PMC’s new nearfields push the boundaries of what a compact, two‑way speaker can achieve.
I reviewed the three‑way PMC6‑2 back in SOS April 2022 and was hugely impressed, not only with the monitor’s performance but also with PMC’s new Soundalign browser‑based monitor configuration app.
The two‑way PMC6 reviewed here effectively borrows half that monitor’s low‑frequency elements (one bass driver rather than two, in a half‑sized Advanced Transmission Line enclosure) and also its tweeter. The resulting monitor is not only significantly more compact dimensionally than the PMC6‑2, with comfortably nearfield proportions, it’s also significantly less expensive — though still not exactly cheap. The PMC6 is in the upper price tier of similarly dimensioned two‑way nearfield monitors, and in fact, is pretty much a direct competitor for the PSI A21‑M that I wrote about in the September 2022 issue.
Business End
The PMC6’s appearance very much follows the template set by the PMC6‑2. Its edge‑softened and black‑painted MDF enclosure, black plastic tweeter plate, and similarly black plastic transmission‑line flow conditioner present an aesthetic vibe that says “tool for the job” rather than “feast for the eyes”. You perhaps don’t buy one of PMC’s profesional monitors for its looks, you buy it because you need it for work. And the work in the case of the PMC6 primarily covers traditional stereo nearfield (approaching midfield) monitoring, and secondary roles (for example, surround, rear and height speakers) in multi‑channel systems like Dolby Atmos. And speaking of Atmos, PMC appear to have successfully positioned themselves as major players in the Atmos mix studio market, with multiple high‑profile installations in both Europe and the US. I think it’s likely that a lot of PMC6s will find themselves in Atmos studios.
The PMC6 front panel carries a nominally 150mm diameter bass/midrange driver fitted with a coated‑paper diaphragm that, rather than carrying a conventional central dust cap, is a one‑piece item. A one‑piece diaphragm makes driver assembly more complex because the traditional technique for centring the voice coil around the pole piece from the front using a circular shim isn’t possible; however, one‑piece construction avoids the sometimes negative acoustic effect of a dust‑cap glue joint, and also results in a much neater appearance. The concave dome form of the diaphragm is also relatively unusual and is likely to display a mechanical resonant signature somewhat different from that of more conventional curved conical diaphragms. The diaphragm roll surround is a large‑diameter rubber affair that provides generous low‑frequency displacement. One of the significant challenges presented by the need for the same driver to cover both low and midrange bands is that a surround that works for one is not necessarily optimal for the other. At low frequencies, the surround needs to impart as little mechanical resistance as possible over significant degrees of movement, while at mid frequencies it needs to terminate and dissipate the vibrational energy in the diaphragm. One of the advantages of electro‑acoustic engineering‑led organisations such as PMC is that they have the in‑house skills and resources required to make such compromises work.
The PMC6 sounds spot‑on from the first note.
Mentioning the bass/mid driver brings me to the PMC6’s Advanced Transmission Line low‑frequency loading technique, because by its very nature, transmission line loading makes significant demands on bass driver diaphragm displacement. I included a reasonably comprehensive description of how transmission lines work, and how they’re implemented by PMC, in my review of the PMC6‑2, so I won’t repeat that material here.
Despite my comment earlier that the PMC6 behaves at low frequencies like ‘half’ a PMC6‑2, it’s not quite that simple (nothing with speakers ever is). In terms of its enclosure, the PMC6 is in fact a little under half the size of a PMC6‑2, so that means the length and cross‑sectional area of its internal transmission line are also both a little smaller. That’s reflected in a 3dB drop in maximum sound pressure level and an increase, from 33Hz to 39Hz, in low‑frequency cutoff (at the ‑3dB point). Thirty‑nine Hertz still represents impressively extended low‑frequency bandwidth for a relatively compact monitor — especially as it’s achieved without the troublesome time‑domain and port‑compression issues that reflex loading might bring.
Drivers
Along with enclosure dimensions, the other significant difference between the PMC6‑2 and the PMC6 is the latter’s two‑way format. Without a separate midrange driver to handle the band from around 350Hz to 2kHz, the PMC6 bass/midrange driver takes on that responsibility. And as with any such driver, it’s primarily the relationship between the behaviour of the diaphragm and its surround that will define its success at the job — in particular at higher frequencies (above, say, 500Hz), where the diaphragm stops moving as a whole and enters a more resonant mode. Resolving these sorts of design challenges, where material science and acoustics meet, is a fundamental aspect of speaker design. One of the great advantages that companies such as PMC, who design and engineer their own drivers rather than buying OEM items, possess is the ability to control and specify all the significant components.
