A study of the scattering of the fundamental guided wave SH 0 at a through-thickness narrow notch directed along the wave's propagation in a plate is presented. The results are obtained from Finite Element simulations and experimental measurements. Good agreement is found between the simulations and the measurements. The results are shown for a range of crack lengths and shapes. The scattered wave field consists of the reflected and diffracted SH 0 mode and also contributions from mode conversions to the S 0 mode. It is found that the coefficient of direct reflection of the SH 0 mode has an undulating nature depending on the length of the crack. This is caused by interference phenomena that are related to the interaction of different surface wave types generated on the crack surfaces and their diffractions at both tips of the crack. It is shown that the dominating part of this reflection is generated by the delayed "Rayleigh type" surface waves reflected from the far tip of the crack.

During last decades, anthropogenic underwater sound and its chronic impact on marine species have been recognised as an environmental protection challenge. At the same time, studies on the spatial and temporal variability of ambient sound, and how it is affected by biotic, abiotic and anthropogenic factors are lacking. This paper presents analysis of a large-scale and long-term underwater sound monitoring in the Baltic Sea. Throughout the year 2014, sound was monitored in 36 Baltic Sea locations. Selected locations covered different natural conditions and ship traffic intensities. The 63 Hz, 125 Hz and 2 kHz one-third octave band sound pressure levels were calculated and analysed. The levels varied significantly from one monitoring location to another. The annual median sound pressure level of the quietest and the loudest location differed almost 50 dB in the 63 Hz one-third octave band. Largest difference in the monthly medians was 15 dB in 63 Hz one-third octave band. The same monitoring locations annual estimated probability density functions for two yearly periods show strong similarity. The data variability grows as the averaging time period is reduced. Maritime traffic elevates the ambient sound levels in many areas of the Baltic Sea during extensive time periods.

The acoustic scattering from a fluid-loaded stiffened cylindrical shell is described by using elasticity theory. The cylindrical shell is reinforced by a thin internal plate which is diametrically attached along the tube. In this model, cylindrical shell displacements and constraints expressed from elasticity theory are coupled to those of the plate at the junctions, where plate vibrations are described by using plate theory. The present model is first validated at low frequency range (k1a approximately 5-40) by comparison with a previous model based on the Timoshenko-Mindlin thin shell theory and by experimental results. Theoretical and experimental resonance spectra are then analyzed in a high frequency range (k1a approximately 120-200). Only resonances due to the S0 wave are clearly observed in this frequency range, and their modes of propagation are identified. Furthermore, A0 wave propagation is detected, because of the presence of the reflection of this wave at the shell-plate junctions.

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An analytical solution is derived for the acoustic response of submerged thin-walled ring cylindrical shell containing lengthwise stiffening members: internal stringers and walls. On the basis of the analysis of the acoustic pressure versus time diagrams the stiffener-borne wave-generation mechanisms are traced. Shown is that the shell/stiffener junctions act as additional entry and exit points of circumferential waves circulating in the shell and the fluid. The stiffening members cause transformations of circumferential waves from one propagation type to another.

Through an experimental approach, this paper explores the acoustic wave scattering processes involved in the acoustic backscattering from air-filled submerged cylindrical shells having axial discontinuity. The discontinuity is caused by lengthwise soldering along the tubes in the course of manufacture from flat sheets, and is represented as an internal axial mass layer. The tubes are excited by a plane wave at normal incidence. In the time domain, different echoes on short pulse responses (echo wave forms), are identified by the arrival times and related wave type. This enables the localization of additional wave generating sites and scattering centers due to the solder. A number of detected echo series are thus identified. In the frequency domain analysis, the influence of supplementary echoes and that of inter-echo interferences on both backscattering and resonance spectra is presented. The influence of the angular position of the solder, relative to the direction of the incident sound wave, on the amplitudes of spectra, is analyzed. The key phenomena in this study are wave generation at the solder when it is directly insonified (prevalent over classical wave generations of considered waves) and the wave type conversions observed when propagating waves encounter obstacles in the shell, geometric or material discontinuities (in this case the solder). The wave types studied are the A wave (Scholte–Stoneley) and the S0 compressional wave, in the midfrequency region, ka=25–90, where k=ω/C1 is the wave number in water, C1 is the sound velocity in water and a the external radius of the tube. This analysis of the influence of discontinuities in the propagation medium on the acoustic scattering is carried out with a view to the investigation of scattering of assembled objects such as cylindrical shells with hemispherical endcaps.

This paper presents theoretical and experimental studies of axisymmetric longitudinal guided wave L(0,2) interaction with the free edge of the pipe. A numerical method based on normal mode superposition is applied to predict the edge resonance by an analysis of dispersion relations of separate modes. In parallel, the finite element analysis and experimental measurements prove the existence of edge resonance in the pipe in case of L(0,2) wave incidence. It is shown that the edge resonance is mainly caused by the first pair of complex modes. Additionally the behavior of edge resonance phenomenon as a function of the curvature of the pipe is studied. The displacement amplitudes measured at the edge demonstrate that the edge resonance is affected by the frequency and thickness to midradius ratio of the pipe, and it is losing its strength in thicker pipes, as the growing difference between the outer and inner radii destroys symmetry. The reflected energy amplitudes show that at the resonance frequencies the incident wave is strongly converted to L(0,1) and L(0,3) modes, depending also on the curvature parameter of the pipe.

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