high efficiency fullrange driver requirements
|Extended range units must have, in order to achieve
their goal, almost contradictory attributes; their cones
must be ultra light and stiff to reproduce treble and at
the same time heavy and soft to perform decent bass. To
be real fullrangers they must radiate wavelenghts from
17m to 1.7 cm (20-20.000Hz). Any fair attempt to explain
electrodynamic full range loudspeakers must begin with
the fact that there is no such thing, there are just
better or worse approximations.
Cone material can be made from paper (the classic material), plastic (often polypropylene), synthetic fiber/resin such as kevlar, and metal (often aluminium). Paper tends to have less stiffness than some other materials (kevlar and metal) but doesn't exhibit the ringing resonance often found in kevlar and metal. Some paper cones can be stiffened up with varnish-like substances such as Damar.
Full/wide range drivers come in two general types: the center cap and the whizzer cone. The center cap type looks like your normal cone speaker with a dome center cap. The whizzer cone type has two cones, a large cone (as in normal cone speakers) and a smaller whizzer cone inside the large cone, which has outer edge unattached.
cone schould exhibit pistonic motion throgh its entire FR
Rather, such drivers are systems of two or more drivers on the same motor assembly. This may seem to be splitting hairs, but it is a very important concept in understanding drivers designed to operate full range. Many smaller broadband drivers (200mm and under) are rear loaded into horns, in order to extend bass.
For example, if enough mass is added to the main cone to reduce the cone resonances, the higher frequencies will be attenuated. If the cone is too light, it probably won't be strong enough to act as much of a piston at low frequencies.
As mentioned before, there are more anomalies with the free cone approach than with a central dome, yet free cones predominate. This is because they offer much greater dispersion, and more adjustability.
A free cone design is more problematic than design with a center cap. Free cone is attached with ony only one surface, resonances of a cone are therefore present at 1/2 the frequency of a cone with the same parameters, but with both ends attached. Accordingly to this is not so unusual that Lowthers and similar designs exhibit specific peaks which originate in free cone resonances. The inside of the free cone is its own conical horn, while the outside is horn loaded by the main cone, which is an entirely different horn formulation. Anomalies also result from the outer edge of the free cone as the front and rear waves collide along this diffractive edge.
Speaker's center plug can be mounted to magnet's center pole or to main cone itself. Drivers with a plug usual feature cone, which is curved similarly to horn expansion rates. This kind of a cone can reproduce wider frequency range than flat or conical cones.
A phase plug works to prevent out of phase HF energy from causing peaks and nulls and at the same time exhibit horn loading for upper frequencies. The same contours can be used to obtain the best result (tractrix, hyper or expo).
Many modern drivers use a very short phaseplug. Lowthers and similar fullrangers use longer plugs, which seem to improve smoothness.
Low end (bass response) is all about how much air is displaced. This can be easilly described with this formula: V=Sd*Xmax. So there are only two parameter to play around, drivers effective cone area and its excoursion. Long throw chasis can be good for woofer design, but for fullrange units, where IM distorsions in mids and treble should be small, increasing of the cone area is the only solution.