For the last few weeks, I have had the pleasure of being joined in my studio by a beautiful pair of PMC 6-2 monitors. I'll get straight to the point and say that these monitors are completely delightful and sound awesome. If you're looking for a quick takeaway then there it is. If you can afford these monitors, buy some. You’ll be very happy. If however you’d like a little more detail, read on.

The 6-2 is the middle model in the Studio range of monitors from PMC. I attended the launch event for these monitors back in 2021 and heard all three models. The range comprises the two way PMC6, the three way 6-2 and the larger three way 8-2. All of them were very impressive, but my conclusion from the day was that the 6-2 hit the sweet spot for me. The two way model was a fine sounding speaker, but you could really hear the contribution of that midrange driver in the three-way designs. The all important pricing is as follows - PMC6 £6,048.00, PMC6-2 £9,828.00, PMC8-2 £15,057.00 Inc Vat, UK, Retail for a pair. These monitors are very much at the higher end of this sector and share that space with some very well respected designs, so if someone is looking at investing in a monitoring system of this quality, these boxes from PMC have some formidable competition. If you are in that market, why would you choose these?

A point of difference is a very attractive feature in a landscape in which there are so many designs which are fundamentally very similar. Anyone who knows anything about PMC will know that such a difference exists in the form of their Advanced Transmission Line cabinet design. In a market dominated by ported cabinet designs what is it about transmission lines which makes them different and maybe even better?

As a self professed speaker nerd, I've had an interest in transmission line designs for many years. I’ve never owned any PMCs but curiosity about transmission lines led me to buy a pair of TDL floorstanding hifi speakers twenty or so years ago, which I still have somewhere. If you read up on the subject online, you'll possibly begin to wonder why if transmission lines are so good, why they aren't more common? In short, the reason is that while the theory looks very appealing transmission lines have always been difficult to achieve. The woodwork is complicated and unless the cabinet is correctly sealed (all panels glued and airtight) and damped with specifically designed acoustic foam then the line doesn't work as effectively as it should. These designs aren’t easy to pull off and PMC, having stayed the course for decades, have gained invaluable and considerable experience in mass producing transmission line speakers.

Cabinet Designs And Transmission Lines

Now would be a good time to take a brief detour into why speakers have cabinets at all, and what the problem is that the transmission line seeks to solve. A loudspeaker driver creates sound waves of opposing polarity from the front and back respectively, and unless steps are taken to stop this out of phase energy from the rear of the driver reaching the front and interfering with that energy, the performance of that driver is seriously affected. Particularly at lower frequencies. Putting a barrier between the front and back of the driver achieves this, but always with some degree of compromise. As anyone who's ever seen an actual loudspeaker knows, this is achieved by putting that speaker in a cabinet. A sealed box works very well, but the box has to be comparatively large for efficient production of low frequencies. Porting that sealed box can extend the low frequency response significantly, allowing adequate bass from an acceptably small speaker, and when done carefully, does so without compromising performance to too great an extent. There is a great deal more to be said about loudspeaker cabinet design, but I'm not going attempt to say it all here!

As with most things, there is no such thing as a free lunch and this extension in bass response produced by putting a speaker in a box also colours the response of that speaker. The inherent springiness of the air contained within a sealed box is a significant source of distortion and the effects porting a cabinet has on the time domain response of a speaker compromises the advantages in the frequency domain - there is no one simple solution. Like so many things it’s about balancing compromises and the more you learn about speaker design, the more surprising it is that they work as well as they do!

There is also a source of distortion which is common to both ported and sealed designs. That is the contribution of the back-wave, the sound coming off of the back of the drive unit’s cone doesn’t disappear and is radiated backwards inside the cabinet. As more and more sound waves are produced by the drive unit these sound waves then reflect off of the surrounding cabinet surfaces and start to cause inconsistencies in phase and amplitude. If only there was some way to fully absorb that….. Enter the Transmission Line.

It is theoretically possible to design a cabinet which can absorb the entire back wave, providing an inert platform from which to hear the driver. However to do that the line has to be infinitely long or to be completely absorbent so the energy is propagated out from the rear of the driver, into the transmission line never to return. In the real world, practical designs don't usually attempt to absorb the entire back wave. They usually have an open end and are of a length of a quarter the wavelength of the target lowest frequency. Below that frequency the acoustic output is allowed to exit the cabinet through a vent located on the front of the cabinet. Because of the specific length of the line this energy is out of phase with the rear of the driver, and is therefore in phase with the front of that driver i.e. the vent acts as a second driver for the extreme low end.

The upper bass & lower midrange frequencies are absorbed by the carefully specified absorbent filling of the line, dramatically reducing colouration. As the frequency approaches a quarter wavelength the changing phase relationship between the driver and the vent at the end of the transmission line behaves as a crossover, with the vent producing more and more acoustic output with the cone excursion dropping as the frequency drops. There are two distinct forms of bass loading happening here and it is this different response in the upper vs lower bass which explains why transmission lines of this kind are so advantageous. The upper bass is almost completely absorbed while the lower bass is extended, with lower distortion compared to other designs. The downside is that these designs are difficult to implement and aren’t well understood compared to sealed box and bass reflex designs which, when done well, can sound excellent.

