In this episode of Hi-Fi History, Tom Martin talks about the IMF Reference Standard Professional Monitor, its history, and why transmission line speakers, if they're so effective, are no longer in vogue.

The seminal 1965 work that I mention and which appears to have triggered John Wright and IMF to pursue transmission line speakers was written by Arthur R. Bailey in Wireless World magazine:

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Bailey for this paper, and subsequently in 1972, did some nice reasoning and empirical measurements to help guide the design of TL speakers. Very importantly, he realized that the wool fiber in the line could slow the rear wave and hence allow shorter lines — a significant element in practical designs.

When I mention in the video that the mathematics of TL designs was not fully characterized, I have this in mind:

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Many people contributed to the “full” models of ported and closed-box speakers, but the work culminated in a series of papers in the early 1970s by Richard H. Small extending the work of A.N. Thiele. These parameters have henceforth been called the Thiel-Small parameters.

My main point, as you may intuit, is that it is one thing to have some mathematics applied to a speaker concept and quite another to have a full parameterization of a design concept. Bradbury, for example, attempted the former in the late 1970’s for transmission line loading and he described the results as “unsatisfactory”.

As far as I can tell, a Thiel-Small type characterization of the transmission line speaker was not published until the 1999 work of George L. Augsperger of JBL. Around this time, others were involved in this “project” including Locanthi, Dickason, and King. A very nice summary is here:

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This kind of mathematical work is challenging, and I intuit that the IMF and related products were so good that they spurred slow but continued work in this area.

About Tom Martin:
Tom is a long-time audiophile. He began his reviewing career after acquiring the The Absolute Sound magazine in 1997 and then hi-fi+ magazine in 2002. He has worked closely with Harry Pearson and Robert Harley at TAS and with Roy Gregory and Alan Sircom at hi-fi+. Since Tom and his teams expanded the TAS and Plus platforms in the digital domain, he has served as Chief Content Officer.

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