An Enhanced Sharpness on Acoustic Focus Via Artificial Iterative Phase Conjugation Processing

Abstract

For a low-frequency source in a homogenous medium, when the source is focused by phase conjugation (also referred to as the time reversal in the time domain), the focal spot is large because of the half-wavelength limit. In this paper, to reduce the focal spot size, artificial iterative phase-conjugated processing is proposed based on passive phase conjugation to focus a point-like source. As AIPCP operates in iteration mode using a computer, the iteration loops of each transducer are calculated independently, and then, AIPCP is achieved by summing the iteration outputs of all transducers together. Numerical simulations and experiments on an audible sound field are employed to illustrate that AIPCP generates a narrower focal spot than passive phase conjugation. Further analyses considered the theoretical change in focal spot size with the iteration number using Fraunhofer far-field approximation, and the result shows that the focal spot size decreases with increase in the iteration number. This finding is predicted in the near field and validated in the far field by simulations and experiments. Moreover, the half-wavelength limit is overcome at a distance from the sound source equal to the wavelength.