Atlantic Technology AT-1 loudspeaker Measurements
Sidebar 3: Measurements
I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone to measure the Atlantic Technology AT-1's behavior in the farfield. For the nearfield measurements, I used an Earthworks QTC-40 microphone. The AT-1's voltage sensitivity is specified as 89dB. My estimate was actually a little higher than that, at 90dB(B)/2.83V/m. However, the AT-1 draws 2W from the amplifier at this voltage level rather than the expected 1W, its impedance (fig.1) averaging 4 ohms rather than the specified 6 ohms. The value of the AT-1's impedance in the treble depends on the setting of its rear-panel Tone switch. The highest impedance between 2 and 20kHz is with the switch in the "" position, the lowest with it in the "+" position. Though the electrical phase angle is generally low, the impedance drops to 1.6 ohms above the audioband, and there is a minimum value of 3.3 ohms at 200Hz. A good amplifier rated at 4 ohms will work best with this speaker.
The traces in fig.1 are free from the wrinkles that would indicate the presence of enclosure panel resonances. However, I did find several strong modes. Fig.2, for example, is a cumulative spectral-decay plot calculated from the output of a plastic-tape accelerometer fastened to the center of one of the sidewalls, 10" from the speaker's base; a single strong mode can be seen at 246Hz. However, this mode was present over only a small area. It disappeared when the accelerometer was placed on the same sidewall level with the tweeter, replaced by a slightly stronger mode at 324Hz. The rear panel had a strong resonance at 281Hz. Fortunately, however, I could find no resonances on the front baffle, which directly faces the listener, and any resonances produced by which would have the most effect on the sound. EL did note that he couldn't hear any cabinet-derived coloration.
In his review, Erick Lichte discusses Atlantic Technology's H-PAS loading, and you can see from the exploded diagram of the AT-1 that the vent at the foot of the front baffle is not so much a reflex port as the end of a folded transmission line. Even so, the AT-1's impedance plot has the double-humped low-frequency behavior typical of a reflex design tuned to 40Hz, though the third small peak in fig.1, at 105Hz, suggests that something more complicated is going on. With a conventional reflex speaker, the woofer and port step responses occur simultaneously but are in opposite polarity to one another. With the AT-1, however, you can see (in fig.3) that while the port's step (red trace) is in opposite polarity to that of the woofers (blue), it is delayed by the length of the line, and emerges approximately 4 milliseconds after the woofers' step, to blend smoothly with the latters' decay.
Nevertheless, the nearfield frequency response of the woofers (fig.4, blue trace) does have the minimum-motion notch at the port tuning frequency that is typical of a reflex design, and the vent's output (red) does peak in the same region. The vent's upper-frequency range is more extended than with a classic reflex design, however. There is also some low-level liveliness evident in the midrange, with a small peak on the outputs of both vent and woofers at 324Hzthe same frequency as one of the side-panel resonances noted earlier.
Higher in frequency in fig.4, the woofers' farfield response is very even and is crossed over to the tweeter a little higher than the specified 2kHz. The rolloff is steep, and though a resonant peak is visible at 18kHz, this is well suppressed by the crossover. The tweeter's farfield output with the Tone switch set to its central position (green trace) is impressively flat, and extends at full level to the 30kHz limit of this graph.
Fig.5 shows how these individual outputs sum in the farfield, averaged across a 30° horizontal window centered on the tweeter axis. The AT-1's upper-frequency response is superbly flat. At lower frequencies, while the summed nearfield response extends almost to 30Hz, 6dB, there is not quite as large an upper-bass boost as I was expecting from the nearfield measurement technique, which implies that the AT-1's lows are a little overdamped. But the benefit of this is that, as EL found, the AT-1's low frequencies are "not only deep and powerful, but taut and tuneful."
The responses in figs. 4 and 5 were taken without the perforated metal grille. Fig.6 shows the effect of the grille on the farfield tweeter-axis response: It introduces some peaks and dips in the mid- and high treble that render the overall response less flat in these regions. Fig.7 shows the effect of the three-position Tone switch on the rear panel. The "" position, which shelves the tweeter down by 2.3dB, was how EL preferred the sound; the "+" position shelves it up by 1dB. Both figures are different from the +1.8dB, 1.5dB specification, though the overall range of change is the same: 3.3dB.
The AT-1's lateral dispersion without the grille (fig.8) is wide and even, though with a touch of off-axis flare at the bottom of the tweeter's passband. This may contribute to EL's finding the AT-1's treble to sound "a touch hard in the low treble when pushed to realistic concert levels." The tweeter also does get very directional above 12kHz or so, which possibly correlates with EL's finding that the speaker's top octaves sounded a touch airless. The tweeter axis is 34" from the floor with the speaker sitting on its feet, a couple of inches below what we've found to be the ear height of the typical seated listener. The plot of the AT-1's vertical dispersion (fig.9) indicates that the speaker maintains its flat balance over quite a wide (+5°/10°) window centered on the tweeter axis, but that a suckout in the crossover region develops above that window. As with almost all speakers, don't listen to this one while standing up.
In the time domain, the AT-1's step response on the tweeter axis (fig.10) indicates that all three drive-units are connected with positive acoustic polarity, with the smooth transition between the tweeter's and woofers' steps suggesting optimal crossover design. The cumulative spectral-decay plot (fig.11) is clean overall, but with a slight ridge of delayed energy just below the crossover to the tweeter.
Atlantic Technology's AT-1 offers a superbly flat frequency response with surprisingly extended low frequencies for a pair of 5.25" woofers. But I shared EL's difficulty with the recessed binding posts.John Atkinson