Design of switching amplifier used in negative impedance dispoal for the active control of transducer's acoustic impedance

We present a concept of "active material" with variable acoustic properties, being constituted of an electroacoustic transducer which acoustic impedance can be changed by an active mean. Among the different ways to obtain variable acoustic properties on an electroacoustic transducer's voicing face is the shunting of the transducer's electrical input. With such shunt devices, the acoustic impedance that the transducer's membrane presents to the acoustic field comprises an acoustic equivalent of the electrical load that can take many values within a specified range. The shunt strategy can either be a passive, with a simple resistor for instance, or active, by way of a negative resistance circuit including at least one operational amplifier. The advantage of the active mean is that it should theoretically allow obtaining very high values of acoustic impedances, in other words to even obtain perfect isolating acoustic material. Though, the use of the operational amplifier presents many drawbacks such as operational instability, high sensitivity to adjustments, and inefficient electrical energy transfer in the electronic circuit. Moreover, when it comes to absorbing/reflecting very high sound pressure levels, these devices are pushed towards their functional limits. In order to counteract the aforementioned problems, a switching amplifier has been designed for the negative impedance disposal. The present work will describe the design of such a switching amplifier for enhanced stability and performances of the active material concept. Theoretical considerations will be compared to experimental (acoustical and electrical) assessments of a prototype, highlighting the main advantages and drawbacks of such disposals, and leading to concluding remarks on general behaviour and possible means of enhancements.