Bass drivers come in a variety of designs and specifications depending on the type of speaker system they are intended for.
As with all loudspeakers, the materials used in its construction greatly influence its capabilities and its sound definition, giving it a focussed range of movement and frequency response.
For example, the bass driver found in a 3 way speaker system will be designed to focus specifically on low frequency reproduction only, as the unit has a dedicated midrange driver to handle the low mids, which means the bass unit can be engineered with just the longer cone movements of bass in mind, so will use a stiffened material, heavier top suspension roll, larger magnet and larger dust cap, which all help it mute unwanted highs and to keep the sound controlled at higher volume.
In a 2 way system, this design will differ as you're obviously channelling both the bass and midrange through the one driver, so it has to be engineered to handle a much wider frequency spread and be able to produce defined mids on top of the low end, which is a big task from one cone. This will often be achieved by using things like vented magnets, and multiple ridges on the cone surface which add detail and brightness by disrupting the surface area. Cones will often be paper with a smaller dust cap.
The common materials
A standard speaker driver consists of a stamped or pressed frame, which is either aluminium or steel. A large magnet of ceramic ferrite construction with a soft Iron core, and a paper cone thats treated with glue as a binder. The central voice coil will be a plastic bobbin with fine copper wire wound onto it.
This design has remained fundamentally unaltered in decades, just being tweaked for performance and as new materials and processes became available.
Many newer high performance drivers will be strengthened with man made materials such as glass fibre or carbon kevlar, polypropylene or urethane surrounds and will feature lightweight ceramic composite magnets. Hifi manufactures and speaker manufacturers in general will spend a great deal of time and money on refining the old techniques and trying out new ones in the never ending search for better sonic detail and clarity, combined with efficient use of the incoming signal and the cancellation of unwanted harmonics.
These newer materials also mean that the drivers longevity and durability has been vastly improved, and not just from wear and tear in usage, but also from environmental conditions such as cold and damp which was the death of many a paper cone that had been transported about or stored.
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