Acoustic filter design for precise measurement of parametric array in near-field region

A parametric array is a nonlinear conversion process that generates a narrow beam of low-frequency sound using a small aperture and that is used in active underwater SONAR and communication systems. A parametric array generates a highly directional difference frequency wave (DFW) by nonlinear interaction of bi-frequency primary waves at higher frequencies. However, it is difficult to measure the parametric array effect in the near distance of the transducer, because the high-pressure level of the primary waves may cause the receiving transducer to provide with pseudo-sound signals of difference frequency by its nonlinearity. The pseudo-sound signals make it difficult to measure the real DFW due to the parametric array process. In this study, we confirmed the existence of pseudo-sound and its effects on measurement of the near-distance DFW by the parametric array using numerical simulations based on the KZK equation. Also, a newly designed acoustic filter was proposed to eliminate pseudo-sound signals. This new acoustic filter uses the resonance of a thickness extensional mode. With the acoustic filter, low frequencies (DFW) were passed and high frequencies (primary frequencies) were effectively reduced. Using this acoustic filter, the precise characteristics of the difference frequencies could be measured. [Work supported by ADD(UD100002KD).]