On the use of virtual sensing for the real-time detection and active control of a scattered acoustic field

There is an interest in many situations to achieve a perfect acoustic cloak, which renders an object acoustically invisible. Although significant contributions have been made to realising passive acoustic cloaks, there is significant potential in the use of active control technologies for cloaking. However, this requires that an accurate estimate of the scattered pressure can be obtained in real-time to provide an error signal to the controller. This is non-trivial, since a standard pressure sensor would detect the contributions from both the incident and scattered fields. The measured pressure must, therefore, be decomposed into these two constituent parts, which has previously been achieved using a double layer of pressure sensors enclosing the scattering object. This paper proposes an alternative method of estimating the scattered component of the sound field, which does not use a double layer of sensors. The proposed virtual sensing method is based on an adaptation of the Remote Microphone Technique that has previously been used in active noise control applications. The proposed method filters the measured pressures using an optimally designed filter to estimate the scattered component of the sound field. The paper first formulates the proposed virtual sensing method for scattering detection and then presents an investigation into the accuracy of this estimation procedure using a series of measurements taken in an anechoic chamber. The effect of varying both the number of sources in the incident sound field, and the number of microphones used in the estimation is investigated. Finally, the practicability of designing an active control system using the estimated scattered field is discussed, and results from offline simulations of active control are presented.