Controlling the upper-frequency characteristics of velocity-feedback loudspeaker systems

A treatment already given permits velocity-feedback systems to be designed for adequate stability margin at low frequencies. At the higher frequencies allowance must be made for the modifying effects of the relatively impure blocked voice-coil "inductance." Because of the nature of this inductance, a design procedure was developed in terms of closed-loop voltage response at the voice-coil terminals. A relatively simple compensation could be provided to produce a suitable balance in the acoustic frequency response at the low, middle and upper audio frequencies. Stable operation also was obtained with no signs of parasitic oscillations. Practical examples, on testing, established the closeness of certain simplifying approximations used in the design. Results indicate how this type of feedback can suppress the fundamental resonance and extend the bass response. An important feature of the design is that it is based upon measurements of voice-coil impedance of the loudspeaker in its enclosure but without the need to block the voice-coil motion.