Near-field error sensing for active directivity control of radiated sound

Near-field error sensing is beneficial to the compactness and stability of an active noise control system. This paper proposes an error sensing strategy based on the spatial Fourier transform to achieve active directivity control of radiated sound. The error microphone array is located on a plane close to the primary source and the cost function is the weighted sum of the error signals from the microphones. The weighting factor is related to the phase shift from the error microphones to the plane perpendicular to the direction where noise reduction is required. The geometric configurations of the error microphone array for effective directivity control are investigated. It is found that the distance between neighboring error microphones must be less than approximately half the wavelength of the frequency of interest and the equivalent size of the microphone array should be larger than twice the size of the primary source. Numerical simulations and experiments demonstrate the feasibility of the proposed strategy.