PSB Imagine T2 Tower loudspeaker Measurements
Sidebar 3: Measurements
I measured the PSB Imagine T2 Tower using DRA Labs' MLSSA system and calibrated DPA 4006 and Earthworks QTC-40 microphones. The T2 is specified as having an anechoic voltage sensitivity of 88dB; my estimate was slightly lower, at 87.3dB(B), almost within the margin of error. PSB specifies the impedance as being 6 ohms, with a minimum magnitude of 4 ohms. However, as fig.1 shows, the impedance drops below 6 ohms for almost the entire midrange, with a minimum value of 2.4 ohms at 570Hz. The electrical phase angle is also quite inductive in the upper midrange. Overall, it would be best to use a good 4 ohmrated amplifier or receiver with the Imagine T2.
A small wrinkle at 23kHz in the impedance traces suggests that this is the frequency of the tweeter's fundamental "oil-can" resonance. A similar discontinuity in the traces just below 300Hz suggests that there is some enclosure resonance present in this region. Investigating the vibrational behavior of the cabinet's walls with a plastic-tape accelerometer revealed a fairly strong resonance at 328Hz, with lower-level modes at 359 and 281Hz (fig.2). The exact proportions of these three resonances varied depending on where I placed the accelerometer. Though Robert Deutsch noted some minor "box resonances" at very high levels, he didn't mention hearing any midrange congestion. It's possible, therefore, that this behavior measures worse than it sounds.
To the left of fig.3 are shown the individual nearfield responses of the midrange drive-unit, the three woofers, and the three ports; on the right are the farfield responses of the tweeter and midrange (black trace) and the woofers (red trace above 350Hz). The upper crossover appears to occur around 600Hz rather than the specified 500Hz, with steep filter slopes. The woofers all cover different passbands; to my surprise, it was the middle woofer (red trace) that extended highest in frequency, the top woofer (blue) having a more restricted bandwidth; the bottom woofer's bandwidth was even more restricted (green). The ports (gray, brown, and dark red traces) all peak between 35 and 75Hz, and the top port (gray) and the middle port (dark red) have peaks visible in their midrange outputs. All three woofers have minimum-motion notches apparent between 47 and 49Hz, the frequency of the saddle in the impedance-magnitude plot.
The Imagine T2's manual implies that the optimal response will be obtained on the tweeter axis. Fig.4 shows the speaker's farfield response, averaged across a 30° horizontal window on the tweeter axis and spliced at 300Hz to the complex sum of the nearfield responses plotted. The T2 is superbly flat in the midrange and treble, with just a slight amount of excess energy apparent above 9kHz. Much of the low-frequency boost will be an artifact of the nearfield measurement technique, which assumes a 2pi (half-space) acoustic environment for the drive-units rather than the usual 4pi. But there does appear to be a little too much bass energy. As RD said of the T2's bass, it was "deeper and more powerful than I would have expected, given the driver complement and the size of the cabinet."
As I expected from its narrow baffle and small-diameter drivers, the Imagine T2's lateral dispersion was textbook perfect below the top octave (fig.5), with a smooth, even, controlled rolloff at higher frequencies to the speaker's sides. However, the fact that more top-octave energy is apparent off-axis might exacerbate the on-axis excess in the same region. Even so, it's fair to note that RD found the T2's highs to sound sweet, smooth, and extended. The T2's tweeter is 35" from the floor; the speaker's plot of vertical dispersion, normalized to the tweeter-axis response (fig.6), indicates the development of a suckout at the upper crossover frequency (which appears to be 2.4kHz) more than 5° above that axis. As is almost always the case, this is not a speaker to be listened to when standing up.
The Imagine T2's step response on the tweeter axis (fig.7) shows that all five drive-units are connected in positive acoustic polarity, and that, as is usually the case with a flat-baffled speaker, first to arrive at the microphone is the tweeter's output, followed by that of the midrange, then that of the woofers. The decay of the midrange step smoothly blends with that of the woofers, implying optimal crossover design. However, a slight discontinuity in the decay of the tweeter's step suggests that the optimal blend between the tweeter's output and that of the midrange driver will actually occur just above the tweeter axis. The T2's cumulative spectral-decay plot (fig.8) is clean in the treble.
As has every other PSB speaker we have reviewed at Stereophile, the Imagine T2 Tower's measurements offer a clean bill of measured health.John Atkinson