Take the H-PAS
It began with Phil Clements, longtime designer and principal of Solus/Clements Loudspeakers, a small, audiophile “boutique” brand of transmission-line designs. A classic acoustic transmission-line enclosure is like a ported design in that it features an opening to the outside world. But instead of a tuned port to recoup low frequencies otherwise lost to the interior of the box, the opening here is the mouth of a relatively long, labyrinthine passage with the woofer(s) at its opposite end. The idea is to absorb the backwave resonances completely, leaving the forward-radiating woofer output unimpeded (and uncolored by internal reflections).
Transmission-line design and construction is complex, and thus relatively unpopular. But its bottom line can be said to be excellent bass extension and low distortion from moderately sized enclosures and drivers — though at a certain sacrifice in efficiency and thus speaker sensitivity.
Some years ago, Clements began fooling around with designs incorporating an “extra” resonating chamber — a sort of cul-de-sac branching off the main transmission line — and found something very interesting: Depending on the size and shape of the cul-desac, and on the placement of its entryway, bass extension could be increased dramatically with no sacrifice in efficiency.
This seemed to contravene the often repeated iron law of loudspeaker design, most simply stated thus: “Deep bass, small size, good efficiency. Pick two.”
To make a long story short, conversations with Atlantic Technology’s Peter Tribeman led to a joint research project to develop what eventually became H-PAS. (This in turn spawned an independent business entity to license the technology.) Rather than being a single design or family of designs, H-PAS is in fact an algorithm — a very complex, difficult algorithm, mapped out via computer finite element analysis (FEA), a computationally intensive sort of mathematical trial-and error grid. (Boaz Shalev, a mathematically inclined engineer at Atlantic Tech, did most of the heavy mousing on H-PAS.)
The elements of an H-PAS enclosure, then, are these: a relatively compact transmission- line enclosure with an internal resonating chamber (the cul-de-sac) that serves to null the unwanted primary resonances of the main line. In addition, the main line terminates via an inverse-horn throat whose opening is still much larger than a typical vented-system port. (An inverse horn is just that: The path is wide at the origin, in this case the upper enclosure housing the woofer, and narrows progressively — but not necessarily linearly — to the terminus, which is the opening to the room.)
A lot of variables to this electromagnetic/ acoustic incantation are in play, beginning with all the usual driver attributes — the Thiele–Small parameters beloved of speaker geeks worldwide. To these, H-PAS adds many more, principal among them being the length and cross-section of the main transmission line; the location and angle of bend(s) in the line; the size and shape of the internal resonating chamber; the size and placement of the (internal) opening to same; and the amount and density of dampening material (“fluff” to use the technical term) in both the main line and the resonator.
Months of laborious FEA-ing yielded a complex algorithm that permits the H-PAS folks to plug in driver and enclosure elements (several pages’ worth) and output a family of predicted results in the form of response plots in both amplitude and time domains. These, as represented and demonstrated to me, predict measured results very closely indeed.
In practical terms, H-PAS is said to work its magic by accelerating pressure in the line with decreasing frequency. Thus, the efficiency of the system rises with lowering frequency, making more and more bass for less and less cone movement. (And indeed, the AT-1’s small woofers can be seen to vibrate barely visibly even when low organ notes are played at high volume.)
Just how repeatable — and scalable — the H-PAS mojo will prove to be remains to be seen, though its owners have some “sandbox prototypes” that show impressive promise. But we shouldn’t have to wait too terribly long to find out: In addition to bringing additional Atlantic Tech H-PAS models to market, the outfit already has a couple of additional licensees on board, with designs proceeding apace.
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