I joined NXT (now Tectonic Elements) in 2001 as a member of the Special Projects team to design & develop new, high power exciter technology. Using 2d and 3d electro-magnetic finite element analysis, I developed several innovative magnetic circuits and established an assembly and test laboratory to prototype and validate these designs.

Using in-house AkAbak FEA software, these new exciter designs were modelled for application specific products for commercial licensees.

Distributed Mode Loudspeakers

Loudspeaker development has largely revolved around identifying, understanding and then suppressing diaphragm resonances and their resulting coloration. Distributed Mode Loudspeakers (DMLs) use a technology that, rather than attempting to eliminate diaphragm resonance, encourages and exploits it.

The basic components of a DML are the same as those of a pistonic loudspeaker: a transducer, usually electromagnetic, and a suspended diaphragm (or panel) to radiate the sound enegy. The difference lies in how energy is transferred to the diaphragm.

Conventional techniques attempt to energise the diaphragm as a single, rigid whole across the loudspeaker’s operating bandwidth. However, all diaphragms exhibit unwanted resonances and ‘break up’.  DML technology exploits the resonances inherent to the panel and the design goal is to excite these resonances evenly throughout the bandwidth.

Exciter Design

A variety of exciter technologies are appropriate for energizing the diaphragm but the most preferred option is a moving coil motor. This has three main advantages. First, it ensures compatibility with conventional amplifiers. Second, it is widely known technology for the existing manufacturing base. Third, it allows exploitation of the full bandwidth potential of a DML panel.

Conventional, low cost exciters use a small rare earth (neodymium) magnet to energise a 25mm diameter voice coil. This type of exciter generally has a low force factor of around 1 Tm and a power handling of 3W to 7W. If a loudspeaker is to generate sufficient sound power to fill a domestic living room, multiple exciters would be required.

The development of a low profile, dual magnet exciter consisting of a central disc magnet and an outer ring magnet increased the force factor to 3.2 Tm and a power handling of 20W. For applications that will accept a slightly deeper exciter, a radial, rare earth magnet design was developed that had a force factor of 4.5 Tm and a power handling of 30W.