Article taken from Haute Fidélité, December 99 - January 00 TECHNIQUE Ocellia Tilia Acoustique et lutherie 
This cabinet is long overdue! Although it has been more than two years since PHYHP launched its first model, the acclaimed H21 LB15, a 21cm (8”) wideband, the offer of a cabinet based on this model has been a long time coming. Initially, the problem was that matching the H21 LB15 with market tweeters didn’t produce good results (the tweeters, having a very low impedance, acted as a short circuit on the wideband PHY which has a very high impedance, especially at the top end). Yet even with the advent of the PHY tweeters about a year ago, the various suggestions which have appeared here and there, while not lacking in quality, have not always fostered the spirit in which the speakers were conceived: a humble yet total regard for the most ancient and proven traditions of hi-fi technology, extremely ambitious objectives, a systematic refusal to cut corners in achieving them and, finally, a tremendous depth of knowledge, an uncompromising sense of perfection and sometimes genuine product innovation. This is poles apart from the kind of dogmatic innovation which characterizes the big manufacturers. The PHY speakers, like the Ocellia cabinets, remain basically traditional while at the same time achieving levels of musical performance which are, in our opinion, without precedent. They might well have been constructed in the same way three-quarters of a century ago, and therefore do not risk becoming outmoded. The PHY spirit applied to the cabinets produces a result which is in radical opposition to anything else in current production. Yet the basic principles which gave rise to the Ocellia Tilia are an integral part of a tradition which, though somewhat forgotten today, has indeed had its hour of glory: thin-wall cabinets. Contrary to what many so-called “modern” cabinet technicians might say, this choice has not been made out of a total absence of technical culture, neither of a refusal to take the most basic realities of acoustics into consideration. Conversely, the choice is perfectly borne out on a theoretical level. So why is it so poorly represented in current acoustic cabinet production? It is because the perfecting of a thin-wall cabinet is a real headache, an endless ordeal with the constant risk of ending up with a result worse than if a “wise” or “sensible” choice had been made from the beginning, in favor of a “thick” cabinet. So Samuel Furon, the designer of the Ocellias, like Bernard Salabert with his widebands, set himself the task of achieving the best musicality possible, and that by an extremely arduous route, but one likely to lead much further than if he had chosen more classical and safe solutions, as maintained in the production of virtually all other cabinets on the market. The particular feature of the thinwall cabinet is that it allows the walls to resonate under the action of the waves emitted by the driver, rather than trying to suppress vibrations at any cost. The difficulty is, evidently, in making these vibrations pleasant to the ear, and making sure they don’t blur the musical message. On a theoretical level, Bernard Salabert, who was involved in the conception of the cabinets with Samuel Furon, justifies the choice, describing his own experiences in his auditorium. It is a huge room, in a building designed specifically to be as acoustically neutral as possible. The concrete slab floor is laid directly on the foundations, without an underfloor space. The walls are made of special breeze blocks, filled with sand. The ceiling is dampened with 12 tons of sand.Apart from the rather unusual appearance of the PHY drivers, the Ocellia Tilias are almost normal to look at. However, their original construction, close to that of a musical instrument, gives them a unique sound quality. The finish is available in several colors. The rear port is adjustable, allowing the cabinet’s resonance to be adjusted to suit the room or the other components in the system. Inside this auditorium, which is probably unique in France, testing is carried out on a wall which acts as an open baffle, itself weighing 16 tons. In total, no less than 144 tons of different material has been used to construct this extraordinary musical tool, a  priori resistant to all vibration. And yet when Bernard set up his 21 cm (8”) driver on the 16 ton wall to listen to a disc, one offering little bass, and at normal domestic volume, it made the floor vibrate under his feet. The conclusion is simple: if 144 tons is not enough to suppress the vibrations emitted by a humble 21 cm (8”) driver, how can one expect to dampen whatever there might be within the limited volume and weight of a simple cabinet? In the second stage of his reasoning, Bernard highlighted the adverse effects of an only partial dampening of vibrations: high frequencies are easily suppressed but bass much less so, and it is exactly this type of imbalance that the human ear despises. Furthermore, to dampen means to dull. In fact, it is not the actual mass of material, but its cubic mass which is used in the equation of vibrational energy emitted, which means, above all, that the time necessary for dampening is lengthening. Put simply, in attempting to dampen the waves behind the loudspeaker, those waves concentrated in the middle and low frequencies, which last much longer due to the considerable mass of material used, are generally allowed to persist. But note, the thin-wall solution has not proved to be free of pitfalls either. The challenge is in getting the cabinet to vibrate across the whole audible frequency range with the same intensity, and “pleasantly”. If this objective is achieved, the musical result can be staggeringly accurate, but if not, then it is a catastrophe. A failed thin-wall cabinet will always sound worse than a thickwall one, even mediocre. If their final appearance is roughly the same, except for their weight, the role the two types of cabinet play, and the techniques used in their construction, are completely different. For one it is all about damping, for the other about resonating pleasantly. For one the walls are inert, solid, heavy, with lots of damping material. For the other, types of wood are selected for their beautiful tone, with thickness reduced to a strict minimum and very little damping. In the construction of a “heavy” cabinet, a small modification generally has