PMC has finally developed a more affordable studio monitor so the rest of us can get in the transmission line.
Review: Brad Watts
It’s not often a set of PMC monitors make their way onto my monitor bridge. I believe the last PMC monitors I auditioned were Digidesign RM-1s — a collaborative effort between the two companies from way back when. Regardless, each time I do sit in front of a pair of PMCs, I need to re-calibrate my perception of lower frequency reproduction. Why is that? Well PMC does things a little differently to your usual monitor manufacturer.
All PMC’s designs utilise what’s known as a transmission line as part of the monitor cabinet design. The upshot of incorporating a transmission line within the cabinet is the taming of reverberation caused by the low-end driver interacting with the rear of the cabinet — as you’d find with typical bass reflex designs, ported or otherwise. Consequently, the monitor projects bottom end in a truer fashion. With a non-ported cabinet the transmission line will dampen and absorb unwanted lower frequency anomalies, and with a ported transmission line those lower frequencies can be altered to leave the open port in phase with frequencies emanating from the front of the low-end driver. The transmission line can be any shape, and is often ‘folded’ to increase the length of the line to a suitable distance, and to keep it within the confines of a usable cabinet size.
The effect is an extremely tight, uncoloured, and balanced low-end, which as I mentioned, takes a little getting used to. Initially the effect is almost as if there’s low-end actually missing, but once you settle in to the PMC sound you realise everything, and more, is there as it should be — it’s just more faithfully reproduced. Such has been my experience.
LINE ’EM UP
Transmission line cabinets aren’t new. The concept was pioneered by Benjamin Olney during the early 1930s while working for Stromberg-Carlson Telephone Co. This design was dubbed as an ‘acoustical labyrinth’. Contemporary designs evolved during the mid 1960s with research and design by Dr A.R. Bailey and A.H. Radford. While these designs were initially based on the acoustical labyrinth work, it was Bailey who hit upon the idea of using dampening to further reduce resonances along the transmission line. From there the history gets a little murky, with American, Irving M. ‘Bud’ Fried and a British group comprising John Hayes, John Wright, and David Brown designing and marketing transmission line designs primarily under the IMF brand (Irving M. Fried’s initials). Unfortunately the IMF company name became a point of conjecture, with the British developing IMF transmission line cabinets and speakers, while Bud kicked about the U.S. distributing the British-designed speakers along with selling lesser quality designs. Once the legal dust settled, Bud created the Fried company in the States, while the three Brits entered a joint venture with driver manufacturer Elac, naming the company TDL.
Into the mid 1970s, a chap by the name of L. Bradbury commenced a spate of work researching the effect of different absorption materials for lining transmission line walls. Unfortunately he kept hitting a wall, with his dampening materials, primarily wool and glass fibre, resulting in quite variable measurements. Climate, humidity, and changes to the material over time rendered many unrepeatable characteristics. It turns out the choice of dampening materials have as much to do with tuning the cabinet and transmission line as the cabinet itself, consequently transmission line cabinet design becomes a bit of a dark art. Understandably there’s very few manufacturers keen to sink research and development resources into such a finely tuned system. At any point in the history of transmission line speakers there’s only a handful of designers prepared to tackle the concept. For those that are the outcomes are impressive, which is why you’ll find it’s primarily esoteric audiophile manufacturers developing transmission line speakers.
The standout player in this tiny field is PMC. Not only does the British firm aim designs to the audiophile market, with its deeper pockets and disposable income, its grass roots are firmly footed in the studio and recording markets. Since 1991, PMC founders Peter Thomas and Adrian Loader have dragged transmission line design firmly into the 21st century, and have dubbed their designs ‘ATL’ or advanced transmission line. Peter’s BBC training led him to create speakers initially for the BBC and London’s Metropolis Studios. Those first speakers are still in use today at the BBC and Metropolis, and now thousands of other top-shelf studios worldwide.
HEAR THEM AT YOUR PLACE
As you can tell from Brad’s review, if you’re not experienced with transmission lines, it can take a little time to adjust. To help you make a decision, PMC Australia has demo pairs of its range available to AudioTechnology readers. If you want to try them out in your own space, just give them a buzz or email on: (03) 9426 3660 or firstname.lastname@example.org
TRICKLING DOWN THE LINE
Knowing the history and some of the design fundamentals lets you realise transmission line designs aren’t cheap. It’s why it’s taken 27 years for PMC to develop an affordable monitor design it could stick its badge on — the result6. Like all PMCs the result6 is a transmission line design, or ATL as PMC do, so it’s a little deeper than your average bass-reflex cabinet. But more on that later; let’s have a look through the specs and build.
