Foreword
After radar and sonar were founded, problems of the “visibility” reduction for physical bodies in air (electromagnetic waves) or in water (acoustical waves) have immediately become serious in physics and technics. Facilities of the "visibility" reduction are oriented physically for the reduction of radiation field and reduction of scattering field of physical bodies. Now many problems of this circle have become classical, and many researchers thought that these problems cannot have solutions for a practical scenario. New technologies (sensors, actuators) of high spatialtemporal resolution plus new computers with fast and accurate calculations in combination with untraditional versions of solutions of above boundary problems, allow to obtain the success in some cases. Traditional thick and weakly absorbing coatings are unacceptable today because of their greater size and weight. Interference coatings have a thickness about 1/4 wavelength, but they have narrow band of working directions and frequencies. The next attempt was the active damping (with radiation of cancellation anti-wave), suggested and developed by G. Maljuzhinets, M. Jessel, G. Mangiante. This idea permits to design coating of thickness much smaller than the wavelength. The works by B. Widrow on adaptive filtering have been applied by C. Fuller, C. Hansen, S. Elliott successfully for active noise control with adaptation ("training" during real time) to a prior unknown boundary value problem (which can also change). Training of active control system requires enough big time, because it must have total acoustical information (vast amount) on the boundary value problem. In real conditions (elastic shell in various depths and temperatures, with vibroacoustical characterictics, which were investigated by M.C. Junger and D. Feit) this vast volume of information deviates more quickly than the process of training. In recent times the idea of cloacking becomes very popular, suggested by J. Pendry. This presents the solution of above general problem for some body with cavity. Incident wave does not penetrate the cavity and, on the other hand, can not be scattered by the external surface of a body. Therefore any body can be spaced into this cavity, and will be invisible for outside observer. This result is achieved, due to special microdistribution of parameters of body material (called "metamaterials"). The cancellation of radiation and scattering field of shell in liquid is connected with following serious problems: (a) wideband acoustical fields to be damped; (b) neutral floatability of a shell in liquid; (c) the absence of dynamical support (consequence of (b)); (d) the absence of inertial coordinate system to measure shell's surface displacements (consequence of (b)); (e) the uncertainty of many parameters of shell and incident wave; and (f) problem of compactness of active control system. The book by V. Arabadzhi presents a rare attempt to solve the problem as a whole. Author uses stable technological tendencies in miniaturization and acceleration of sensors, actuators, computer components. On the other hand the lengths of waves to be damped, were constant due to the constant condition of their far propagation. Taking into account the technological progress, the author also used several untraditional approaches to the problem: conversion of the waves to be damped, into the waves of spatial and temporal frequencies which are "invisible" for the used receivers. I hope this book will be useful for a lot of readers with various interests in physics of waves. This book can help them to find new approaches and solutions of a wide area of problems.
Mal’tsev, A.A.
Nizhny Novgorod State University,
Russia.
