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Genius!2: Mark Dodd and the AXiPeriodic driver #29

The driver uses a 5” voice coil to produce a compression driver with an extremely wide bandwidth, smooth response and low distortion

My interest in audio started young, writes Mark Dodd. I remember as a four-year-old my mother trying to explain a loudspeaker and how it seemed like an incomprehensible miracle. Soon after, my parents bought a Dansette [portable mono vinyl player] and I had my first contact with reproduced music. Rather than satisfy my curiosity, it grew… and after completing my exams, I spent several months working for Theatre Projects, getting my first taste of professional audio.

For my Physics dissertation at Southampton University, I devised a method of mapping wavefront shapes in an exponential horn using a microphone hooked up to a phase meter. This project led to a job at (military PA specialist) Vitavox in the early 1980s, using slide rules and log tables. By the time I progressed to working at Tannoy, where I designed a wide-dispersion coaxial driver, we were using 286PCs for CAD, and I could calculate simple horn models using 1D models. Such limited tools relied heavily on an intuitive understanding of the physics – and much experimentation.

After joining Celestion in the mid ’90s, I progressed to using FEM models [Finite Element Method, a maths technique for finding approximate solutions – Ed] of compression drivers. Celestion had pioneered scanning laser velocity meters, so we could see an animated picture of how the diaphragm moved.

When I became head of group research for Celestion parent GP Acoustics, this gave me the considerable resources of working for two companies, Celestion and KEF, and someone who was prepared to invest in long-term technology.

At this time, improvements in software and hardware enabled us to create ‘virtual 3D FEM models’ of complete loudspeakers including sound, vibration, electrical and magnetic domains. Having gained these ‘key’ technological tools, and with a team writing sophisticated software, we could now explore concepts for new designs and, more importantly, try to understand the underlying physics. The power of modern computing means that we have been able to improve both our understanding and product performance.

The work carried out by the research team concerned phase-plug design for compression drivers. Initially we studied conventional designs and derived a method that extended the approach from a flat piston to a more realistic spherical ‘dome’. A similar approach was applied to radial channel phase plugs but these proved more suitable for coaxial designs.
The next step resulted from a colleague’s PhD thesis on compression drivers which provided a method mapping out how badly the mechanical resonances excite the acoustic resonances. It showed how an annular diaphragm – which has movement that increases from the edges to a maximum in the centre – could avoid resonant output. At the time, I was working on a wide-band coaxial compression driver and finding unavoidable performance limitations.

We needed a new type of diaphragm – the 50-year-old loudspeaker engineer’s paradigm of a rigid piston was no longer appropriate! I then realised that some kind of corrugated diaphragm could allow for this type of motion and the team analysed my various designs which allowed me to produce the current geometry….
The new ‘axiperiodic diaphragm’ not only delivered a smooth response above the first mechanical resonances but also was linear in response, resulting in low distortion, flexible in the right way for a low resonance but also light in weight.

The resulting driver – the Axi2050 – uses a 5” voice coil to produce a compression driver with an extremely wide bandwidth, smooth response and low distortion. Although it is early in the life of this new type of compression driver it is already clear that there is much potential for this new approach.

Pictures: Top: Mark Dodd. Second and last: Various takes on the Axi2050.

Published earlier this year and sponsored by QSC Audio, Genius!2is the second edition of Genius!, celebrating those clever people whose inventions have transformed the world of professional audio. The 30-page supplement is also available to read in a handy digital-edition form