Circumventing Efficiency and Bass Extension Limitations

Abstract

This chapter perhaps the most fascinating material in the book, provides an in-depth discussion of the highly acclaimed Watkins Dual-Motor Woofer, which gained commercial fame in the 1970s and 1980s in multiple audiophile speaker systems produced both by the Watkins firm and also by Infinity Systems of Chatsworth, CA. The revolutionary technique was granted U.S. Patent 3838216 in 1974 and was further documented in a landmark article by the same name in Audio Magazine in December 1974. Fundamentally, the concept permits the decoupling of the normally inherent tradeoff between the flat-band efficiency of an acoustic-suspension (closed-box) woofer and its relative efficiency in the region of the fundamental low-frequency resonance. The surprisingly simple solution is implemented by the use of a secondary voice coil on the woofer’s former, fed from a series-resonant LC network tuned to the fundamental acoustic resonance of the driver in the box and driven from the same source as the main coil. For flattest response and bass extension, the Q of the secondary circuit must be matched to the dynamic response of the woofer in its enclosure. The key to understanding the invention lies in the behavior of the speaker’s back EMF, especially around the in-box cone resonance. The scheme so effectively reduces the typical peak in the back-EMF near resonance that the woofer’s drive impedance appears to be largely resistive, thus coupling much better to most modern audio amplifiers and also providing appreciably improved transient response and lower bass distortion. The chapter also provides extensive performance measurements of the dual-motor system to further document the superiority of the approach over conventional woofers.