Listening #38

The Hartleys I wrote about last month may be the loudspeaker drivers that time forgot, but the venerable Lowthers of Sidcup, England, reign supreme as the horseshoe crabs of the loudspeaker world: strange, ungainly things that have scarcely changed since the days when Franz Schmidt and Robert Johnson walked the earth. Literally.

206listen.jpgThat's not the only reason the Lowther holds my imagination. Arguably more than any other, the classic 7" full-range Lowther is a driver whose potential has yet to be fully exploited. Surely I'm not the only person who's been charmed by their uncanny presence, clarity, and sheer musical dynamism—not to mention their ability to do all that with just two or three watts of amplifier power—yet put off by their weak bass and peaky lower trebles.

Enter the Danish audio designer Tommy Hørning, whose domestic speakers are among the most recent attempts at making a more perfect Lowther-based product. For two years running, his entry-level Perikles ($8500/pair) has provided some of the most convincing music at Primedia's Home Entertainment shows, as you've no doubt read in this column and elsewhere. Now I've finally had a chance to try a pair of them in my own listening room (footnote 1).

The Hørning Perikles isn't perfect. There's congestion here and lumpiness there, and a lack of ultimate scale overall. But for a very efficient full-range speaker—a feat that itself seems to exhaust the talents of most designers who try—the Hørning Perikles does everything else at least acceptably well, and in some cases very well indeed. This may be the best off-the-shelf, high-sensitivity loudspeaker you can buy that's also of a reasonable size and price: at least as good as the Beauhorn Virtuoso and the Lamhorn, and possibly even better.

This shouldn't work
The most critical aspect of virtually any Lowther-based design is its bass-loading scheme, and the Hørning Perikles is no exception. In the Perikles, a 7" Lowther driver fires toward the listener, two 9" woofers fire in the opposite direction, and the rear waves of all three play into something the Hørning website, www.horninghybrid.com, calls an H.D.A.Q.C.S., for Hørning Double Asymmetric Quarter Wave Cabinet/Enclosure System. At first jot, meaning no offense and noting that English is not the Hørning company's first language, the phrase seems nonsensical. Then the words Quarter Wave jump out, and seldom-used wheels begin to turn: P.G. Voigt, the Lowther engineer whose genius found flower in the most basic elements of their timeless designs, once created an enclosure that some adherents dubbed the Voigt Pipe. Other people tagged it with a more descriptive term: the Tapered Quarter Wave Pipe, or TQWP.

A TQWP is sometimes described as a cross between a transmission line and a horn, and I suppose that's true, depending on one's definition of transmission line. But it's a better idea—and a safer one, in snippy waters—to begin with the even simpler concept of the science behind literally any loudspeaker enclosure wherein the rear wave of a woofer plays into a tunnel or tube of considerable length and reasonably consistent cross section (noting also that soundwaves are dumb things that always travel in a certain way, regardless of where a clever designer tells them where to go, or what name he gives to the path he's laid out for them). As a reproduced tone descends in frequency, the size of its wavelength goes up, of course, and when this progresses to where the wavelength is so long that half of it is equal to the length of the tube, then the tone coming out of that tube is perfectly out of phase with the tone that went in. But because that tone entered the tube from the rear of the driver, and because the rear wave is perfectly out of phase with the front wave, then the bass tone coming out of the tube is perfectly in phase with the tone coming out of the front of the driver. The effect is additive, and bass reinforcement occurs, which is nice.

However: As the reproduced tone continues to descend in frequency, its wavelength becomes so large that a quarter wave is the same size as the length of the tube. This is where something interesting and altogether different happens: The movement of the woofer's diaphragm is impeded. Normally, I'd say that's not at all nice, but this is a different case: If you design the tube so that its length is equal to one quarter the wavelength size at the woofer's resonant frequency, then the impedance peak associated with that resonant frequency is drastically damped, and power transfer and bass response flatten out nicely.

Let's have a closer look at the Hørning Perikles and see if it fits the theory.

Like so many other contemporary loudspeakers, the Perikles is a good deal deeper than it is wide, but in this instance the depth is chosen to accommodate an unusual three-sectioned labyrinth. The rear section is formed by an interior MDF wall that slants away from the inside top of the enclosure, and which is also open to the rest of the interior at the top. The front section is formed by a similar wall, also open at the top but noticeably taller than its forward counterpart. The center chamber is the space between those two interior walls, which also happens to be open at the bottom—what we'd call the mouth of a horn, if it were such a thing (and it may well be).

Let's return to the rear of the enclosure, where a pair of Spanish Beyma BR60 woofers, each nicely made with paper cones and butyl rubber surrounds, fire their rear waves into the Perikles' rearmost chamber. If you consider the rear and center chambers as a single, continuous tube of gradually widening bore, then what you have is a gradually tapered pipe with an effective length of about 74". And the resonant frequency of the Beyma BR60 woofer, according to the company's website, is 45Hz—a frequency whose quarter-wave is approximately 73.3" long. Ka-ching.

Because the very-low-frequency waves will follow the path of least acoustical impedance, it seems they'll tend not to load the front chamber, thanks to its taller interior wall and consequently smaller opening (although on that point I'm open to correction by more knowledgeable souls). And that raises the question: What, then, is the nature of the loading for that front-mounted Lowther, whose rear wave fires into the front chamber of the labyrinth?

Well, the area of the chamber below that driver isn't very large, and I suppose that, and the constricted opening at the top of the chamber, might allow it to function as the (high-pressure, high-acoustical-impedance) throat of a horn, again opening at the mouth at the bottom of the enclosure. The whole of the enclosure may also be seen by the Lowther as a tapered quarter-wave pipe of an effective length different from that of the pipe driven by the woofers—an effect mitigated by the fact that the more tightly suspended Lowther has a very different resonant frequency from the loosely suspended Beyma woofers. Maybe not quite ka-ching, but at least Thank you, sir, and please come again.

In other words, what we have here is your basic Double Asymmetric Quarter Wave Cabinet. I think.



Footnote 1: Hørning Hybrid Systems, Lykkendalsvej 125, 8220 Braband, Denmark. Tel: (45) 8626-4079. Web: www.horninghybrid.com. US distributor: High Water Sound, 274 Water Street, New York, NY 10038. Tel: (212) 608-8841. Web: www.highwatersound.com.
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