Page 2: PMC6-2

Diagrams

I’ve endeavoured to illustrate the principles of transmission‑line loading with some PMC6‑2 FuzzMeasure data displayed in Diagram 2. The orange curve shows the 20Hz‑1kHz frequency response data from a measuring mic positioned very close (<0.5cm) to one of the 6‑2 bass drivers. The most interesting feature to note is the shallow dip in the driver’s high‑pass roll‑off centred just below 40Hz. This dip, if you examine it closely, actually extends from about 25Hz to just under 60Hz — more than an octave. I’ve marked this in the diagram with a grey region. It reflects the band over which the quarter‑wave resonance of the transmission line results in output and loads the driver, reducing its diaphragm displacement. Close‑mic driver measurement of a reflex‑loaded speaker will show a similar kind of dip at the port resonance, but the dip will typically be much sharper (higher Q) and extend over a significantly narrower band.

Diagram 2.

The blue curve of Diagram 2 shows the 20Hz‑1kHz frequency response data from a measuring mic positioned 50mm down one of the 6‑2’s honeycomb‑shaped port flow conditioner holes. It very clearly shows the broad hump of the quarter‑wave transmission line resonance centred on the same frequency as the dip in the orange driver curve. The blue curve also shows a hint of the change in transmission‑line mode at around 150Hz, where it becomes fully absorptive and its output drops. The rise in output towards 200Hz is the leakage of output from the forward radiation of the bass drivers into the measuring mic (These sorts of close‑mic measurements tend to be usefully illustrative only over a relatively narrow band of frequencies. Sadly, you can’t drive the transmission line without having the bass drivers working, so ‘crosstalk’ will always happen).

The Rest Of It

I’ve devoted many words to the 6‑2’s transmission‑line loading, so it’s high time to move on, not least because the 6‑2 also sports a couple more particularly unusual and interesting elements. Firstly, as I mentioned, the 6‑2’s 55mm‑diameter midrange dome is an unusual‑looking driver. In constructional terms, behind the dome, the driver is relatively conventional (apart from the fact that dome midrange drivers are generally outliers), however the plastic moulded waveguide in which the dome sits is anything but. Waveguides appended to the front of dome drivers have two functions. First, they help to control and define the driver’s dispersion characteristics, and second, they can provide a degree of horn loading that increases the radiation impedance the driver ‘sees’, which consequently increases its efficiency. That extra efficiency can be deployed as increased power handling, and/or to allow the driver to operate to a lower frequency (a more efficient driver run at an attenuated level can be used over a wider bandwidth). The diameter, depth and profile of a waveguide are all significant in terms of the effect on dispersion and efficiency — both of which may also vary significantly with frequency.

Diagram 3: PMC6's midrange driver and waveguide construction.

In choosing to run with a relatively small midrange dome in order that it can reach further up the audio band, the 6‑2’s designers needed the midrange waveguide to offer good efficiency gain at lower frequencies so that the driver could also reach down to a sub‑400Hz bass/mid crossover. They found that a hyperbolic waveguide profile provided the required lower‑frequency efficiency gain, however it didn’t offer the right high‑frequency dispersion characteristics. Conversely, an exponential waveguide profile gave the required higher‑frequency dispersion but not the efficiency gain at lower frequencies. The solution was to design a waveguide that incorporated both hyperbolic and exponential elements, and that explains the unusual shape. The eight inclusions around the dome add hyperbolic regions to a base exponential profile. It’s an unusual and innovative solution that PMC calls ‘N‑compass’. The detail and complexity of the waveguide shape can be seen clearly in Diagram 3.

Panel Show

Describing the second unusual element of the PMC6‑2, and in fact all the 6 Series models, is best done by first describing its rear panel. So, around the back of the monitor there’s the traditional panel that carries connection sockets, mains socket and configuration controls. As far as signal connection facilities go, the 6‑2 offers both balanced analogue and AES3 digital inputs, and an AES3 output for daisy‑chaining. The 6‑2 internal signal flow operates in the digital domain at 96kHz.

Controls fitted to the 6‑2’s rear panel comprise a couple of rotary encoder knobs along with an associated LCD panel that displays a menu structure comprising a variety of configuration and EQ options. However, the second unusual element of the 6‑2 is that it can also be configured over a TCP/IP network via an Ethernet connection to a rear‑panel RJ45 socket. Plugging each 602 of a pair into a network switch and browsing to their IP address opens PMC’s new Soundalign web interface. And once you have access to Soundalign, there’s really no need ever again to look at the 6‑2 rear panel LCD display or touch its rotary encoders.