Adjacent to the PMC6 bass/midrange driver, but arranged slightly asymmetrically (pairs of PMC6s are ‘handed’), sits the PMC6 27mm‑diameter soft‑dome tweeter, which rolls in at a relatively low 1800Hz. A low crossover frequency takes some of the pressure off the higher‑frequency performance of the bass/mid driver diaphragm, but it requires a tweeter that has particularly good power handling and distortion performance at the low end of its band.
The PMC6 tweeter is the same newly developed driver as employed in the PMC6‑2 and it’s mounted in a similar ABS moulded trim plate. The trim plate incorporates a very short waveguide profile around the dome that’s probably most significant in terms of softening the edge that results from mounting the tweeter body to the back of the trim panel. The tweeter dome has a notably small surround component. This probably helps with response linearity but also might tend less helpfully to raise the tweeter’s fundamental resonant frequency. Surround size against resonant frequency is another of those pesky speaker design compromises that has to be managed. Designing a successful high‑performance dome tweeter has always been mostly about getting the small details right rather than making huge strides in, say, material science. This is why, even though it’s perfectly possible these days to create a tweeter dome in beryllium, diamond or even graphene, a traditional soft‑dome driver can still be competitive. Success is as much about how you build a tweeter as it is about what you make it from.
Making Connections
Around the back of the PMC6, in the traditional location, sits the connection and heatsink panel. Connections comprise a mains socket, an XLR socket that can be configured for either balanced analogue or AES3 digital inputs, an AES3 through socket, and an RJ45 Ethernet socket. The rear panel also carries a two‑line LCD display and a couple of rotary encoders that together enable access to the various monitor configuration menu items. It is far, far easier, however, to configure PMC6s via a network connection and the PMC SoundAlign browser app. If that option is available, the rear‑panel encoders and display are effectively superfluous.
I covered SoundAlign in my PMC6‑2 review so I won’t repeat myself, except to say that SoundAlign continued to impress with its combination of intuitive operation and comprehensive functionality — including its provision of seven EQ bands (two shelf and five parametric). It’s not easy to go back to the old way of configuring active monitors, reaching around the back to search blindly for the correct switches and knobs, and never being sure that both monitors of the pair are in the same configuration.
Downstream from the PMC6 input and DSP‑based EQ and crossover stages are Class‑D power amplifiers rated at 200 Watts for both bass/midrange driver and tweeter.
Hot Fuzz
As is my usual custom, as well as listening to the PMC6 I indulged in a little FuzzMeasure acoustic analysis. Diagram 1 shows the axial frequency response curve I measured from the PMC6, along with response curves taken ±30 degrees vertically off axis (with the monitor in portrait orientation). The response curves are fundamentally very tidy. The axial curve is mostly free of major discontinuities, and the off‑axis curves show well‑controlled driver integration (as witnessed by the sharpness of the suck‑outs caused by off‑axis path‑length differences between drivers). The tweeter response also holds up particularly well above 10kHz in the off‑axis curves.
Diagram 1: The PMC6’s frequency response, measured on axis (red trace), and 30 degrees above and below (green and blue traces, respectively).
There is one intriguing feature in all three curves, and that’s the dip at just over 300Hz. This is within the bass/mid driver’s band, where its diaphragm will be working as a simple piston, so if the discontinuity is not driver‑borne, some unknown phenomenon must be at play. I had my suspicions, but instead of speculating I enquired of PMC’s Principal Acoustic and R&D Engineer Toby Ridley, and he explained it thus: “The dip around 300Hz is caused by the fifth harmonic of the transmission line’s resonant frequency. We typically employ several techniques to minimise the effects of harmonics bleeding from the transmission line and impacting the overall sound field, but some of these techniques become more challenging to implement and less efficient as the cabinet dimensions shrink and the transmission line length is reduced. For example, we can completely remove the third harmonic by offsetting the driver one third of the way down the transmission line, but this leaves the fifth harmonic as the first really challenging one. In the case of the PMC6 though we were really trying to push the boundaries of bass extension and headroom from a small cabinet, and this made it much more challenging to completely remove the fifth harmonic from the vent output. In the end we decided that a small dip around 300Hz was a small price to pay for the improvements we were able to achieve in the extension, headroom, impact and clarity of bass.”
