MartinLogan Prodigy loudspeaker Measurements

Sidebar 3: Measurements

As is my current practice, the MartinLogan Prodigy was measured out of doors, in my enclosed backyard. I usually raise the speaker being measured on a high stand so it is well away from the ground. In the case of the Prodigy, though, its 133-lb weight and awkward bulk meant that I couldn't get it off the ground. (Apparently, it is impossible to acquire suitable skyhooks in my neck of the Brooklyn woods.) I indicate in the text where this practical difficulty impacted specific measurements.

The Prodigy's voltage sensitivity was a little below specification but still good for a panel speaker, at an estimated 88dB(B)/2.83V/m, and this will be aided in-room by the Prodigy's line-source behavior. (The interaction of the tall source and the floor and ceiling results in an in-room sound that does not fall off with distance as quickly as with a conventional point source.) However, the plot of the MartinLogan's impedance magnitude against frequency (fig.1, solid trace) indicates it to be, as specified, basically a 4 ohm load through much of the audioband. This will mean the speaker will actually draw 2W from the amplifier at that voltage level, not 1W.

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

As with other electrostatics, the Prodigy's impedance drops to a very low value above the audioband: to 1 ohm at 25kHz, which I suspect is the series impedance of the internal and external wiring. Fortunately, even with wideband media like SACD and DVD-Audio, there is very little energy in this frequency region, so the partnering amplifier will not be required to drive much power into this low impedance. But look at what happens around 8kHz, a frequency where there is significant musical energy: an impedance of 3.55 ohms coincides with a capacitive phase angle of -58 degrees, which results in a much lower effective impedance in this region. A powerful amplifier capable of handling 2 ohm loads will work best with the Prodigy.

The two traces in fig.2 show the individual responses of the Prodigy's panel and its dynamic woofer section. The former was taken on-axis halfway up the panel, on its centerline, at a distance of 50"; the latter is actually the complex sum of the front- and rear-woofer nearfield responses, taking into account the differences in distance between the two drive-units and a nominal farfield listening position. (The two woofers are wired out of phase; but because the front and rear woofers face in opposite directions, an electrical pulse results in both cones moving in the same direction.)

Looking first at the panel, a strong reflection of its output from the ground between it and the microphone leads to a narrow but deep suckout at 1.6kHz, and to a rather "furry"-looking trace at higher frequencies. Ignore those characteristics, which are specific to the measuring situation, and the response trend is basically smooth and flat up to 10kHz, above which the speaker's output shelves down by 3-4dB, due to the proximity effect you get with a physically large speaker like the Prodigy. (This effect was described in my measurements accompanying our May 1997 review of the MartinLogan SL3—see Gayle Sanders' "Manufacturer's Comment" on that review, also included in the website reprint.) Moving the microphone out to 100" gave a flatter top-octave balance (not shown).

Fig.2 indicates that the panel rolls off with an approximate 18dB/octave slope below 300Hz, the woofer section coming in smoothly below that frequency. The woofers peak at between 50 and 70Hz, with a typical sealed-box rollout at 12dB/octave reaching -6dB in the low 30s—a little higher than I was expecting from the specification. However, checking my notes revealed that the bass control had not been switched to the "+3dB" position for this measurement. In addition, the close proximity of the woofers to a listening room's floor will tend to boost the low bass. LG did note in his review that, judged with the 1/3-octave warble tones on Stereophile's original Test CD, the Prodigy's bass extended down to 31.5Hz in his room—what I would have expected.

Fig.2 MartinLogan Prodigy, anechoic response on axis midway up panel at 50", averaged across 30 degrees horizontal window and corrected for microphone response, with complex sum of woofer responses plotted below 300Hz.

The presence of the early ground reflection makes the MartinLogan's lateral dispersion plot (fig.3) look a little hashy in the high frequencies. Nevertheless, the panel's dipole behavior can be clearly discerned from this graph. Interestingly, the top two audio octaves feature wider dispersion than the region between 500Hz and 2kHz, presumably due to the panel's curvilinear shape. In normal-sized listening rooms, this will tend to compensate for any top-octave depression resulting from the proximity effect. Though the vertical dispersion plot (fig.4) also looks rather hashy due to the unavoidable early reflection, the speaker's response doesn't change significantly over the height of the panel, meaning that it will be relatively insensitive to changes in listener ear height.

Fig.3 MartinLogan Prodigy, horizontal response family at 50", normalized to on-axis response, from back to front: differences in response 90 degrees-5 degrees off-axis; reference response; differences in response 5 degrees-90 degrees off-axis.

Fig.4 MartinLogan Prodigy, vertical response family at 50", normalized to on-axis response, from back to front: differences in response 15 degrees-5 degrees above reference axis; reference response; differences in response 5 degrees-10 degrees below reference axis.

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