Soundalign offers control over multiple speakers (an Atmos array, for example), as well as a visual parametric EQ interface.The Soundalign interface is great to look at but also easy to use. The UI designers at PMC have, I think, found a good balance between intuitive accessibility and aesthetic appeal. All of the functions available from the 6‑2’s rear‑panel controls are duplicated within Soundalign. There’s input selection and level gain/trim, room position compensation, desk filter, orientation (landscape or portrait on the two‑way PMC6), subwoofer integration filtering, polarity inversion, and low/high‑shelf filtering, followed by five discrete stages of parametric EQ. Configuring a monitor through Soundalign is simply a matter of selecting the required one by clicking on its Select button (this will usefully switch the colour of the monitor’s front‑panel indicator to the same colour as displayed in Soundalign), and adjusting the required options. The first three Soundalign screenshots show all the configuration options available.

The Soundalign web interface gives you comprehensive control over the monitors via an Ethernet connection.In addition to providing a full suite of configuration options, Soundalign also has the ability to go far beyond a simple stereo monitoring system. PMC have, over recent years, been particularly busy equipping Dolby Atmos studios, and Soundalign very much reflects how significant multichannel audio has become. The fourth Soundalign screenshot shows how multiple network‑connected monitors can be added to the system and selected. Monitors can also be grouped and their configuration parameters copied and pasted. The final Soundalign screenshot shows the shelving and parametric EQ stages. The settings displayed reflect those developed for the 6‑2s working in my monitoring space following its acoustic measurement using Room EQ Wizard. Although it doesn’t include any automatic monitor/room optimisation functionality, Soundalign reminds me of Genelec’s GLM software approach in terms of its look, feel and overall sense of professional competence. It’s a huge advance on reaching around the back of a monitor and fiddling with some switches, and its ease of use I think makes it much more likely that a monitor installation is likely to end up working optimally in any given room.

In Use

Speaking of working optimally in any room brings me to how the PMC6‑2 worked in mine. The answer is that they worked fantastically well. The overwhelming subjective character of the 6‑2 for me is a feeling of wide bandwidth accuracy and dependability at any volume level, all the way up to very loud. Any monitor the size of a 6‑2 that clearly aspires to a midfield role has to be able to make that significant jump towards main monitor volume levels without losing its cool and starting to sound rattled, or more subtly changing in character. A genuine midfield monitor is not simply one that’s a step bigger than a nearfield monitor. It has to back up the scaled‑up dimensions with an equivalent scaling up of volume level capability. The 6‑2 pulls that off without any question.

I’m reminded of PMC’s roots, as I described in the Golden Gear feature on the KEF KM1 a couple of months ago (https://sosm.ag/kef-km1). Having become so familiar with the KM1 in his role at the BBC, Pete Thomas’ motivation to found PMC was built on a desire to design monitors able to reach very high volume levels while at the same time achieving genuine tonal accuracy, and combine that with wide bandwidth, low distortion and time‑domain accuracy too. The PMC6‑2 to my ears achieves all of that, but I don’t want to give the impression that it’s a one‑trick pony, only good at high volumes. The 6‑2 also does subtle and informative at everyday monitoring volume levels, with a finely focussed portrayal of stereo, and detailed, insightful high frequencies. In those terms it’s an extremely capable mix tool, it just happens also to be able to play genuinely loud.

The 6‑2 demonstrates just how much of a performance step skilfully designed and engineered midrange drivers can bring to monitoring.

While I’ve skewed the content of this review towards describing how the 6‑2 plays low frequencies, it was actually the midrange performance that most often caught my attention subjectively. Said midrange is particularly notable for a great sense of realistic, unhyped accuracy, especially on naturally recorded human voices, where it quickly reveals if something isn’t quite right, or fools you into thinking there’s another person in the room. Once again the 6‑2 demonstrates just how much of a performance step skilfully designed and engineered midrange drivers can bring to monitoring.

I’ll finish with some final thoughts about transmission‑line bass as implemented in the 6‑2. To my ears there’s little doubt that the way the 6‑2 plays bass is right up there with some of the best monitors I’ve had the pleasure of hearing in my own listening space. 6‑2 bass is extended in terms of bandwidth so well able to reveal those little quirks of recording that bandwidth‑limited nearfield monitors often miss — the thud of an accidental foot tap on a microphone stand, or the low‑frequency thump of some acoustic guitar techniques. But the 6‑2 combines extended bandwidth with great dynamic, pitch and temporal accuracy. It could be said to offer the bandwidth of reflex loading with the accuracy and punch of closed‑box loading, and in doing so it has something of a rare mythical beast about it.