PSB Imagine Mini loudspeaker Measurements
Sidebar 3: Measurements
I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone to measure the PSB Imagine Mini's frequency response in the farfield, and an Earthworks QTC-40 for the nearfield and spatially averaged room responses. With its ¼" capsule, the Earthworks mike is small enough to not significantly affect the tuning of the port when used for a nearfield measurement.
As expected from its small size, the Imagine Mini is not very sensitive, my estimate coming in at 85.5dB(B)/2.83V/m. This is a couple of dB lower than average, but within experimental error of the specified 85dB. The specification lists the impedance as 4 ohms and, as shown in fig.1, the magnitude does drop below 6 ohms for the entire midrange, reaching a minimum value of 3.15 ohms at 300Hz. The combination of 4.4 ohms and 45° electrical phase angle at 177Hz means that an amplifier rated into 4 ohms should be used if the little PSB is to sound at its best.
The impedance traces appear free from the small discontinuities in the midrange that would imply the existence of cabinet resonances of some kind. Nevertheless, when I investigated the enclosure's vibrational behavior with a simple plastic-tape accelerometer, I found a fairly strong mode at 488Hz on all surfaces, as well as a second resonance an octave higher that was present only on the sidewalls (fig.2). This graph was taken with the speaker sitting on upturned spikes, which allows the resonances to develop to their fullest extent. Taking the cones away so that the speaker sat on its hard rubber base reduced the amplitude of these resonances a little, but they remain the Mini's Achilles' heel; some experimentation will be necessary to minimize any coloration they give rise to.
The saddle centered on 78Hz in the impedance-magnitude trace in fig.1 suggests that this is the tuning frequency of the small port on the rear panel. This is confirmed by the nearfield response of the woofer (fig.3, blue trace), which has a notch at the same frequency. (The woofer cone is being held stationary by the back pressure from the port resonance; therefore, all the output at this frequency comes from the port.) The port's response (red trace) peaks between 60 and 120Hz, with textbook 12dB/octave rolloff slopes above and below that region, disturbed only by a low-level resonance just below 1kHz. The sum of the woofer and port outputs, taking into account acoustic phase and the different distances of the two radiators from a nominal farfield microphone position, is shown as the black trace below 300Hz in fig.3. The slight rise in the upper bass is entirely an artifact of the nearfield measurement technique; the Mini will be flat down to 100Hz or so, and 6dB at the port tuning frequency.
Higher in frequency in fig.3, the Imagine Mini's response, averaged across a 30° horizontal window on the tweeter axis, is basically flat, though with slight excesses of energy in the upper midrange and top two octaves. Paul Barton sent me a graph taken in the NRC's anechoic chamber, displaying the average of the on-axis response and the responses 15° to either side (fig.4). This was almost identical to the trace in fig.3 above 400Hz. The tweeter's titanium dome has a fundamental resonance at 24.8kHz, which results in a large peak above the audioband. While this will have no audible consequences with CDs, which will not excite this resonance, I do wonder if the presence of this resonance contributed to the increased audibility of clicks I thought I noticed with LP rips sampled at 192kHz.
The PSB's lateral dispersion (fig.5) is smooth and even in the midrange and presence region, but the radiation pattern narrows a little above 4.4kHz. In the vertical plane (fig.6), a suckout develops in the crossover region more than 10° below the tweeter axis, suggesting that shorter stands will work better than tall ones. (PSB's own PFS-27 stands are 26" high.)
I performed my usual spatially averaged response measurement in my listening room. Using SMUGSoftware's FuzzMeasure program, I average 20 spectra taken for the left and right speakers individually in a rectangular grid centered on the positions of my ears. This eliminates the more egregious effects of room acoustics on the measurement, and integrates the direct sound of the speakers with the in-room energy to give a curve that correlates quite well with the perceived tonal balance. The PSB's in-room response taken in this manner is the red trace in fig.7. It is extraordinarily flat between 100Hz and 3kHz, with then a smooth, gentle rolloff due to the increasing absorptivity of the room furnishings in the top two audio octaves.
The blue trace shows the spatially averaged response of the Emotiva XRT-5.2 towers, with which I compared the PSBs. You can see why I felt the two speakers sounded so similar in the midrange and trebleother than the Emotivas having a touch more energy between 3 and 5kHz and the PSB's ultrasonic tweeter resonance adding a small peak above the audioband, the blue and red traces overlay one another. The XRT-5.2s extend lower in frequency but have considerably more energy apparent in the upper bass, which correlates with my finding the towers to sound a little boomy. The green trace in fig.6 is the spatially averaged response of my 1978 pair of Rogers LS3/5As. With their sealed-box woofer alignment (and aided by the 32Hz diagonal mode in my room), these extend usefully lower in frequency than the reflex-loaded PSBs, but their midrange and low treble are less well balanced than the Minis'.
The Imagine Mini's step response on the tweeter axis (fig.8) indicates that both drive-units are connected with positive acoustic polarity. Because a tweeter is physically shallower than a woofer, its acoustic center, when both are mounted on the same flat baffle, is closer to the listener or microphone. Its output therefore arrives first. But as long as the decay of the tweeter's step blends with the start of the woofer's step, their outputs will correctly add in the crossover region, and the ear/brain should be unable to detect the different times of arrival. Other than a ridge of delayed energy at the frequency of the ultrasonic tweeter resonance, the Imagine Mini's cumulative spectral-decay plot on the tweeter axis shows impressively clean decay (fig.9).
As I've come to expect from loudspeakers designed by Paul Barton, the measured performance of PSB's Imagine Mini is almost beyond reproach.John Atkinson