little effect: a little more or less damping here or there makes little difference. An extra millimeter on an inside surface already 25 thick won’t be noticed. But when the cabinet should be sounding like a musical instrument, the slightest alteration of material, thickness, assembly technique or even of glue and varnish, stands out a mile. It is for this reason that a thin-wall cabinet proves to be infinitely more difficult to fine tune than a classical cabinet. In the case of the Ocellia Tilia, Samuel Furon has spared no expense. Two and a half years of work, and no less than 25 prototypes have been necessary to achieve such a result. Types of wood, thickness of walls, assembly techniques, the shape of the casing, the speaker mounting, the quality and quantity of dampening material, the glues, the varnishes: everything has been tried, listened to and judged. Each choice has been meticulously weighed up, tested and endorsed to achieve a final result. Despite their ultra-simple appearance, the Tilias are the result of a very long process of fine tuning, and of meticulous construction achieved, in the main, by the use of spruce panels assembled within a beech frame. Furthermore, the varnishes chosen are traditional, as used by instrument makers, in a range of colors directly inspired by the subtleties of musical instruments. The cabinet shown in the photos displays a relatively bright color, however more natural colors are also available and, conversely, some closer to red or brown. The casing, quite a sensible size, includes an adjustable port at the base of the back panel, and two fixed ports, one at the bottom of each side. The purpose is to reach a compromise between bass reflex and open baffle: with the port in the closed position, the cabinet acts as a bass reflex; with it open it is closer to a folded flat baffle. However, the principals of bass reflex, perfectly fine tuned for “thick” cabinets, had to be adapted considerably for the thin-wall cabinet. Damping is reduced to a minimum with two small cloths of double-thickness cotton, one on the back panel at the base of the loudspeaker, the other at the bottom of the cabinet next to the ports. Samuel Furon explained that the quantity of damping necessary decreased drastically as the cabinets came closer to perfection. As we shall see shortly, damping reduced to a strict minimum gives absolutely astonishing results under test. Connection is by way of a double terminal, accepting banana plugs. Obviously little can be said about the filter since the wideband is not affected and the tweeter contains the standard filter, perfected by Bernard Salabert and incorporated into the unit itself.
Performance If we have given you the impression of having clearly come down in favor of thin-wall cabinets it is because listening to the Tilias totally convinced us. We are familiar with the PHY H21 LB15, since we were among the first to get hold of a pair, and have tried all sorts of set up, from the huge open baffle (with a pair of 38cm (15”) drivers to give a little support for the 21cm (8”) in the bass) to the big closed cabinets with two 21cm (8”) drivers incorporating tweeters. Furthermore, we have had the opportunity to listen to all cabinets incorporating the PHYs. Whether own our personal configurations, or market cabinets, everything was blown away by the Tilias which offer the added luxury of retaining perfectly reasonable dimensions! The first quality of these cabinets is entirely objective, and all critics will undoubtedly agree on this, even if their opinions differ on other aspects of the Tilias’ performance: these small cabinets reproduce an absolutely astounding level of detail. In our opinion, they have no rivals in the category of electrodynamic loudspeakers. To find such levels of detail you have to turn to the large horn systems, with which the Tilias have other things in common. Such definition means infinite sound details which render the music extraordinarily fresh and alive, very finely differentiated tones and an unprecedented richness of reverberation which allow the Tilias to construct extremely variable sound images from one disc to another, with superb depth. The second great quality of the Tilias is their precision in low level detail, again reminiscent of the big horn systems. With “normal” cabinets, the sound tends to disappear into a woolly mist as it approaches silence. With the Tilias, it seems even more precise when intensity is weak. Notes don’t seem to stop fading out, and the Tilias carefully dissect the ephemeral boundary between music and silence with a sort of delectation. This is quite obviously the result of the virtual absence of damping material within the cabinet. But the real tour de force is in having succeeding in creating a cabinet which achieves such a minimal level of damping while sounding so remarkably precise and neutral. The third quality of these cabinets is, contrary to all expectations, their bass. We are not exaggerating in saying that we have never heard bass as balanced, lively and melodious. One benefits from all the theoretical qualities of the wideband in the bass, and what one hears is effectively a large, light membrane, contacted directly to the amplifier without any filtering components and the losses and phase differences which result. Considering the size of the cabinets, this achievement is verging on the miraculous. For classical music, the clarity and vivacity of this bass, destined to become classic, works wonders in bringing out melodies usually drowned in an undefined hubbub. Under the heading of limitations, all we can point to is the few minute colorations of the HB21 LB15 which still persist in this cabinet, though are reduced to their simplest expression because the casing, as unbelievable as it may seem, is absolutely neutral. Furthermore, it must be remembered that the extraordinary transparency of the Tilias makes them very demanding on the electronics, musical source and recording. The Tilias are in no way obliging or accommodating; if something isn’t quite right, first blame the other components in the system and try another setup. One day or another you will chance upon the right one, and then you will understan. Needless to say, these cabinets, with their output and high impedance, are well suited to valves. But certain small integrated transistor amps, readily available, also give astonishing results. Alexandre Geoffroy | |
