Thomas J. Norton

If Canada has emerged as a hotbed of loudspeaker production in the past few years, the folks at Audio Products International must be positively sizzling. Of their three lines—Mirage, Energy, and Sound Dynamics—Mirage is perhaps the best known in the US, with Energy running a distant second. Mirage, at least in their flagship M series, features rather esoteric bipolar designs, while Energy sticks to the more conservative, forward-radiating approach. Stereophile has had extensive exposure to the various Mirages (a review of one of the M-series babies, the M-7si, is scheduled for a future issue); our exposure to Energy has been virtually nil, save for the odd Hi-Fi Show and CES. And thereby hangs a tale.

Americans might be forgiven if they haven't heard of Epos. A small, British specialist loudspeaker manufacturer, it was founded in 1983 by its designer, Robin Marshall—an ex-BBC engineer and a onetime consultant to Monitor Audio, according to Roy Hall of Epos's US importer, Music Hall. Their first model, the ES 20, didn't make it to the US, and was apparently a bit too pricey in the UK to make much of a splash there (about £700, $1300 at the current rate of exchange—a mint in a country with the patent on sub-$500/pair (UK prices!) two-ways). In 1986 or thereabouts, the new, smaller ES 14 was introduced, using updated versions of the same drivers in a smaller, ported cabinet. Music Hall only recently began importing the Epos into the US.

If anyone can be said to be the guru of the transmission line, that would have to be Irving M. "Bud" Fried. He has been promoting the design for years now, first with the made-in-England IMF designs, later with the designs of Fried Products, made right here in the US of A. He has long been convinced of the basic superiority of the design, and still uses it in his top-of-the-line systems. But true transmission lines are invariably big, heavy, hard to build, and, for all of those reasons, expensive. Essentially, they involve a long, convoluted, heavily damped tunnel behind the bass driver which channels the back wave to the outside world. The length and cross-sectional area of the tunnel are of some importance, although the technical basis for the transmission line, as applied to a loudspeaker enclosure, has never been firmly nailed down. Certainly there is no mathematical model for the transmission line as complete as that developed over the past two decades for the sealed or ported box (footnote 1).

But Bud Fried has clung to the transmission line, for all of its complexities. In order to bring at least some of its touted advantages to a lower price point, he had to come up with a variation which would work in a smaller enclosure. That variation was the "line tunnel," which, according to Fried, originated in an early-1970s Ferrograph (a British company specializing in tape recorders) monitor which was later adapted by IMF. Basically it consists of a short (compared with a transmission line) duct from the inside to the outside of the heavily damped enclosure. The duct is designed with approximately the same cross-sectional area as the loudspeaker cone.

Boy, do we ever get letters. From readers angry that we review too many expensive products. From readers depressed that we review too many affordable products. From readers bemoaning our digital coverage. From readers asking when we're going to get with the 21st century and stop gushing over analog. From readers wanting more coverage of tube products. From readers wanting more coverage of MOSFET amplifiers designed for high voltage gain on the output stage.