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.

Fig.1 PSB Imagine Mini, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).

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.

Fig.2 PSB Imagine Mini, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of side panel (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

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.

Fig.3 PSB Imagine Mini, anechoic response on HF axis at 50", averaged across 30° horizontal window and corrected for microphone response, with nearfield responses of woofer (blue) and port (red) and their complex sum (black), respectively plotted below 350Hz, 1kHz, 300Hz.

Fig.4 PSB Imagine Mini, anechoic response on HF axis at 50", averaged across 30° horizontal window in NRC anechoic chamber.

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.)

Fig.5 PSB Imagine Mini, lateral response family at 50", normalized to response on HF axis, from back to front: differences in response 90–5° off axis, reference response, differences in response 5–90° off axis.

Fig.6 PSB Imagine Mini, vertical response family at 50", normalized to response on HF axis, from back to front: differences in response 45–5° above axis, reference response, differences in response 5–45° below axis.

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.

Fig.7 PSB Imagine Mini, spatially averaged, 1/6-octave response in JA's listening room (red); of Emotiva XRT-5.2 (blue); of BBC LS3/5A (green).

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 treble—other 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).

Fig.8 PSB Imagine Mini, step response on HF axis at 50" (5ms time window, 30kHz bandwidth).

Fig.9 PSB Imagine Mini, cumulative spectral-decay plot on HF axis at 50" (0.15ms risetime).

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

PSB Speakers International
633 Granite Court
Pickering, Ontario L1W 3K1
(905) 831-6555

remlab's picture


zomin8tor's picture

The Imagine Mini's are small speakers; ideally suited for desktop or another nearfield experience. As such, and when connected to the NAD D7050 (desktop) amp, and when combined with the PSB 125 sub, I would put these speakers up against almost any floor standing speakers, in an apples to apples comparison, eg, floor standing in a living room and the Mini's in a nearfield environment. For the reviewer to expect that these beautiful little gems were ever going to fill up a room -- especially without a matching sub, was naïve at best. Although, to his credit, the floor stands do insinuate that you could use these in a living room -- but to me that application is more as a subcomponent part of a 5.1, 7.1, etc., system, NOT as the mains.

keith_h's picture

When my wife and I auditioned these some years ago, we had never heard of PSB. We were sure B&W is what we needed. But these sounded much better and the ability to mount neatly on the wall easily got this over the line for our sensibilities.

OK so its an old review, no problem. But it has many of the tropes of audio reviews, pointless observations when a certain piece of equipment is unsuited to a task that its clearly not intended to fulfil. Important looking charts and graphs that tell you nothing of how it will perform in your environment, only how it performed in a lab test. Or how it compares to "reference equipment" which most of us will never listen to let alone own. I never understand the need to supply this information.

Additionally the reviewer shows they have no clue about how to most effectively utilise these speakers and so makes some unusual assumptions and generalisations while trying to figure it out.

Here's what I've learned as a long time owner. Despite these speakers being around a looong time as I write they still kick butt sonically, when setup right. They are small for a reason and work well in a small space. They will not fill an auditorium, and while they do have a matching floor stand, this is not where they shine. We had them on floor stands for a while, but when we mounted them on the matching wall stands, amazeballs. This positions them where the designers intended and brought them to life compared to using floor stands.

The wall stands take care of the way the wiring is configured as well, something that seems to confound all reviewers. Its a simple thing folks, a neat touch and works as intended concealing the wiring. I managed to easily get banana plugs in there, unsure what everyone else's issue is with this configuration other than its not typical.

Paired with a small sub like the PSB 200, you get full and pleasing sound reproduction entirely suited to a small space. High up on the wall they are well out of the way and pretty much invisible as well in the white finish.

They are not especially efficient, but paired with a decent amp, they light up nicely. You won't get epic dynamics, but you will get a nice smooth clean sound that punishes poorly recorded material, and will bring out things you've not hear before in some recordings. Poorly recorded material will sound truly terrible, flat and lifeless. Well crafted recordings are eye opening, these speakers are quite revealing in that way.

You won't get PA levels of volume of course, but its good to know you can rock the house if you must, especially with a sub.

After some years of ownership, they continue to deliver a satisfying listening experience.

keith_h's picture

A further observation following environmental changes likely unforseen when these speakers were first reviewed. First the rise of streaming and digital music which has all but seen the demise of the traditional record shop as we once knew it. Vinyl is on the rise for enthusiasts, but digital is largely how things get done these days, either streaming or digitised recordings of some sort. My CD collection for example resides on a NAS as FLAC files.

The next thing is the rise of the single box streaming solution, compact powerful devices that connect all your digital music making it accessible via a mobile device of some sort. And finally, the implementation of room correction software like Dirac.

Imagine all this in one box and you have the NAD M10 in my case, the only input is an ethernet cable and connected to these venerable speakers, elevates them even further. It's a testament to the original design and implementation of these speakers as to how good they sound on the newest equipment all these years later.

Dirac performs some sort of incredible magic with them, sound just comes from everywhere, almost like a pair of Maggies. There is greater detail and dynamics present in music we have been listening to for years. In some cases, the music just soars. It's an interesting experience from two small boxes high on a wall and a small sub which now has its crossover controlled by Dirac. Hence low bass is present and deeply satisfying.

Long story short, all the things that were always very good, now raised to a new level. The PSB Imagine Mini combined with the PSB Sub Series 200 have been an excellent choice. Combined with the NAD M10, it now delivers hours of next level enjoyment.