PSB Image 4T loudspeaker Measurements
The elegantly proportioned PSB Image 4T tower was surprisingly sensitive, at an estimated 89.7dB(B)/2.83V/m. Its plot of impedance magnitude and phase against frequency (fig.1) implied that it is also a relatively easy load for the partnering amplifier or receiver to drive, with a minimum value of 3.8 ohms in the lower midrange and a mostly moderate electrical phase angle. The saddle at 47Hz in the magnitude trace indicates the tuning frequency of the twin ports—not that low, given the speaker's size.
Fig.1 PSB Image 4T, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)
The wrinkle at 23kHz is due to the tweeter's ultrasonic resonance and is of no subjective consequence. The wrinkles between 100Hz and 300Hz, however, will be due to cabinet-panel or air-space resonances, and might give rise to audible problems. Fig.2, a waterfall plot calculated from the output of a plastic-tape accelerometer fastened to the side wall 12" from the top, reveals a strong resonance at 430Hz, which might be just high enough in frequency to be benign—I note that BJR wasn't bothered by a lack of clarity in the lower midrange. However, a lower-level pair of modes at around 200Hz might be more bothersome, particularly as they were present on all cabinet surfaces. (The 430Hz mode was evident only on the speaker's side wall.)
Fig.2 PSB Image 4T, cumulative spectral-decay plot calculated from the output of an accelerometer fastened 12" from the top of the cabinet side wall. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)
Fig.3 shows the individual on-axis and nearfield responses of (from left to right) the pair of ports, the pair of woofers, and the tweeter. The last is a little boosted in its passband, but rolls off sharply below 2.3kHz. The lower woofer rolls off earlier than the upper one, which tilts up their summed bass response a little. More important, a combination of a narrow cancellation notch and a peak can be seen between 1kHz and 2kHz, which I suspect is due to acoustic cancellation between the two woofers at this measuring distance on the tweeter axis. I initially wondered if this behavior was at the root of the "powdery" coloration BJR mentioned, but interference phenomena don't generally sound like additive problems. The woofers' minimum motion point lies at 37Hz, a little lower than the tuning frequency of the ports. (The port response is the bandpass peaking between 37Hz and 62Hz.)
Fig.3 PSB Image 4T, acoustic crossover on tweeter axis at 50", corrected for microphone response, with the nearfield woofer and port responses plotted below 350Hz and 500Hz, respectively.