Constant beamwidth beamformer for difference frequency in parametric array

The sound reproduction in air by using a parametric acoustic array [P.J. Westervelt, 1963] has been reported for a few decades. Two inaudible ultrasonic frequencies are produced from the parametric array. Due to the nonlinearity of air, it is possible to produce an audible frequency with its frequency equal to the difference in the two ultrasonic frequencies. However, there is not much work done in controlling the beam pattern of the difference frequency generated by the primary waves. In this paper, an algorithm is proposed to control the sidelobe level of the difference frequency directivity. By making use of array signal processing techniques, the algorithm is also capable of producing a constant beamwidth for broadband difference frequency.