Back in the Dark Ages, loudspeaker design was commonly based on semi-enlightened experimentation, with new enclosure configurations appearing almost monthly in professional and consumer journals. One of those, published in Wireless World in October of 1965, was A.R. Bailey’s transmission line, a long, selectively resistive, folded-and-sometimes-tapered tube that loaded the back of the (woofer) driver for low-bass reinforcement. The term “transmission line” was misleading; a transmission line in a loudspeaker is designed to absorb all but the very lowest frequencies of the driver’s rear-going output, while the thing it was named forthe electrical transmission lineis designed to preserve as much of a signal as possible. Oh, well.
Shortly after the first commercial transmission line products emerged, from Radford Electronics in the UK, IMF Electronics adopted the concept and launched a commercially successful loudspeaker line. Infatuated by their products, I built several loudspeakers, both passive and active, based on IMF principles.
But the loudspeaker landscape was changing, largely as a result of two developments: the continuing rise of acoustic suspension (sealed box) designs, which had started in the 1950s, and, especially, A.N. Thiele’s codification of the parameters for loudspeakers with ports (footnote 1), which occurred at about the same time that the transmission line was introduced. Now one could design speakers that were more compact and, in many cases, more efficient, with more predictable results. So, transmission lines quickly faded in popularity. As far as I know, The Professional Monitor Company stands today as the technology’s most avid and important advocate.
As befits its name, most of PMC’s products are made for the studio and pro-audio markets, and their large, matte-black boxes seem strong and serious (footnote 2), especially when stacked with matching subwoofers.
PMC’s consumer products are a stark visual contrast with their professional ones, their lines svelte, sleek, and elegant. The fact.8 signature ($12,000/pair, footnote 3) is all that. According to PMC, the fact.8 signature and its larger three-way sibling, the fact.12 signature, are based on “trickle down technology applied from the fact fenestria,” the top model in PMC’s fact line. The crossover network, which employs noninductive Mundorf MResist Supreme Resistors and high-grade British-built ClarityCap Capacitors, derives from the development of PMC’s flagshipand, indeed, the presentation and the construction of the compact fact.8 signature seem comparable.
I slipped the fact.8 signatures from their boxes to find a pair of impeccable-looking slabs, finished in PMC’s Metallic Graphite lacquer (Silk White is also an option) with decorative front grilles that attach with magnets and fit firmly and precisely. No seams, joints, or attachment hardware to be seen. Simply and cleanly beautiful.
The speakers came with bright chrome outriggers and floor spikes attached. The spikes were too sharp to spare my wood floor, so I inverted them to expose the ball tips and hen added the provided plastic boots. If there was going to be a lot of repositioningand there wasI wanted to make sure the experience would not be permanently inscribed on my floors.
The three drivers are stacked closely near the top of the front baffle, with the 0.75″ (19mm) Sonomex-dome tweeter above the dual 5.5″ (140mm) mid/bass drivers. Cut-away diagrams show the mid/bass drivers back-radiating into the cabinet space in front of a vertical divider. The air pressure from these drivers passes above that divider, then down in the space behind it and through another chamber below all this, which extends to the front of the enclosure. The racetrack-shaped port of this 9.8′ (3m) transmission line is close to the bottom of the front baffle.
It is not clear to me how PMC’s “Advanced Transmission Line” (ATL) differs from classic designs. The company says that they’ve optimized various physical elements (drivers, crossovers, etc.) thanks to their “bespoke” acoustic simulation software, which they say allows them “to tune the cross sectional area of the transmission line at the start and end to be essentially as small as possible without compromising low frequency performance and upper bass absorption.” The company’s description continues: “The rate of taper is another factor that can be determined and optimized with the simulation so as to tune our transmission lines to absorb best the upper bass frequencies which are not desirable to transmit from the ATL Vent.” The chamber at the back of the drivers is larger, relative to the driver area, than I would have expected, and there is a dilatation in the chamber just behind the narrow terminal vent. Otherwise, PMC did not provide technical information about what makes their ATL “advanced.”
PMC promotional literature also mentions the effect of ATL on higher frequencies: “Because the low end is clean and clear, it doesn’t mask the rest of the music. Vocals, in particular, are beautifully projected and ultravivid.”
On the back panel, two pairs of silver-capped, multiway binding posts are linked by a pair of silver bridging bars, the removal of which allows for biwiring or biamping. Just above the connectors are two toggle switches. The lower one switches the LF response from its upper, “flat” position to either of two amounts of bass rolloff. The upper switch allows the HF output to be increased or decreased from its central, “flat” position.
To start, I placed the fact.8s in the same positions as my resident speakers, but the results there were disappointing. The treble was in my face, the midbass was on vacation, central images were distant, and the only evidence of a bottom end was some indeterminate bumps. There was clearly a lot of work to do. PMC suggests 50 hours of “running in” time for optimal performance, so maybe that was part of the problem.
I found that the fact.8s needed to be much closer to the wall behind them than other speakers that I have had in this room. Pushing them back to less than 1′ from the wall filled in the bass and restored some harmonic balance.
Footnote 1: Thiele, A. Neville (1961), “Loudspeakers in Vented Boxes,” Proceedings of the Institute of Radio Engineers, Australia, 22(8), pp. 487-508. Reprinted in Journal of the Audio Engineering Society, 1971, 19(5 & 6), pp. 382-392 & 471-483.
Footnote 2: I reviewed their IB-1S in 2005.
Footnote 3: I’ll respect the manufacturer’s avoidance of standard capitalization.
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The Professional Monitor Company Limited
US distributor: Motet Distribution Inc. (a division of XLO International Inc.)
90 Nolan Court, Unit 30-32
Markham Ontario L3R 4L9, Canada
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