A known issue of the dynamic loudspeaker is the instability of the average working position for frequencies above the resonance frequency. In this range, because of the phase relationship between force and position, the variations in force factor (Bl) vs position drive the moving assembly away from the Bl maximum. Intuitively, the loudspeaker tends to “slide” down the slopes of the Bl(x) curve. This is referred to as DC offset.
Since the rest position is the optimal average working point, DC offset leads to several undesirable consequences: reduced excursion capabilities, increased mechanical stress, and increased distortion. These are due to the loudspeaker working in a region where nonlinearities are larger, and thermal dissipation and power handling are lower. Generally speaking, a large amount of DC offset leads to poor performance and shortened loudspeaker life.
All B&C loudspeakers are designed with DC offset reduction in mind, especially our large excursion subwoofers. Our motors are designed to have a large plateau around the rest position, both through magnet assembly optimization and voice coil design. Reducing the slope of the Bl curve, especially in the central region, will reduce instability and therefore the amount of DC offset. A special winding technique has been adopted in the most critical cases. Our suspensions are also designed to counteract DC offset before it degrades performance.