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LOUDSPEAKER

- KALMAN RUBINSON

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 for—the electrical transmission line—is 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,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.