Low frequency performance of loudspeaker systems has been the subject of extensive analysis and debate over several decades now. Some of the well-known results are the following:
- According to theory and neglecting non-linear effects vented systems offer the advantage (when compared to closed boxes) of a higher efficiency factor, translating into more output and / or extended frequency range for a given volume.
- On the other hand, they may be suffer at higher levels from effects of tuburlent air-flows, inducing power compression and distortion. Another drawback of vented systems is that they are prone to midrange coloration due to back-wave radiations, ill-absorbed inside the enclosure, and radiated through the port.
The passive radiator concept has been introduced a long time ago, in order to overcome the drawbacks of vented systems while retaining their benefits in tems of efficiency and therefore acoustic output. However, to achieve this result a number of conditions must be fulfilled, and this is where careful engineering is required:
- The passive radiator must have a high, well-dimensioned excursion capability, in line with the expected output power.
- It must have a suspension combining low mechanical losses, high compliance, but high radial stability.
The passive radiator especially designed for the SM11 uses a wide, single roll rubber surround, with dimensions and shape allowing maximum distortion-free excursion, and material selected among several varieties to provide the best posible compromise for linearity, stability, and long term reliability. This allows the use of a flat W-piston, saving as much as possible of the available volume.
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