Gradient Revolution loudspeaker Measurements

Sidebar 2: Measurements

DO estimated the Revolution's sensitivity at 82dB/W/m (footnote 1). My B-weighted measurement was a little lower, at 81.7dB/2.83V/m. This speaker will definitely need plenty of watts to come to life—leave your low-powered single-ended tube amplifiers in their shipping cartons! The impedance curve (fig.1) will also make life hard for the accompanying amplifier, dropping to 3 ohms in the power region of the upper bass. The high range of the impedance magnitude—from 3 ohms at 60Hz to 26.5 ohms at 2.1kHz—will also make the tonal balance of the speaker very dependent on the output impedance of the partnering amplifier. (See Martin Colloms' explanation of why this should be so in last month's Stereophile, Vol.18 No.4, p.59.) With an amplifier with a highish source impedance—DO's preferred Air Tight ATM-3 monoblock offers 0.8-1.0 ohms from its 2-4 ohm output transformer tap, depending on frequency, up to 1.75 ohms from its 6-8 ohm tap—the Gradient's mid-treble will be pushed forward at the listener, the midbass shelved down.

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

This will exacerbate the speaker's already suppressed top octave, which can be seen in fig.2. This shows the response of the coaxial midrange/tweeter unit on the tweeter axis at a microphone distance of 50" (footnote 2), spliced to its nearfield rolloff and the nearfield response of the woofer. The sharp dip on-axis at 16kHz is presumably a diffraction effect due to the tweeter's symmetrical acoustic environment. The crossover from the midrange to the woofer occurs at 200Hz, with the low-frequency driver's output peaking between 30 and 80Hz. This implies reasonably good bass extension: the -6dB point lies at 17Hz, though the dipole-loaded drive-units will not be able to reproduce any significant power levels in the low bass.

Fig.2 Gradient Revolution, acoustic crossover on tweeter axis at 50", 34" from the floor, corrected for microphone response, with nearfield woofer and midrange responses plotted below 1kHz and 300Hz, respectively.

Fig.3 shows the Revolution's response averaged across a 30 degrees horizontal window on the tweeter axis, again spliced to the woofer's nearfield response. The upper midrange is both a little forward and uneven, but the audible-region balance is otherwise impressively flat. The tweeter's "oil-can" resonance peak has an amplitude of almost 20dB, but its frequency is 26kHz—above the hearing range of anyone I know.

Fig.3 Gradient Revolution, anechoic response on tweeter axis at 50", averaged across 30 degrees horizontal window and corrected for microphone response, with nearfield woofer response below 120Hz.

The plot of the Gradient's horizontal dispersion (fig.4) reveals that the on-axis notches in the top octave fill in to some extent. Below that frequency region, although the shelving-down of the response is very even with frequency, the speaker's radiation pattern is more directional than usual—as is to be expected from its cardioid design. While this will reduce the effect of side-wall reflections, as DO correctly points out, it will also make the speaker sound a little subdued in all but the very liveliest rooms—as, indeed, DO found. Due to the midrange/tweeter's symmetrical construction, the Revolution's vertical dispersion was very similar to its horizontal behavior. I haven't shown it, but it should be noted that this speaker is therefore relatively noncritical regarding listening axis. A listening-ear height of anywhere between 28" and 43" will give the designed-for tonal balance.

Fig.4 Gradient Revolution, horizontal 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.

In the time domain, the Revolution's step response (fig.5) reveals that all the drive-units are connected with the same polarity—the tweeter output is the little up-down spike just before the 4 millisecond mark, followed by the midrange just after, then the woofer 2ms or so later. Despite the coaxial drive-unit construction, the speaker is not time-coherent.

Fig.5 Gradient Revolution, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Finally, the Revolution's cumulative spectral-decay, or waterfall, plot (fig.6) is superb. There's only some residual energy storage in the upper midrange.—John Atkinson

Fig.6 Gradient Revolution, cumulative spectral-decay plot at 50" (0.15ms risetime).

Footnote 1: Other than impedance, all acoustic measurements were made with the DRA Labs MLSSA system and a calibrated B&K 4006 microphone.—John Atkinson

Footnote 2: To minimize reflections from the test setup, the measuring microphone is flush-mounted inside the end of a long tube. Reflections of the speaker's sound from the mike stand and its hardware will be sufficiently delayed not to affect the measurement.—John Atkinson

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