Acoustic resonance scattering by cylindrical shells with consideration of material attenuation

Abstract

Acoustic resonance scattering by an air-filled cylindrical shell immersed in water is studied. Complex wave numbers are introduced in order to consider the effects of material (shell) attenuation. Attenuation coefficients are treated as a linear function of incident wave frequency. Scattering form function is numerically calculated and resonance characteristics are analyzed. Firstly, in case where shell attenuation is neglected, resonance width versus resonance frequency is calculated up to 230, or fd=6 MHz.mm for five modes (S0, A1, S1, S2, and A2 circumferential wave modes not only for selecting appropriate circumferential wave modes and frequencies, but also for determining frequency regions in which shell attenuation effects should be intensively investigated. Obtained were resonance frequency regions which have relatively narrow resonance width (less than 0.5) caused by radiation attenuation. Secondly, the effects of material attenuation on acoustic resonance scattering are investigated in the frequency regions determined in the above for three modes, namely S0, A1, and S1. From these results, ultrasonic attenuation brought the increase of resonance width and the decrease of resonance amplitude. However, the change of resonance frequency is negligibly small. In addition, the component of resonance width due to shell attenuation for each mode is linearly increased with k1a and ultrasonic attenuation coefficients. Resonances can be represented by a simple function which is defined by resonance center frequency, peak amplitude, and resonance width. The square of the function representing resonance form is Lorentzian type. The relationships between the change of resonance width and that of peak amplitude due to shell attenuation is as follows. The value of total resonance width times peak amplitude when shell attenuation is taken into account equals to that of resonance width times peak amplitude when shell attenuation is neglected. Therefore, the variations i...