Shunt loudspeaker technique for use as acoustic liner

In this paper, the underlying concept of acoustic liner is an electroacoustic transducer which acoustic impedance can be changed by electrical means, be it passive or active. 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 takes account of an acoustic equivalent of the electrical load that can take many values within a specified range. Shunt strategies can either be passive, with a resistor, or active, with a negative resistance circuit including at least one operational amplifier, or with more complex impedance control systems. These active strategies allow variable acoustic impedances at the transducer’s voicing face, assuming linearity of the systems. This presentation aims at giving an overview of the shunt loudspeaker techniques for use as liners in an aircraft engine nacelle, and discuss on possible means of enhancement.