Impulse response of a PM1 bass/midrange driver fitted with a fixed bullet (blue) and the new Anti-Resonance Plug (green).
Unlike the majority of dust caps, this mushroom-shaped device is a tight fit inside the voice coil bobbin.The stiffness and resistance properties of the EVA polymer are adjusted to damp down the tendency of the coil former to go out of round under the influence of cone break-up modes.
Damp the flexing of the bobbin and you in turn reduce the cone break-up, leading to a smoother response at the upper frequencies of the driver and a commensurate reduction in sound overhang.
Impulse response of a PM1 bass/midrange driver fitted with a standard dust cap (red) and the new Anti-Resonance Plug (green).
As bass drivers get smaller, it becomes increasingly important to prevent residual noise from turbulence as air is forced through the magnet system from pressure inside the cabinet.
The bobbin needs to be totally sealed and the open design of the fixed ‘bullet’ approach, effective in midrange-only applications, becomes less viable. A standard dust cap will seal off the bobbin as well, but, unlike the solid construction of the plug, its membrane structure does not damp resonances on the cone itself.
The resulting driver, with its smoother response, is more suited to the low roll-off rate that results from minimal component crossover designs.
Find out what Bowers & Wilkins customers and audio enthusiasts are talking about on our blogs, and read in-depth articles in the Sound Lab.
John Bowers: A Lasting LegacyDecember 08,2016
Production Notes from Slowly Rolling Camera – LondonDecember 01,2016
Faber Social present: the sonic world according to Alan WarnerNovember 29,2016
The making of Bowers & WilkinsNovember 25,2016