ADAM Audio Classic Column MK3 loudspeaker Measurements
Sidebar 3: Measurements
I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone to measure the ADAM Classic Column MK3's frequency response in the farfield. For the nearfield responses I used an Earthworks QTC-40, with its small, ¼" capsule. I left the speaker's grille off for these measurements.
My estimate of the ADAM's voltage sensitivity was 88.5dB(B)/2.83V/m: slightly below the specified 90dB. The impedance remained below 6 ohms for much of the midrange and bass (fig.1), with a minimum magnitude of 3.35 ohms at 155Hz. The electrical phase angle was generally low, but the rising impedance and increasingly capacitive angle below 25Hz suggest the presence of a DC-blocking capacitor in the woofer feed.
While there is a suspicious-looking discontinuity at 600Hz in the impedance trace, I found no cabinet resonances at that frequency. The enclosure's sidewalls seemed well damped; only the rear panel had some significant resonant modes. Fig.2 shows a cumulative spectral-decay plot calculated from the output of a plastic-tape accelerometer fastened to the rear panel's center; modes of delayed energy can be seen at 313, 457, and 668Hz. I doubt these will have any audible consequences, given that this panel faces away from the listener.
The minimum value of 3.6 ohms at 44Hz in the impedance-magnitude trace suggests that this is the ports' tuning frequencyhowever, the nearfield responses of the upper woofer (fig.3, green trace) and lower woofer (blue) both have the expected minimum-motion notch a little lower in frequency, at 41Hz. This is where the back pressure from the port resonance holds the woofer cone stationary, and the outputs of the upper (red) and lower (brown) ports peaks narrowly between 30 and 60Hz. Both ports roll off sharply above that region, but also apparent in the midrange output of each port is a peak at 600Hz, the frequency of the glitch in the impedance graph. These peaks are well down in level, however.
Fig.3 shows that the lower woofer (blue trace) rolls off above 150Hz, while the upper woofer (green) extends two octaves higher in frequency, before crossing over to the X-ART units (black) just below 1kHz. The upper woofer rolls off sharply, and while a peak at 4kHz is apparent in its output, this is well suppressed by the crossover. The response of the AMT drive-units is extremely flat, with just a slight excess apparent around 10kHznot a region that adds brightnessand an output that extends at full level to the 30kHz limit of the graph. Fig.4 shows how these individual responses sum in the farfield, averaged across a 30° horizontal window on the tweeter axisthe low frequencies extend to the port tuning frequency of 44Hz, with then a steep rolloff. Higher in frequency, this speaker is extraordinarily flat.
Whether so flat an on-axis response translates to a neutral tonal balance depends a great deal on a speaker's behavior off axis. The Classic Column MK3's horizontal dispersion is shown in fig.5. The speaker's off-axis behavior is relatively uniform up to 12kHz, above which the radiation pattern dramatically narrows, which, in a typical room, will compensate for the slight excess at the base of the top octave. In the vertical plane (fig.6), the ADAM's response hardly changes at all over a ±5° window centered on the tweeter axis, which is significant considering that the tweeter is a high 42" from the ground.
Turning to the time domain, fig.7 indicates that the tweeter and woofers are connected in positive acoustic polarity, the midrange driver in inverted polarity. However, the decay of each drive-unit's step blends smoothly with the start of that of the next-lower unit, which suggests optimal crossover design and correlates with the superb integration of the drive-unit outputs seen in fig.4. Kal Rubinson commented very favorably on the Classic Column MK3's upper-frequency transparency, and the ADAM's cumulative spectral-decay plot on the tweeter axis is indeed superbly clean (fig.8). (Ignore the black ridge of energy just below 16kHz in this graph, which is caused by interference from the measurement computer's video circuitry.)
Measuring audio components can sometimes be a frustrating business, as anomalous behavior requires further investigation to make sure that what is being observed is intrinsic to the device under test. By contrast, examining the ADAM Classic Column MK3's performance was an unalloyed pleasure: Each measurement indicated sensible and effective audio engineering.John Atkinson