About Some Misunderstandings in the Modern Theory of the Sound Isolation and Discrete Models of Sound Transmission

The present paper is a short critical review of the modern theory of the sound isolation by single-layered protections. It is shown that dependence of insulation upon angle of sound incidence is untenable. The author disproves existence possibility of "spatial frequency" resonance at wave coincidence and of "loss coefficient of a design" defining the abatement of the sound isolation on resonances.The substantiation of application of discrete models of sound transmission through a plate, in which the equations of laws of conservation of momentum and kinetic energy are used, is explained. On the basis of discrete models is offered a practical way of calculation of the sound isolation, not conceding accuracy to modern standard methods, but distinguishing itself in a greater universality and simplicity. Acoustics theory as a section of mechanics of continua which had been based at the beginning of the 18th century had been finished as a whole by the middle of the 20th century. However its applications in some areas of technics have not found the necessary decision also for today. It had happened so with building acoustics and particularly with the sound isolation theory basic results of which [1] are not deprived of contradictions and rather poorly are proved by experimental data. Development of electronics since the middle of the 20th century has allowed creating perfect measuring acoustic devices by means of which are made many experimental and natural measurements of various soundproof constructions. Statistical processing of results of measurements allowed creating practical methods of calculation of the sound isolation. At this stage, there had been covered the primary inquiries of consumers of the theory of the sound isolation - basically civil engineers and transport workers. Therefore the sound isolation theory did not develop the last fifty years. And by-turn it had hampered the development of practical acoustics in the field of working out of new soundproof constructions. The central constructs of the modern theory of the sound isolation by single-layered protections consist in the following: the sound isolation of protection is defined as valueR = 10lg 1/tau(theta)dB (1), where tau(theta)is coefficient of passage of energy of the sound wave falling at angle 0 to normal of surface of a protection.The protection is considered as a homogeneous isotropic plate incompressible on its thickness, making bending vibrations under the sound influence. For definition of the coefficient of sound transmission the plate is accepted as one of unlimited sizes. Further, for isolation definition on own frequencies to a plate are given concrete values of length and width. There are considered three frequency ranges: 1) Not over the boundary frequency of wave coincidence (f <f(b)). The length of the bending wave in a plate on the considered frequency is less than a sound wave in the medium; 2) Over the boundary frequency of wave coincidence (f >= f(b)). The length of the bending wave in a plate is equal or more than lengths of a sound wave in the medium. Wave coincidence consists in that the trace of a sound wave falling at angle of coincidence is equal to the length of bending wave in the plate;