Direct-Radiator Loudspeaker Systems with High Bl *

Direct-radiator loudspeakers typically have a very low efficiency since the acoustic load on the diaphragm or cone is relatively low compared to the mechanical load, and in addition the driving mechanism of a voice coil is quite inefficient in converting electric energy into mechanical motion. The drivers have a magnetic structure that is deliberately kept mediocre so that the typical response is flat enough to use the device without significant equalization. Some decades ago a new rare-earth-based material, neodymium-iron-boron (NdFeB), in sintered form came into more common use. It has a very high flux density coupled with a high coercive force, possessing a B-H product up by almost an order of magnitude. This allows drivers to be built in practice with much larger total magnetic flux, thereby increasing Bl by a large factor. An earlier paper of the authors has outlined some features of normal sealed-box loudspeakers with greatly increased Bl [1]. That work focused mainly on the efficiency of the system as applied to several amplifier types, but it also indicated several other avenues of interest. An important conclusion of that earlier work is that a high Bl value causes the loudspeaker to become quite reactive with attendant very high impedance. This means that although somewhat more voltage is necessary for similar acoustic output, the required current is very mush less. For an increase in Bl by a factor of 5, the power taken by the loudspeaker is reduced by more than an order of magnitude, and the amplifier dissipation is approximately halved. The increased reactive nature of the system can be seen in Fig. 1 [1]. For each Bl value the loudspeaker has been equalized to produce precisely the same acoustic signal. Note that at the higher Bl value the instantaneous power is often negative, as would be expected for a highly reactive system. It might be expected that the power dissipated in the