Invisible speakers in home windows for simultaneous auxiliary audio playback and active noise cancellation

The use of a transparent thin film speaker in a home window can provide both an invisible audio playback device and an active noise cancellation system. However, several key challenges need to be addressed for development of such a system. A traditional feedforward active noise cancellation system uses direct microphone measurements for both reference and error signals. Such a system can degrade both the audio playback and the noise cancellation performance. Hence a wave separation algorithm is used in this paper for separation of the audio playback component from the external noise component for the reference signal. Further, an online adaptive secondary transfer function estimation method is used for accurate removal of the audio component from the error signal. Two approaches are attempted for the secondary transfer function estimation - use of source audio signals and use of additive white noise. The developed algorithms are evaluated in a scaled cabin equipped with a window and a transparent thin film actuator. Experimental results show that the developed system can preserve the auxiliary audio sound while cancelling external noise effectively. The system based on the use of source audio signals for secondary path estimation is found to yield the largest cancellation of external noise while producing the least amplitude of control inputs that cancel desired audio signals.