Plasma-based Electroacoustic Actuator for Broadband Sound Absorption

Environmental noise, mostly related to human activities, has an immense impact on public health. The development of noise reduction technologies is paramount in addressing this problem. Because of practical and economic reasons, a compact, broadband, lightweight, and mechanically robust solution is often compulsory. Existing noise reduction methods typically fall short meeting these requirements. Passive absorbers are bulky and inefficient in the low frequency range or present only a narrowband performance. Active noise reduction methods, including active noise cancellation and acoustic impedance control, appear more promising as they allow extending the bandwidth of operation and remain small compared to a wavelength. An electrodynamic loudspeaker is conventionally a favourable choice for a controlled transducer. However, it limits efficient technology application due to its fragile diaphragm, relatively high weight, and inherent resonant nature, bounding the bandwidth of control.

This thesis is devoted to the development of a fundamentally different plasma-based electroacoustic transducer for active sound control applications. The acoustic field is manipulated by the partial ionisation of a thin air layer with an atmospheric corona discharge and its further control with an alternating electrical field. The transducer consists of a set of high voltage wires, separated with a grounded mesh by an air gap. Analytical and numerical models are first developed to design and characterise the corona discharge actuator. Several feedback impedance control strategies are adapted and implemented with the corona discharge actuator, resulting in achieving broadband impedance and sound absorption. A prototype of a plasma-based active acoustic liner for noise reduction under grazing sound incidence is proposed and assessed experimentally in laboratory facilities. A model-based feedforward approach for broadband control of acoustic impedance is then developed. Exploiting the unique physics of the corona discharge actuator with the help of the analytical model, perfect sound absorption and tunable acoustic reflection under normal incidence are achieved over two frequency decades, from several Hz to the kHz range. Such unprecedented bandwidth and compactness of the developed system, along with the simplicity of construction, lightweight, and flexible design, opens new doors in noise control applications, and acoustic metamaterials, among others.