WM-4 Midrange Driver

Concept

We had three principal objectives in designing the midrange driver: wide bandwidth, freedom from ‘cavity effect’ and low distortion.

We’ve achieved this by using our made in house FMWD (Flat Membrane Wide Dispersion) units, optimised for high-performance applications, designed by and exclusive to FINKTEAM. These operate pistonically on-axis but exploit bending resonances at 2, 6, 12 and 18kHz to bolster off-axis output. These resonances are controlled by two damping pads mounted either side of the flat diaphragm to ensure a wide, flat on axis frequency response and smoothly decaying power response. The wide bandwidth allows optimum choice of crossover frequencies and simplifies crossover design. Because the diaphragm is planar, the driver does not cause the diffraction effects that result when grazing radiation (sound travelling close to the baffle surface) encounters the cavity formed by a conventional cone diaphragm. This removes a distinctive colouration.

The third objective was achieved in significant part by minimizing back EMF induced currents in the motor system. Two aluminium tubes below and above the magnet gap and a copper shield on the inner pole piece keep third harmonic distortion low, which is otherwise generated by eddy currents travelling in the motor’s metal structure. The suspension system has been designed to generate symmetric reaction forces in the operating range and to match the motor’s force versus displacement (Bl(x)) characteristic.

WM-4 Midrange Driver

Basket

Die-cast aluminium chassis featuring three threaded holes for rear mounting.

Cone

5mm thick paper honeycomb panel for minimum mass and carefully dimensioned bending stiffness.

Motor

The motor system features a neodymium magnet located inside the voice coil. Two aluminium

demodulation rings inside and outside the magnet gap minimise inductance variations with cone excursion, while a copper cap on the inner pole piece reduces overall inductance. Both measures assure low distortion.

Voice coil

75mm diameter copper clad aluminium wire
(CCAW) voice coil wound on a GRP (glassfibre)
former.