Acoustic impedance synthesis at the diaphragm of moving coil loudspeakers using output feedback control

This paper discusses a time-domain technique for synthesizing acoustic impedance at the diaphragm of a loudspeaker using a proportional-plus-derivative output feedback. The dynamics of electroacoustic transducers such as moving-coil loudspeakers can be readily controlled either by direct feedback principle on acoustic quantities, or by plugging a shunt network at the electrical terminals. Any conventional loudspeaker first intended to be a sound transmitter may then become a versatile electroacoustic resonator capable of absorbing (or of reflecting as much) the incident sound energy in a frequency-dependent way by simple electronic controls. Instead of counteracting some unwanted sound by using superposition principle, as is the case for conventional active noise control, such actuator-based strategy aims at monitoring the reaction of a loudspeaker embedded into walls so as to control the proportion of reflected sound waves on this boundary. After a short description of the dynamics of moving-coil loudspeakers giving emphasis on the advantage of electromechanical coupling reversibility, a proportional plus derivative output feedback combined to a feed-forward action is proposed for synthesizing of desired acoustic impedance. As a conclusion, the overall performance of the proposed method is presented along with computed results and general discussions on practical implementation.