Normally, low-frequency sound reproduction with small transducers is quite inefficient. This is shown by calculating the efficiency and voltage sensitivity for loudspeakers with high, medium, and, in particular, low force factors. For these low-force-factor loudspeakers a practically relevant and analytically tractable optimality criterion, involving the loudspeaker parameters, will be defined. Actual prototype bass drivers are assessed according to this criterion. Because the magnet can be considerably smaller than usual, the loudspeaker can be of the moving-magnet type with a stationary coil. These so-called low-Bl drivers have a high efficiency, however, only in a limited frequency region. To deal with that, nonlinear processing essentially compresses the bandwidth of a 20–120-Hz bass signal down to a much more narrow span. This span is centered at the resonance of the low-Bl driver, where its efficiency is maximum. The signal processing preserves the temporal envelope modulations of the original bass signal. The compression is at the expense of a decreased sound quality and requires some additional electronics. This new, optimal design has a much higher power efficiency as well as a higher voltage sensitivity than current bass drivers, while the cabinet may be much smaller.
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