ACTA ACUSTICA UNITED WITH ACUSTICA Vol. 105 (2019) 1251 – 1257 DOI 10.3813/AAA.919402 Acoustic Scattering from Bi-Layered Cylindrical Shells: Influence of the Layers Thicknesses on the Guided Waves Younes Khandouch, El Houcein Aassif, Said Agounad Laboratory of Metrology and Information Processing, Faculty of Science, Ibn Zohr University, B.P 8106 Agadir, Morocco. khandouch.younes@edu.uiz.ac.ma Colour Figures: Figures in colour are given in the online version Summary The current work focuses on the study of acoustic scattering from bi-layered stainless steel-copper and copper- stainless steel cylindrical shells filled with air and immersed in water. This paper is interested in revealing the effects of physical and geometrical characteristics of the layers constituting the shells on the scattering phe- nomenon. The object of this work, is to study the influence of the layers thicknesses on guided waves, the overall thickness of the shells is fixed. The plane of modal identification was chosen to analyze the scattering phenomenon. We investigate the resonance trajectories of the guided waves, especially the curves change. The investigation and comparison made on resonance trajectories, show a shape change, a gradual deviation, or both, appear on the resonance trajectories of different guided waves, for the reduced cutoff frequencies of guided waves a sliding to higher and lower value are noticed. The interaction between guided waves is also manifested in the scattering phenomenon. The findings for the bi-layered cylindrical shells are then compared with those obtained for the mono-layered stainless steel and copper cylindrical shells. Then, this work is completed by an investiga- tion on the reduced cut-off frequencies of the A 1 wave, that have been extracted for different possible values of the intermediary radius. In this part, to understand the observed phenomena, other examples of bi-layered cylindrical shells are introduced. The obtained results are analyzed and investigated. PACS no. 43.20.Fn, 43.20.Mv 1. Introduction If an air-filled elastic cylindrical shell immersed in wa- ter, excited by a plane acoustic wave perpendicularly to its axis, two types of circumferential waves were gener- ated automatically in the shell and in the water-shell inter- face (Symmetric waves S i and Antisymmetric waves A i ). For some frequencies, these circumferential waves make standing waves on the circumference of the shell consti- tuting resonances [1, 2, 3, 4, 5, 6]. These resonances are observed in the spectrum of the scattering acoustic pres- sure. The previous studies shows that the acoustic reso- nances of the shell are related to its physical and geometri- cal proprieties [7, 8, 9, 10]. Some of these studies, done on submerged air-filled cylindrical and spherical shells, have shown the bifurcation of the dispersion curve for the guided wave A 0 wave into two waves denoted by A 0 − and A 0 + [11, 12]. Works in Refs. [13, 14] are concerned with the acoustic scattering from polymer coupled with a thin layer of water or from cylindrical shells filled with water. Furthermore, some research has investigated the acous- Received 13 April 2019, accepted 9 December 2019. tic scattering from bi-layered cylindrical shells and multi- layered spherical curved plates [15, 16, 17]. In the first part of this work, we focused on study of the acoustic scattering by an air-filled stainless steel- copper and copper-stainless steel bi-layered cylindrical shells immersed in water. The stainless steel and cop- per one-layered cylindrical shells are taken as a refer- ence for studying the acoustic scattering from bi-layered cylindrical shells. This paper reveals the effect of physi- cal and geometrical characteristics of the layers constitut- ing the shells on the scattering phenomenon. The work is done from the calculated form function, the obtained res- onance trajectories have shown the changes appeared on the guided waves. In this research, we investigate the res- onance trajectories of symmetrical and anti-symmetrical guided waves, especially the curves change. The findings are compared with those obtained for the stainless steel and copper mono-layered cylindrical shells. In the second part, we focus on investigation the re- duced cut-off frequencies of circumferential A 1 wave, that have been extracted for different possible values of the in- termediary radius. This treatment is done for three cases of bi-layered cylindrical shells; copper/stainless steel, alu- minum/stainless steel and copper/aluminum, in the object to have a good understanding of the observed phenomena. © S. Hirzel Verlag · EAA 1251