The result6 is a two-way nearfield design incorporating a 6.5-inch doped fibre low-end driver and a 27mm soft-dome high-frequency driver. Amplification is a fairly modest 100W for the low end and 65W at the top end. However, transmission line designs make the bass driver work more efficiently, requiring less power to deliver the same low end as a bass-reflex design. Both amplifiers are a Class-D design, which we know offers the advantages of low heat production along with smaller size and less weight. There’s little else in the way of published specifications for the amplification apart from the mention of the amplification compensating for driver impedance variations due to frequency changes. On the other hand, there seems to be more information regarding the upper level twotwo series monitor’s amplification, so I suspect this is an area that has helped PMC keep the final price down. Suffice to say, there’s no changing the amplification — it’s built in and does the job well.
FIN BODY WORK
Looking around the largish cabinets you see a rubberised strip encircling the cabinet at both the front and rear. This provides a non-slip and de-coupled footing for the monitor and would function whether the result6 is sitting vertically or on its side, although PMC suggest vertical positioning is the correct procedure, as would I. Regardless, if horizontal placement is your thing, the rubberised bands will work just as well in this configuration.
The entire cabinet is almost as deep as it is high, with measurements of 380mm high by 199mm wide, with the depth stretching out to 360mm. Larger than the PMC twotwo.5 monitor and closer in size to the twotwo.6. They take up quite a bit of space so this will be a consideration if you’re tight on control room real-estate. The depth is obviously exacerbated by the size of the transmission line architecture, which occupies a good 85% or more of the cabinet depth. If you decide to mount the result6 monitors on stands you’ll need to check the stands are up to the task. Otherwise there are dedicated PMC wall-mounts designed specifically for the result6, along with PMC’s high-mass tube104 stands that will keep the monitors stable. That said, the result6 is actually fairly light for its size at 8kg.
Looking further into the driver and porting arrangements, the result6’s transmission line exits the cabinet from the front baffle below the low frequency driver, which is covered with a fine mesh grille. I’m all for front porting in any monitor, but it’s especially important with a transmission line as the line behaves almost as a low-end driver itself. It’s also advantageous if you’re keen on soffit mounting, PMC suggests the result6 to be a perfect candidate for surround monitoring systems, which is where a soffit mount may come into play.
At the high-end is the previously mentioned 27mm soft-dome tweeter. And as soft-dome tweeters should be, it’s not fatiguing, and my personal preference (give me ribbons or soft-domes — I can’t do metal tweeters). All seems relatively standard fare until you notice the radial fins either side of the tweeters. This is PMC’s latest weapon against diffraction effects cause by large amounts of front baffle surface area surrounding a small driver, and indeed, typical cabinet edge diffraction effects. Trademarked as ‘D-Fins’, they jumble high frequency radiation patterns, resulting in a wider sweet-spot and a less ‘smeared’ high-end reproduction. The D-Fins certainly seem to make a difference with a crisper and more precise image than I’d expect from a more typical soft-dome arrangement. And as the manual says, the imaging and sweet-spot is remarkably wide. I found the high-end incredibly accurate, whereas I was initially expecting that high-end wash you find with wave-guide style high frequency dispersion. It seems such a simple idea, but it works.
So why the lower price point, and where have the corners been cut? What’s missing? Again simplicity is the key. Unlike many monitors released over the last few years there’s no DSP. There’s no digital I/O, no tone-shaping, no corner EQ settings, no bass-lift, no room-adaptive spatial enhancement malarkey, and no fuss. If you need that stuff you’ll need to pony up for the twotwo.6 monitors. To be honest I feel the recent array of monitors touting umpteen different methods of balancing a monitor via DSP goes against the grain. It reminds me of the old ’70s idea of running a 31-band graphic EQ prior to the monitor sends to supposedly tweak the speakers to suit the room. If you look out the back of the result6 there’s a power input and switch, an XLR input, and an attenuation pot — continuous — not stepped, which I also like very much. Many a time I’ve wanted finer adjustment in monitor attenuation and have had to pull in ancillary hardware, adding additional connectors and normally unnecessary signal path.
What hasn’t changed? Well, being PMC monitors they are ‘Designed, made and tested in the UK’. I had to double check the result6 wasn’t emblazoned with something along the lines of ‘Designed in the UK and made in China’. But no, PMC are true to the cause, and it speaks volumes for the company’s commitment to its craft. Hats off!
The results are impressive, and yeah, that pun has to remain in this case. Like PMC’s other ATL monitors, the bottom end is controlled, tight, realistic and distortion free, and stable throughout the SPL range. Like I said, it takes a little getting used to at first, but then that’s probably because you, like me, have been listening to bass-reflex designs for too long. PMC has far too many runs on the board to be taken lightly. The company’s website will guide you through how many household names are reliant on PMC monitoring (Studios 301 included) along with many that aren’t — the back-room names behind the names, the mastering rooms, the studios. The result6 is about to make that list a whole lot longer.