V.2 No 1

15

Transversal acoustic wave in gas

As transiting to the far field, the amplitude attenuation degree essentially diminishes, as well as the growth of phase delay, and somewhere about 320 – 340 mm (for this particular device and this particular frequency) the process stabilises completely. The phase delay grows fully proportionally. The experimental points give a smooth straight line. The wave inversion completely stops. The only, the signal amplitude varies very unstable; possibly, this is caused by the basic kind of the experiment. The revealed difference of the wave propagation regularities before and over 320 mm evidence unambiguously the near – far field transition of the acoustic wave as the region of the properties transformation. Thus, the polarisation diagrams shown in Fig. 7b relate also to the far field. In its turn, it also confirms the progressive pattern of the polarisation acoustic wave in gas medium.

On the basis of regularity _d (r), using (4), we plotted the average velocity v of the wave propagation with respect to the distance between the radiator and receiver. This dependence is shown in Fig. 8c. It also corroborates the results obtained before. In the near field, the wave velocity is approximately twofold less than in the far field. Actually, this explains so fast growth of the phase delay. In the far field, the propagation velocity stabilises at the level typical to the longitudinal acoustic wave in free space. It can be explained by the transversal acoustic wave formation by way of longitudinal acoustic waves superposition. However, the fact of essential change of properties in transiting to the far field evidences that as a result of this process, namely the transversal wave has formed, with all the properties typical of it. The disappearance of the wave inversion being typical for the interference process evidences it, as well as the stabilised phase delay growth with the distance between the radiator and receiver.

5. Conclusions

As a result of the carried out investigation, we have indicated experimentally the fact of radiation/reception of the transversal acoustic wave in gas medium.

We have revealed that the general properties of the wave process are inherent in this wave. The wave propagation space clearly divides into the near and far fields.

In the near field, the small propagation velocity, the fast amplitude attenuation and periodical inversions with the increasing distance from the radiator are typical for transversal acoustic wave. None the less, in this field the standing wave is absent, since we undoubtedly indicated that the phase delay increases with the distance from the signal source. In the far field, the wave propagation velocity increases and attenuation degree essentially diminishes. The wave inversions disappear. The phase delay growth diminishes with the distance.

This investigation carried out as a fundamental experiment undoubtedly evidences that the transversal acoustic wave in gas medium exists.