Near-field and Far-field Scattering Characteristics of Underwater Low-frequency Spherical Acoustic Wave

To investigate the scattering mechanism of underwater low-frequency acoustic waves, a uniform asymptotic solution of near- and far-field acoustic pressures is derived. First, the infinite series solution of the scattering acoustic pressure for low-frequency spherical acoustic waves encountering spherical obstacles such as soft and hard impedance is provided. Based on the assumptions of low frequency, near- and far-field, the approximate forms of the Bessel and Hankel functions are employed to derive the asymptotic solution. By assuming that the acoustic source is near the scatter, a uniform asymptotic solution of the near- and far-field scattering acoustic pressures is derived. Subsequently, the near- and far-field scattering characteristics of a spherical acoustic wave are investigated. The results show that the low-fre- quency scattering acoustic pressure can be expressed as a superposition of fields because of a monopole source and dipole sources, the uniform asymptotic solution derived is consistent with the classical solution at low frequency. When the source is near the obstacle and the receiver is far from the source or obstacle, the echo strength of the soft sphere is shaped like a circle, whereas those of the hard and impedance spheres are shaped as “gourds”. When the source is far from the obstacle and the receiver is near the source, the echo strengths are related to the shape of the incident wave. The results are useful for controlling acoustic scattering and detecting low-frequency waves of underwater targets.