Specifications

At 400mm/15.7” x 430mm/16.9” x 372mm/14.6” (HxWxD) the 6-2 is relatively large for the nearfield use it will probably be used for. With a max continuous SPL of 109dB at 1m there is ample level on tap here and pairing the 6-2 with the complementary 8-2SUB, would get this monitor down to 25Hz (-3dB) however the 6-2 certainly qualifies as full range on its own with a frequency response down to 33Hz (-3dB). At 21.3 Kg (47lb) each they are right at the edge of what I’m happy to install single-handed these days and although, because of the distribution of the drivers, the centre of gravity is off-centre, they are well within the capabilities of a pair of good quality speaker stands. The cabinets are very businesslike with quite shiny, black painted HDF woodwork which is reassuringly solid without being hazardously heavy. The back panel, as well as carrying the expected input and power connectors (the input XLR being both analogue and digital) also features a small LCD screen and a pair of rotary encoders for setting up the on board DSP and a blanking plate covers an expansion slot for future alternative input options. An XLR provides a digital through and a network port is provided for setup and control via PMC’s SoundAlign network control.

Drivers

The front baffle features twin six inch bass drivers, a two inch soft dome midrange driver surrounded by a distinctive waveguide, and a one inch soft dome tweeter. The prominent twin apertures of the transmission line include the distinctive hexagonal ‘Laminair’ inserts which are echoed on what is otherwise a clutter-free baffle. These Laminair inserts smooth the flow of air through the vents, reducing turbulence. The illuminated PMC logo glows white and uses flashing and colour to indicate status and warnings.

On to the drivers themselves, the LF drivers are paper and while the task they are achieving, controlling a transmission line, differs from the duties performed by most LF drivers they are conventional enough in design and function. The same can be said of the silk dome tweeter but the midrange dome is set in a very distinctive waveguide which can be best described as crenellated. In testing PMC found that an exponential flare to the waveguide was beneficial for horn loading the driver, improving its efficiency, allowing more flexibility in the lower cutoff frequency. However a hyperbolic profile was advantageous for smoothing the off-axis response. Rather than choosing one over the other, this profile which combines both was shown to produce both benefits - a great example of clear thinking!

Amps

The four amplifiers on board the 6-2 are all 400W Class D units, the bass drivers having one amplifier each. DSP features heavily under the hood of these monitors and the three way crossover is realised digitally with 24dB/Oct filters. And crossover frequencies of 500Hz and 3.8kHz. 

The DSP handles time and phase alignment of the drivers and provides extensive filtering for speaker setup including level trims, orientation and space settings, desk filtering, delay and polarity settings for use in XBD mode with the matching subwoofers for extension all the way down to 25Hz. Additionally there are high and low shelving filters and 5 bands of parametric EQ. This 32 bit DSP incurs 2.4ms of latency. Control is either via the rear panel LCD and rotary pots or via the network port. Which leads us to SoundAlign. 

SoundAlign

SoundAlign is the control software for network control of the Studio series. Usefully the speakers display their IP addresses on power up and using these in the address bar of a browser you can access the DSP settings using a network cable. It’s a nice touch not having to install any additional software. Simultaneous connection of multiple speakers via a network switch is straightforward and once connected the software interface is intuitive. Speakers can be grouped and identifying the monitor being controlled is easy, with the monitor being addressed flashing its logo to save possible confusion in immersive setups. Being able to tweak monitor settings from the listening position is pretty much mandatory in 2024, the days of calling instructions to a friend crawling behind the desk, or worse trying to do it alone from the back panel are thankfully behind us!

Measurement for speaker calibration to the room has to be performed using something like Room EQ Wizard. You can use SonarWorks if you have it but there isn’t at present any way to upload the settings automatically from the measurement software. For immersive installs there are plenty of features to help manage these more complex setups and on the subject of immersive, as well as some bespoke stands which are available there is also a yoke for ceiling mounting of these speakers.

Sound

So after all this discussion of how they work, do they do a good job? How do they sound? As stated very directly in the opening paragraph, these monitors sound wonderful. I’m a fan of the compact three way format and I auditioned these at home for a few days against my Neumann KH310s. Given the relative costs and sizes of these monitors you would expect the 6-2s to outperform them and indeed they do. I really like my KH310s for their tight bass performance and clarity through the midrange, the combination of a sealed box design and a large dome midrange driver putting in a consistently solid performance. So often when comparing different monitors you can end up in a rather confusing world of ‘different but not clearly better’. The 6-2s leave no such prevarication. They sound so much better in every respect. Other monitors just sound coloured by comparison - boxy even.

I took the 6-2s over to Cube Recording to try them in a better room and set up next to Cube’s ATC SCM25a’s we just had to try them next to each other. I love the sound of ATC monitors. This isn’t a criticism of them, but that same absence of ‘not hearing the box’ was evident in the 6-2 compared to the ATCs. It wasn’t subtle.

This lack of cabinet colouration is a consequence of the design. The ATL absorbs the lower midrange colouration in a way other closed box designs don’t and as a byproduct of the design the cabinet has a labyrinth design that inherently braces the cabinet stopping vibration and flexing. The contribution of the deep bass which is allowed to pass through the line is effortless in supporting the bottom end in a smooth way which doesn’t suffer the same issues reflex porting can. The bass performance is consistent at different monitoring levels, being full and deep even at low levels. The midrange clarity is addictive and the HF is smooth and well voiced. Off axis performance is consistent, and there is more level here than I’ll ever need.

Conclusion

I always know that my relationship with monitors I have on test will be temporary, though for speakers I like I’m usually happy to be slow at sending them back. I have been quick to chase PMC to get these 6-2s collected from me because I don’t want to get used to them. I’ll miss them too much. If you’re looking to buy a serious pair of nearfields the PMC 6-2 are required listening. There are lots of very good speakers out there but these are different, and just so good!