by Bob Zurek
January 26th, approximately thirty-five members of the Chicago section attended a presentation by Dr. Earl Geddes of Visteon on his new invention "the Acoustic Lever�." The lever for which Dr. Geddes applied for a patent in 1997, and is currently licensed for Automotive use by Visteon, was described in Dr. Geddes paper in volume 47 number 1/2 of the JAES. The acoustic lever system consists of a loudspeaker mounted in a band-pass configuration, but in place of the port or passive radiator, a system of two passive radiators tied together with a rigid rod is employed. The radiators are enclosed in a volume such that the pressure produced by the speaker in the front cavity of the band-pass drives the smaller radiator, which in turn drives the larger outer radiator, which results in increased efficiency over a port or standard radiator. Dr. Geddes claimed that the system produces a 3-9 dB increase in efficiency over conventional band-pass designs within the pass band of the lever system. Dr. Geddes explained that the amount of gain is limited by the compliance of the volume between the radiators, and the mass of the lever itself. Dr. Geddes showed that the excursion of the speaker in an acoustic lever system is lower than that of either a conventional band-pass system or a closed box design. The excursion of the lever however is much larger than the excursion of a passive radiator in a traditional band-pass arrangement. According to Dr. Geddes, this excursion is very hard on the components of the lever, causing premature failure of the prototype levers. Dr. Geddes is currently looking for a supplier that can manufacture a lever subassembly that can take the punishment the large displacements induce. Dr. Geddes went on to demonstrate the design considerations required for an acoustic lever system, and the types of drivers that work best for this application. Some of the constraints that came are up dealt with the size of the loudspeaker, the best choice of speaker parameters, and the maximum practical lever ratio. The driver should have a diameter somewhere in between the diameters of the inner and outer radiators. The system also works best with drivers that have a high Bl product (magnetic field � length of wire in the speaker gap). Dr. Geddes also pointed out that the largest lever ratio is approximately 2.6:1. Dr. Geddes showed simulated examples of this system using his Speak_32 loudspeaker simulation software. The presentation was followed by an active question and answer session that covered topics ranging from terminology and historical comparisons, to design philosophy and recommended practice in the design of an acoustic lever system.
