The length-limited parameter array loudspeaker (LLPAL) usually consists of two parametric array loudspeakers (PALs) with separate carrier frequencies. Each PAL can be used individually as a directional sound source that produces the difference frequency from ultrasonic carrier and sideband frequencies by the nonlinearity of air. The beam length of the PAL depends on the carrier frequency. Therefore, in the LLPAL, two sound beams interfere with each other, resulting in a length-limited beam. This paper first modifies the far-field Westervelt's directivity and establishes the directivity model of the LLPAL. Subsequently, a beamformer design method is presented for the LLPAL to achieve a constant beamwidth over a wide band of difference frequencies.
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