V.2 No 1

11

Transversal acoustic wave in gas

3. The technique used for the experiment

As we said before, the task of this experiment was to register the fact of transverse acoustic wave existence and to find four regularities: A_m (),   _m () ,  A_m (r)  and _d (r). The experiment was carried out at 7,4 kHz and at the distance between the radiator and receiver from 75 up to 900 mm.

When finding the regularities A_m (),  A_m (r), not absolute amplitude of the signal but its variation with respect to the angle and distance r was of the most interest. To a considerable degree, it facilitated the metrological provision of the experiment, since with it a number of the systematic errors became inessential, and we could measure the magnitude in some arbitrary units (and we did so).

Dependently on _d (r), the main interest we saw in, how the phase delay varies with the distance between the radiator and receiver, since for the wave propagation velocity determined on the basis of this characteristic, namely the phase difference between the indicated values was important, though we already could not measure it in the arbitrary units. So in this case the systematic errors also did not exert an effect on the results of measurement. Besides, it would be reasonable to take the phase delay at a minimal distance between the devices as a zero value of the phase.

The same for the regularity _m (); the only difference was that we needed to fix accurately the parallelism of the radiator and receiver as the reference point. Though in this case, when the polarisation planes parallelism determining, small errors also could not effect on the transversal acoustical wave verification as the fact, since the measurement was executed in limits of the complete period of variation.

The phase delay measurement was effected by the sinusoid horizontal displacement at the oscillograph screen that worked, as we said, in the slave sweep mode, when the radiator – receiver reciprocal location varied.

To calculate the phase, we used the following expression:

where _f x is the sinusoid displacement measured at the oscillograph screen; p_f is the calibrated sweep coefficient, and f is the signal frequency.

As all the complex of the measurement has been effected at the constant frequency f, the phase measurement error was determined by the sweep coefficient error p_f and by the reading values error of the sinusoid displacement at the oscillograph screen   _f x:

According to the maker’s data sheet, the ratio p_f / p_f did not exceed 5%. But the ratio  _f x/ _f x in (3) can be diminished by way of using the maximal possible sweep of the signal, when measuring. Practically / did not exceed 8%, what was quite sufficient for the basic experiment. Though at small values of the signal amplitude, where the high noise level takes place, the error naturally increased up to 15-20%. But these were the narrow domains unable to effect on the studied process pattern as a whole. As our main task was to establish the general regularities of a transversal acoustic wave behaviour, such error values were considered acceptable.