Legacy Audio Focus 20/20 loudspeaker Measurements

Sidebar 4: Measurements

At 185 lbs and 55" high, the Legacy Focus 20/20 was too bulky for me to lift into the air for the acoustic measurements, as I usually do. My speaker turntable therefore had to sit on the ground, which meant that the midrange resolution of the farfield measurements was curtailed by the presence of a floor reflection. However, I don't believe this affects the overall validity of the results.

The Legacy's sensitivity was slightly lower than specified, but still a very high 94.5dB(B)/2.83V/m—this speaker will play very loudly even with a low-powered amplifier. However, its plot of impedance (fig.1) reveals that it needs to be used with an amplifier that can deliver high current into low impedances. Not only are there two minima in the bass of less than 2 ohms, and another of less than 3 ohms in the mid-treble, but there is an amplifier-crushing combination of 3.3 ohms magnitude and 60 degrees capacitive phase angle at 20Hz. Fortunately, it is rare in music to have high levels of energy this low in frequency. However, a good, beefy solid-state amplifier will probably work better than a tube design. (This graph was taken with all of the tone-contouring switches in the up/off position, which was how PB auditioned the 20/20. Switching them on/down changed the shape of the impedance curve slightly, but without changing the minima.)

Fig.1 Legacy Focus 20/20, electrical impedance (solid) and phase (dashed) with all three tone switches up/off (2 ohms/vertical div.).

The impedance traces have a slightly wrinkly appearance between 150Hz and 300Hz, which might indicate the presence of cabinet resonances. However, exploring the panels' vibrational behavior with an accelerometer—a higher-tech equivalent of the traditional knuckle-rap test—found that what modes there were were few in number and low in level. Fig.2, for example, is a "waterfall" plot calculated from the output of the accelerometer when it was fastened to the center of the large sidewall. The four modes visible changed their relationship as the accelerometer was moved up and down the sidewall, but remained low in level. The Legacy's designer has obviously done a good job of arranging the cabinet's internal bracing.

Fig.2 Legacy Focus 20/20, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the center of the cabinet's side panel. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)

While the impedance plot implies that the rear-facing ports are tuned to a low 32Hz or so, the Focus 20/20's use of three different 12" woofers and two midrange units leads to complex behavior in the lower midrange and below. This can be seen in fig.3, which shows the nearfield responses of the midrange units (black trace), the upper woofer or transition driver (red), the lower, front-firing subwoofer (blue), the rear-facing subwoofer (green), and the ports (magenta). The midranges extend down to 80Hz or so, with a broad overlap evident between their output and that of the transition driver. The two subwoofers have almost identical outputs, both peaking between 40Hz and 120Hz. The twin ports do peak at their tuning frequency, but cover a wider passband than I was expecting.

Fig.3 Legacy Focus 20/20, nearfield responses of the midrange units (black), transition woofer (red), front subwoofer (blue), rear subwoofer (green), and ports (magenta).

Fig.4 shows a similar picture, plotted over the full audioband, with (from left to right) the port output, the sum of the subwoofer outputs, and the nearfield output of the midrange units spliced above 400Hz to the farfield output of the midrange units and the dome and ribbon tweeters. Given the LF extension of the midrange drivers, it almost looks as if the transition woofer is unnecessary. However, the red trace in this graph shows the sum of the outputs of the midrange units and the transition driver, showing that, together, they do produce a flat response through the lower midrange and the upper bass.

Fig.4 Legacy Focus 20/20, anechoic response of midrange-tweeter array on ribbon tweeter axis at 50", corrected for microphone response and spliced at 400Hz to the nearfield response of the midrange units, with nearfield responses of the subwoofers and port. Red trace is the complex sum of the midrange units and transition woofer, spliced at 400Hz to the farfield output of the three 12" drive-units.

Fig.4 also shows that the output of the tweeters on the ribbon axis is relatively smooth and even through the low and mid-treble, but that the top octave is a little too hot, with a cancellation notch in the region where the dome tweeter crosses over to the ribbon. The trace in fig.5 is a composite. Above 300Hz it shows the farfield response on the ribbon axis, averaged across a 30 degrees horizontal window; below 300Hz, it shows the complex sum of the nearfield responses, calculated while taking into consideration acoustic phase and the different distances of the drive-units from a nominal farfield point. Some of the boost in the mid- and upper bass will be due to the nearfield measurement technique, but it does appear that the Focus 20/20 will produce weighty low frequencies. Ultimate bass extension is a little less than I would have expected from a speaker of this size, but in-room, with the usual boundary reinforcement, the speaker will produce bass down to the mid-20s, as PB found.

Fig.5 Legacy Focus 20/20, anechoic response on ribbon-tweeter axis at 50", averaged across 30 degrees horizontal window and corrected for microphone response, with the complex sum of the nearfield responses (taking into account acoustic phase and distance from the nominal farfield point) plotted below 300Hz.

In the treble, PB noted that the speaker was, if anything, a bit forgiving and soft, yet fig.5 suggests a rather exaggerated top octave. I suspect that what is happening is that, given the weighty bass, the ear latches on to the level of the Legacy's top two treble octaves as its reference, identifying the lack of energy in the upper midrange and low treble as relative softness and lack of aggression.

The Focus 20/20's lateral radiation pattern (fig.6) reveals that the upper crossover notch quickly fills in to the speaker's sides, which is probably why PB didn't toe the speakers all the way in to the listening position. The speaker is a little more directional than usual in the region covered by the dome tweeter, presumably because of the modification of its dispersion by the wide baffle. In a typical room, this, again, will tend to make the Legacy sound a little polite. In the vertical plane (not shown), the speaker's balance changes quite appreciably with listening height. It's probably best to sit with your ears level with the dome tweeter or just above, which will require a higher-than-normal seat given the fact that the ribbon tweeter is 45" from the floor.

Fig.6 Legacy Focus 20/20, lateral response family at 50", normalized to response on ribbon 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.

The Focus 20/20's step response on the ribbon axis (fig.7) is hard to interpret, but it appears that the ribbon supertweeter is connected in positive polarity; the dome tweeter, midrange units, and transition driver in inverted polarity; and (though it can't really be seen in this graph) the subwoofers in positive polarity. Other than the two high-frequency drivers, the step response of each drive-unit hands over smoothly to the one below it, which correlates with good integration in the frequency domain.

Fig.7 Legacy Focus 20/20, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

The farfield cumulative spectral-decay plot (fig.8) is not as clean as the best speakers I have tested, with small amounts of delayed energy evident in the transition region between the midrange units and dome tweeter and at the bottom of the ribbon's passband. But overall, the Focus 20/20 offers respectable measured performance and features a surprisingly well-integrated response—considering it uses seven drivers in a five-way configuration—at a very competitive price.—John Atkinson

Fig.8 Legacy Focus 20/20, cumulative spectral-decay plot at 50" (0.15ms risetime).

Company Info
Legacy Audio
3370 Route 100
Macungie, PA 18062
(800) 283-4644
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