V.3 No 1

13

Comparison of characteristics of propagation velocities

Of course, the described process has its features. In particular, the transverse velocity of shift of the elements of medium does not induce the shift of neighbouring elements, since it is the consequence of superimposing acoustic fields, not the reason of excitation of transverse oscillations. We should also note that to this transverse shift there will correspond the complex local longitudinal oscillations connected with the pressure redistribution. None the less, as the experiment showed, from this superposition there has resulted an independent transverse wave in gas which had all properties characterising this type of waves. Namely, there has formed the polarisation plane, near and far fields whose properties essentially differed. And should the formation of transverse velocity had a pure interference pattern, in the far field at   r >> , on the basis of (15) and conventional concepts of acoustic potential (see, e.g., [16, p. 24]), we would yield

In other words, the far field simply would not form, and the entire process would localise in the near field. However the experimental results of [15] evidence that the far field of the transverse acoustic wave has formed, in this field the wave polarisation remained, and the amplitude decreased with the distance in proportion to  1/r . This evidences that in the present case there exist some factors disregarded by the standard formalism, and they allow, in the absence of shear deformation in the medium, to form a transverse acoustic wave in the far field, with all salient features of this field - the constant velocity of wave propagation, stable polarisation plane and regularity of the amplitude decrease with the distance proportional to  1/r .

Another feature of the obtained experimental data was the observed fast increase of the delay phase with a large variation within the band in the near field, which does not follow from the conventional theoretical models of the radiation of elementary EM radiators. And this is basically important issue. In particular, in [11] we showed the standard idea - that in the near field "there is no energy transfer; there is only a periodic energy exchange between the electric and magnetic components of the field" [17, p. 99] - grounding on the incorrect disregarding the delay phases at  r <<   . It is easy to show that this incorrect approximation is commonly used. Specifically, "But at the distances small as against the wavelength (kR₀ << 1) we will neglect the terms  1/R₀  and  1/R₀²   and put  exp(ikR₀) 1 ; then

(where    is the unit-vector of the direction of wave propagation,    is the dipole momentum of the system of charges,  is the vector of strength of electric field of radiation, - Authors), which corresponds to the static dipole electric field; naturally, the magnetic field is absent in this approximation" [10, p. 248].

The opposition of theoretical research in the electromagnetism to the experimental results presented in [15] complicates by the fact that, according to [15], the near region of acoustic field extends to r = 10, not  r << , as it is thought conventionally both in electromagnetism and acoustics. This requires, just the experimental characteristics of the field for the near region to be compared in more details. The results of this comparison we present in this paper.