PMC MB2 SE

A three-way design, incorporating a 310 cm woofer, a 7.5 cm midrange and a 2.5 cm tweeter in a cabinet 870 (H) x 380 (W) x 535 mm (D), the MB2se sits on a purpose-made stand for an overall height just shy of 1.3 m. The cabinet is made in the UK from 25 mm high density fibreboard with a 50 mm front baffle. All elements are CNC cut and then hand assembled. Internally, the transmission line, also fabricated from HDF, has an operational length of 2.4 m, and is necessarily folded back and forth adding consequential and very substantial bracing to the cabinet.

This is the latest version of a model that began life as the MB1 in 1993, launched just two years after Peter Thomas left the BBC to form PMC. It was designed as a mid-sized studio monitor and has proven very popular with musicians and studios all around the globe, no doubt because the essential concept was so right from the outset. Over the years and the upgrades separating the ’1 and the ’2se, the primary changes have been to drivers, the crossover, and to internal stiffening, but the basic form-factor remains essentially the same.

The drivers fitted to the MB2 in its se form today are worthy of note. The tweeter is made for PMC by Norwegian company SEAS and uses a treated fabric dome with ferro-fluid cooling. The 75 mm mid-range driver is manufactured by PMC entirely in-house. It has a hand-doped linen dome and weighs a thumping 10 kg. The 310 mm woofer is made for PMC by Dorset driver specialist Volt. Superficially it resembles Volt’s OEM series of exoskeletal drivers, but some of its Thiele-Small specifications are quite different. That’s primarily due to the much stiffer cone specified by PMC to satisfy the more arduous requirements of transmission line loading, rather than infinite baffle or ported cabinets. Well-matched, the driver and the transmission line allow a pair of MB2s to deliver a bass response that – with suitable recorded material – sounds positively titanic. As is PMC’s usual practice across its range of speakers, the crossover is 24 dB per octave fourth order, with crossover points at 380 Hz and 3.8 kHz. Externally the MB2 offers six substantial gold-plated screw terminals for tri-wiring, with flat gold-plated links tot enable less ambitious cabling schemes.

Since the transmission line is such a key part of what the MB2 is about, a quick refresher on this less common method of woofer loading may be welcomed by some readers. It was first described in 1965 in a Wireless World article by A R Bailey who built on research from the 1930s into labyrinth design.

PMC’s Peter Thomas says the ideal transmission line design would absorb all the frequencies emanating from the rear of a woofer, but that would require a line so long as to be quite impractical. Compromise requires that a short line—a quarter wavelength of the lowest desired frequency—is so configured by the use of specialist acoustic filling as to absorb the higher frequencies but not attenuate longer wavelengths. The line therefore operates as an acoustic crossover, below which frequency the woofer is loaded by the mass of the air-column. The specified length of the line inverts the rear output of the driver by 180 degrees so that it now exits a front-mounted port in phase, creating in effect, a second woofer. The line loads the driver over nearly three octaves, reducing cone excursion and thus distortion. PMC’s claim is that ‘all the rear output upper-bass is absorbed’ for a clean output that doesn’t blur the midrange while the lower bass is extended in a controlled and unforced way.