. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)
Footnote 7: See Stereophile, June 1992, p.205; and September 1992, p.162.—JA