Classic audio

IMF Reference Standard Professional Monitor

British-American IMF utilised the rare transmission line principle to shake listeners at their foundations.

By / 3 May 2023 - 8:00 am
IMF Reference Standard Professional Monitor

IMF Reference Standard Professional Monitor MK IV.

The name is almost as imposing as the speakers. And in the 1970s and 80s, IMF was one of those brands that a young hi-fi enthusiast could only dream of. And nothing impressed more than when the heartbeat in the opening track of Pink Floyd’s Dark Side of the Moon was played on the metre-high four-way speakers. Such deep, powerful and, most importantly, clean bass had never been heard before.

The reference speakers were built according to an exotic principle called transmission line. The oval KEF B139 woofers were mounted at one end of a 2.5 metre long tunnel that meandered through the cabinet, eventually opening into two slots at the bottom of the front panel. The tunnel was lined with a mixture of foam rubber and sheep’s wool. And it was this construction that gave the speakers the unique bass response that made the jaws of teenage boys and hi-fi veterans alike drop.

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The IMF Reference Standard Professional Monitor MK IV was a four-way loudspeaker. The upper bass and midrange was covered by a KEF B110 woofer with a polystyrene cone, which was also housed in a short transmission line. The treble was split between a KEF T27 dome and a Coles super tweeter. These were all good drivers, but it was the deep bass that made the IMFs stars.

Nuanced bass

In a bass reflex speaker, the port also adds to the deep bass, but the port only operates in a relatively narrow frequency range – typically less than an octave. At best, this means that the woofer still has to cover most of the bass range on its own. And at worst, it leads to “one note bass,” where the speaker plays lots of bass, but the sound is muddled.

In transmission line loudspeakers, the port provides a contribution over a much wider range, resulting in both more powerful and more nuanced bass, as not only very specific frequencies are boosted. And it is also possible to make a given woofer work deeper than in other types of cabinets, as the mass of the air column in the tunnel adds to the mass of the diaphragm and lowers the resonant frequency.

This is how intricate the interior of an IMF Reference Standard Professional Monitor is (Graphic from IMF brochure)

The IMF was not the first to design transmission line loudspeakers. The principle was described by A.R. Bailey back in 1965 in a construction article in Wireless World magazine. But the IMF was the first to commercialise transmission line loudspeakers. This was through a British-American collaboration, led by hi-fi importer Irwing M. Fried in New York. The loudspeakers were designed in England by a company named with Fried’s initials, IMF, and sold on the American market. But IMF was also available here in Europe. The flagship was the Reference Standard Professional Monitor, but there were smaller and cheaper models based on the same principle.

Success lasted about a decade. But the speakers were laborious and thus costly to build, and in the early 1980s Fried abandoned the transmission line in favour of more traditional speakers built under the Fried name. IMF Electronics of England changed their name to TDL Electronics, but continued to build transmission line loudspeakers throughout the 80s and 90s. The company still exists, but the current models are standard bass reflex speakers.

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Laborious principle

The costs of achieving the deep, accurate bass in a transmission line cabinet are several. Firstly, a several metre long folded tunnel means a large enclosure. The cabinet is complicated to build in terms of craftsmanship, and tuning the transmission line with precise amounts of damping material is also demanding. And finally, transmission line speakers tend to be extremely inefficient.

Wonderful, wasted watts

The latter is not really a fault of the principle, but a woofer that has to reproduce very low frequencies linearly needs either a very large diaphragm – which would mean an even more gigantic cabinet – or a very long excursion. And woofers with a long linear excursion must have a long voice coil, with only a small part of it in the voice coil gap, which in turn means that much of the amplifier power is wasted.

The KEF B139, which was used in many classic transmission line speakers, is a textbook example of this. And because it also has a relatively small magnet, a powerful amplifier is needed to drive the loudspeaker. The IMF Reference Standard Professional Monitor Mk IV is rated at a sensitivity of 80 dB! So forty watts are required to achieve a standard sound pressure level of 96 dB at a distance of one metre!

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Modern successors

For these reasons, the large, expensive and wickedly inefficient transmission line loudspeakers never achieved the widespread use to which they were sonically entitled. And the principle was – with a few exotic exceptions – largely forgotten for decades. Fortunately, thanks to more efficient drivers (and relaxed requirements for the very lowest octave), recent years have seen examples of new transmission line loudspeakers that are both smaller in size and less power hungry. Kerr Acoustics and PMC Twenty are examples of modern transmission line speakers.

Should I buy a set?

Although it’s been four decades since they went out of production, the IMF Reference Standard Professional Monitor MK IV is not impossible to get hold of. The speakers were sold in fairly large numbers in radio shops, so there are still many in circulation. A set in reasonable condition can be found for between €1,000 and €2,000. And if you can make do with the home version, the TLS-80, you may be lucky enough to get them for about half that. This allows for high-quality reproduction at a really reasonable price.

Be aware, however, that there may be unpleasant surprises hiding inside the cabinet. Neither foam nor sheep’s wool are everlasting materials, and if the damping material has shifted or deteriorated, it will drastically alter the reproduction.

John Hvidlykke

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