Electroacoustic absorbers I: bridging a gap between active acoustic feedback control and shunt loudspeakers

Variable acoustic properties can be obtained at the diaphragm of an electroacoustic transducer, with the help of very basic control strategies, among which is the simple electrical shunting of the transducer. These shunt techniques are compared to active acoustic feedback techniques for controlling the acoustic impedance of an electroacoustic transducer. It is shown here that the formulation of feedback-based acoustic impedance control reveals singular analogies with shunt strategies, and highlights an interesting strategy for synthesizing electric networks capable of mimicking actual acoustic feedbacks, bridging a gap between passive and active acoustic impedance control. The present paper describes the underlying theory unifying all these passive and active acoustic impedance control strategies, introducing the concept of “electroacoustic absorbers”. The formal equivalence between shunt and active feedback control is first formalized, on the one hand through the introduction of a 1-degree-of freedom active acoustic resonator accounting for both electric shunts and acoustic feedbacks, and on the other hand through the introduction of equivalent electric networks that mimic the performances of acoustic feedbacks. Simulated acoustic performances are presented, followed by discussions on the design of active electric shunts in view of active sound absorption. At last, experimental assessments of the studied configurations are presented, with general discussions on the potential improvements and applications.