Loudspeakers

Broadly there are two design philosophies regarding the vibrational behaviour of moving-coil drive units. The first desires the diaphragm motion to be pistonic throughout the driver's passband, ensuring there are no in-band breakup modes to colour and smear the output. But directivity will narrow towards the upper end of the driver's working range, resulting in a different form of coloration caused by increased spectral disparity between the direct sound and the early reflections and reverberant sound in-room. To counter this, the alternative design approach arranges for the diaphragm to undergo controlled flexure, so that its effective radiating area decreases at higher frequencies, obviating the narrowing of directivity. But it is very difficult then to arrange for the bending waves travelling out from the voice coil to be completely dissipated in the diaphragm and its surround. Usually there will be some reflection – and therefore resonance. Manger attempts to get this concept to work by using a thin, flat diaphragm with a central damping structure, within the diameter of the voice coil, and external edge-damping material. It performs well – if not quite perfectly. KH