The tweeter remains central to the design of the Sopra, it is the "eye" of the product that captures the eye of the onlooker. Its positioning is not just aesthetic, it is highly critical to the sound as it predetermines the speaker's spatiality, what Focal call "Focus Time".
Due to its incredible rigidity, Beryllium represents the ultimate material for a tweeter dome. Focal, after two years of research and development, produced a world first: a pure Beryllium inverted dome, able to cover more than five octaves (1000Hz 40kHz). You may ask yourself, why strive for an extended response at 40kHz, if the human ear can only hear up to 20kHz? If you can extend frequency response, you will improve the perception of transients and other micro details. As well, the linearity of the speaker's response curve is mainly a function of three opposite parameters: lightness, rigidity and damping. To this day, Beryllium is the only material which permits a joining of these three parameters. The result is a sound wave propagation three times faster than Titanium and two and a half times faster than Aluminum. In the end, the linearity of the frequency response curve, the acoustic transparency and the impulse response of the Beryllium tweeter are maximized and offer near-perfect sound.
The inverted dome tweeter is perhaps the perfect transducer. This design permits very high-efficiency, precision and energy. The particular advantage of the inverted dome tweeter is the optimisation of the mechanical coupling between the voice coil and the dome.
Conventional Positive Dome Tweeter Inverted Dome Tweeter
The voice coil is fixed at mid-height on the dome and uniformly moves the cone entire surface. The positive dome is only joined at its edge, causing it to be inactive beyond 16kHz for a flexible surface. The inverted dome directly radiates into the air, with maximum efficiency, which is translated by an extremely precise soundstage. The dome's response curve is clearly more linear than that of any positive dome. It provides better space dispersion and very low directivity, contrary to ribbon tweeters. The inverted dome tweeter offers unequalled dynamics, enabling sound engineers to very precisely control compression. The image precision (height, width and depth) is outstanding and makes the positioning of each element very easy, as well as allows one to exactly assess the reverb tails.
The requirement for Sopra to be compact meant that as much of the cabinet as possible had to be used for the bass. The Focal Design team had to explore a new system for loading the tweeter. Their Beryllium tweeter features an incredibly light and extremely rigid dome. Its main limitation comes from the compression of air in the cavity behind it. To overcome this, the ideal would be to have a nearly infinite volume.
With the Utopia IAL 2 tweeter Focal worked on improving the flow of the rear-directed wave from the dome. In Utopia III the tweeter has its own enclosure with a large volume to absorb the back wave very gently, as would an infinite space. With Sopra, the need for compactness meant they could not devote the necessary volume for this. So their thinking has led them to imagine a rear acoustic load on the tweeter that approaches the ideal by gradually absorbing the rear radiation. Located on the back of the tweeter a progressively damped horn-shaped duct evacuates smoothly the rear radiation of the dome and has the advantage of being the most effective solution in terms of compactness, thereby preserving internal volume for the bass section.
The rear of the Beryllium tweeter is loaded via a small cavity which is connected to the exterior of the enclosure by a horn. The inside of the horn is filled with a damping material. Thus, sound waves from the rear of the tweeter are delicately and gradually absorbed. Its acoustic impedance approaches zero so that no resistance alters the movement of the dome. This pushes definition to its maximum. Moreover, a lot of effort went into isolating the tweeter from any interference from vibrations. The injected polyurethane mono-bloc tweeter frame provides the optimal mass and damping in a reduced space.
Thus, the Focal design team were able to get the most out of the woofers to enhance bass performance while having an ideally-positioned tweeter, and all this in a reasonably-sized cabinet.
In our next installment we discus the Sopra's Mid-range section.