Shunt loudspeaker techniques for use as liners for engine nacelles

In this paper, the liner concept 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 control systems (“hybrid control”). These active strategies allow very versatile acoustic impedances, assuming the whole device remains linear. Though, the use of the operational amplifier in active devices presents many drawbacks such as operational instability, non-linear distortion, high sensitivity to settings. In order to counteract this, alternative designs have been assessed. This presentation will give an overview of the above-mentioned techniques for use as liners in the engine nacelle, and discuss on their performances and limitations.