PMC has built its brand on a cabinet loading technology that had all but disappeared when the company was launched in 1991. The transmission line became the advanced transmission line or ATL in Pete Thomas and his late partner Adrian Loader’s hands. The advanced refers to the use of damping foam in the line that runs from the back of the main driver to a vent at the bottom of the box, the material being selected so that only the lowest frequencies escape from the vent and the length of the line chosen so that what comes out is in phase (time) with the output of the main driver. PMC has further advanced this loading system by adding fins to the vent that prevent turbulence and allow for a smooth flow of air, further clarifying the sound of the system. This system is called Laminair and first appeared on the twenty5 series when it was launched in 2017, so it’s perhaps surprising that the company has upgraded this range to twenty5i status so soon. But the PMC R&D department, headed up by Pete’s son Oliver, doesn’t rest on its laurels and in the intervening three years it cooked up the biggest PMC yet seen in the Fact Fenestria. This is one of those products that’s so expensive that few of us will ever get to hear it in anger but was used as a means of researching new technologies to trickle down into real world speakers. The twenty5i range is the first place the trickle has made an impression.
It has impacted the twenty5i models in two ways; the most obvious is in the tweeter which is very similar to the model created for the fact models and designed by PMC in collaboration with Norwegian driver specialist SEAS. This has a 19mm dome at the centre of a large roll surround that brings the overall radiating area up to 34mm. There is always a trade off with loudspeaker cones and domes, the larger they are the more power can be produced but the lower the usable bandwidth that can be delivered, eg. a 12 inch bass driver can generate serious sound pressure levels but doesn’t produce clean midrange let alone treble. In this tweeter the small central dome gives very high frequencies and wide dispersion while the extra area of the surround enables high power handling and a lower crossover point to the midrange.
The two floorstanders in the twenty5i range benefit from another aspect of Fenestria research, vibration control. Although the plinth bars on this model look much like those on the preceding twenty5 models there is a vital difference in the form of rubber grommets that provide a degree of isolation between cabinet and metal bar. These sit either side of the bar with a bolt through to the cabinet base, the rubber like material ensuring there is no metal to metal contact and the instructions urging users not to over tighten them, which reduces the amount of isolation provided. PMC are the only major brand to have used Townshend Seismic Podiums under speakers in demonstration and clearly understand the benefit of compliant isolation, but rather than use springs they looked for material that would provide the best sonic outcome with the minimum of impracticality. They discovered that by significantly reducing vibration transmission around the 400Hz point that the quality of midrange was greatly improved and found a compound that provided that isolation. PMC supplies custom machined stainless spikes to fit into the plinth bars and these can be used with or without receptors provided to protect wooden floors, so there is still a firm connection to the floor at lower frequencies.