MartinLogan Prodigy loudspeaker Measurements part 2

In fig.5, you can easily see the ground reflection in the Prodigy's impulse response after the 5ms mark. But the impulse response itself, just before the 4ms line, has a good, time-coherent shape marred only by some low-level high-frequency ringing.

Fig.5 MartinLogan Prodigy, impulse response at 50" (5ms time window, 30kHz bandwidth).

The ground reflection can also be seen in the step response (fig.6), but note what a perfect right-triangular shape the panel's initial step has. The setback of the bass cabinet results in the positive-going step of the summed woofer outputs coinciding in time with the panel step's recrossing of the time axis. This can be seen in fig.7, which shows the individual step responses (the panel is in red, the woofers in blue), and implies a seamless transition between the two sections in the frequency domain.

Fig.6 MartinLogan Prodigy, step response at 50" (5ms time window, 30kHz bandwidth).

Fig.7 MartinLogan Prodigy, step response of panel (red) and woofers (blue) at 50" (5ms time window, 30kHz bandwidth).

Finally, the presence of that early ground reflection interferes with the Prodigy's cumulative spectral-decay plot. (The presence of reflections in this kind of waterfall plot is indistinguishable from delayed energy due to resonances.) But windowing the impulse response just before the reflection gave the plot shown in fig.8. The dotted region in this graph indicates where the data are invalid, due to the necessary truncation of the impulse response. But the dropoff of the initial sound is fairly clean up to the 1.5ms mark, broken only by the presence of some delayed energy at 12.6kHz, this correlating with the frequency of the ringing seen in the impulse response (fig.5). This may well be too high in frequency and too low in level to have an effect on the sound, given that LG commented so favorably on the Prodigy's high frequencies.

Fig.8 MartinLogan Prodigy, cumulative spectral-decay plot at 50" (0.15ms risetime).

As I have mentioned before, measuring large panel speakers is an exercise fraught with practical difficulties and, sometimes, frustration. Nevertheless, the Prodigy is the best-measuring electrostatic speaker I have examined. I am not surprised LG was impressed by its sound quality.—John Atkinson

2101 Delaware Street
Lawrence, KS 66046
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