Narrow-edged beamforming based on individual phase inversion in amplitude-modulated wave for parametric array loudspeaker

•We propose a method for forming a narrow-edged acoustical beam.•Phase-cancellation are utilized in ultrasonic and audible sounds for beamforming.•The directivity can be significantly narrowed with a cost of sound pressure. The parametric array loudspeaker (PAL) can realize sharp directivity using the straightness of ultrasound. Some recent studies have applied phase array approaches to PAL to control the directivity. The PAL is composed of a large aperture emitter consisting of multiple small ultrasonic transducers to exhibits sharper directivity and reproduce audible sound with sufficient sound pressure. In contrast, the directivity of the small-scale PAL may diffuse. In the present paper, we focus on forming a narrow acoustical beam for practical use of the PAL, so that it is able to control the area where the audible sound reproduced by the PAL can be heard. We propose a narrow-edged beamforming method based on individual phase inversion in the amplitude-modulated wave. The PAL is divided into interior part and exterior part, and the phases of the components in the signal fed to the exterior of the PAL are controlled individually. In the processing of the proposed method, the lower frequency component between the carrier and the sideband wave is phase-inverted in the signal fed to the exterior of the PAL. The proposed method uses phase cancellation of both ultrasound and demodulated audible sound to form a narrower beam. This method can also be combined with beam steering techniques to form a narrow steered beam. The experimental results demonstrate the effectiveness of the proposed method in forming a narrower beam than the common PAL.