The ND940 is ultra compact size, high performance, high power handling 4.0-inch diaphragm compression driver with 1.4 inch exit throat. Thin copper ring is precision pressed on to the pole piece in order to modify and lower the inductance characteristics of the magnetic circuit and voice coil providing a controlled extension of the acoustic frequency response. The voice coil assembly is designed using high strength, high temperature Kapton voice coil former and edge wound copper clad aluminium wire directly joined to diaphragm by RCF proprietary Direct Drive kapton technology. The ND940 features 4-slot, optimised geometry, phase plug design. Extended computer assisted mathematical modelling and testing has resulted in a geometry that provides balanced acoustic performance controlling and lowering air distortion and maximizing output.