Diagram 2: The output of the PMC6’s ATL vent, between 10Hz‑1kHz.
Toby’s explanation confirmed what I suspected: that the 300Hz dip is created by out‑of‑phase vent output, and the reason I suspected that is illustrated in Diagram 2. The diagram shows a response curve generated by placing the measuring mic right in the mouth of the PMC6 vent. The expected peak at around 45Hz reflects the fundamental resonance of the vent, above which frequency the output generally drops as the vent lining absorbs the rear‑radiated energy of the driver. However, the output temporarily kicks up again in the 300Hz region, and it’s this feature that interferes with the driver output to create the response dip.
This all raises a few questions. Firstly, if the fundamental resonance is at 45Hz, why is the fifth harmonic not at 225Hz (5 x 45)? The answer, again from Toby, is that while the cross‑section tapering used in the transmission line acts to lower its fundamental resonance, higher harmonics are less affected by the taper, so the harmonic series is stretched. The second question raised is: considering all the DSP power in the PMC6, why not equalise the 300Hz dip? The answer is that, as the dip is caused by a phase cancellation, it will not occur at all positions in space around the monitor. At locations where the path lengths of the outputs from driver and vent approach half a wavelength at 300Hz, they will add and create a peak in the response. So using EQ to correct the dip will result in a peak in the in‑room response of the monitor and a subjectively unbalanced result.
Diagram 3: The PMC6 impulse response.
I’ve included one final response curve, Diagram 3, from my FuzzMeasure analysis simply because it’s impressively clean. Diagram 3 shows the axial impulse response of the PMC6. The impulse response impulse is notably compact, with very little smearing or ringing. Monitor impulse responses are rarely as tidy. A tidy impulse response is very often, to my way of thinking, a positive indicator of subjective performance.
The Joy Of Six
And speaking of subjective performance, as with my experience of the PMC6‑2, the PMC6 sounds spot‑on from the first note. The tonal balance on naturally recorded voices and instruments is convincing (no subjective sign of that 300Hz dip) and trustworthy in translation terms, and there’s a satisfying freedom from boxy or nasal speaker coloration. The PMC6’s low‑frequency bandwidth is clearly extended beyond the traditional expectation of its enclosure dimensions, and the extension is achieved without any obvious cost in terms of time‑domain artefacts. It’s truly impressive bass from a relatively compact box and it remains so even when the volume gets genuinely loud. At the other end of the spectrum, PMC’s new tweeter impresses just as it did in the 6‑2. I praised its “detailed and insightful high frequencies” back in my 6‑2 review and I’ve no reason to revise that opinion.
I was also immediately struck by the PM6’s stereo imaging. I quite often listen in mono when first trying a monitor, and the stability and focus of the phantom mono image created by the PMC6 was genuinely noticeable. This says to me that a monitor is producing little in the way of low‑level noise (non‑harmonic distortions, miscellaneous structural resonance, and so on) that isn’t correlated with the input signal. The quality of the imagery generated by the PMC6 remained really impressive once I moved on to listening in stereo, with great focus of off‑centre as well as central mix elements.
Hand in hand with the stereo image quality, the PMC6 displays great resolving power of fine mix and performance details. I was struck repeatedly by hearing subtle new quirks and perspectives on material I thought I knew. So, to my ears, the PMC6 marks another great success for PMC. It’s just as impressive, I think, as the 6‑2 but, as it’s more affordable, perhaps an even more significant achievement.
Alternatives
If I were in the fortunate position to be considering a pair of PMC6s, on my list also would be the ATC SCM20ASL, the PSI A21‑M and the Genelec 8341A.
Pros
- Trustworthy and extended bass.
- Neutral tonal balance with minimal coloration.
- Exceptional detail‑resolving power and stereo imaging.
- Configurable using SoundAlign.
Cons
- None.
Summary
The PMC6 is a perfect example of the kind of thing that a monitor should do: wide bandwidth with tonal accuracy so that work translates well, combined with great detail‑resolving power so that mix decisions can be made with all the information audible.
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