In the early days of the quest for resonance and reflection free drivers it became abundantly clear that in order to fully complement the transparency of reproduction from a well-designed dome transducer something had to be done to remove any hint of cabinet colouration.
Colouration which could usually be traced back to resonant air pockets, the eigentones of the main cavity or structural modes in the enclosure walls.
Mounting the drivers in the side of a large cylinder proved a very effective way of smoothing the path for the forward radiation but this did nothing for the sound coming out of the driver rear which has to be confined or dissipated in some way. Initial results, using a ring magnet within the voice coil diameter and a small central hole to connect the rear of the dome to the outside world, suffered from a strong Helmholtz resonance problem which imposed a severe peak and dip in the forward response. Replacing the magnet with an external ring and opening out the hole in the pole to the greatest diameter possible allowed the rear radiation to emerge unhindered but when attached to any sort of enclosure this performance was inevitably compromised by the resonant modes of the enclosed air space. Adding damping can improve this greatly but there had to be a better way.
Coupling the driver instead to the end of a long tube excited a classic series of resonances but these were rather easier to damp out with a fibrous filling than with a short wide enclosure but it required rather a long tube to get good results. That wasn’t really an issue for a midrange unit but a 30cm bass driver in the end of a 3m tube was a rather different matter! So it was decided to try an exponentially tapered tube which, above its cut-off frequency, behaves pretty much like a straight tube but occupies about a third of the volume. And when adding the damping material it was found that simply drawing a length of fibre mat into the tube and letting the taper of the tube naturally compress the filling towards the narrow end resulted in a performance which actually exceeded that of the parallel tube.
This principle has now been applied to both the high frequency and mid-range units of Vivid systems and ensures the best performance from both.