SELF

10

S.B. Karavashkin and O.N. Karavashkina

Fig.5. Schematic circuits generating transverse acoustic wave (a) and receiving opposite-in-phase acoustic oscillations (b)

The circuit exciting the radiation is shown in Fig. 5a. This is a variation of a full-wave rectifier circuit consisting of the step-up transformer T1 and two high-voltage rectifiers VD1 and VD2.

According to the circuit, to a moveable membrane of the radiator 1, a direct, as to the earth electrode, voltage 2 kV was supplied, and to the stationary plates 2 and 3 – an alternating voltage of the same amplitude. When the positive half-period, the plates 1 and 2 are charged positively and repel each other, and the plates 1 and 3 are charged oppositely and attract each other. At the negative half-period, the membrane interacts with the stationary plates reversely. The maximal symmetry of these processes was achieved due to the close symmetrisation of the step-up transformer T1.

The receiver circuit is shown in Fig. 5b. It consists of the step-up transformer T1, high-voltage balanced rectifier VD1-VD4, with filters C1 and C2 to whose input the alternating voltage 300 V, 50 Hz was supplied, and of the discriminative contour consisting of the transformer T2 and paired capacitors C3. The contour was connected directly to the stationary plates of the receiver, and its central point – to the membrane.

The rectifier was used to supply a high voltage (2 kV) developed by the membrane and stationary plates of receiver to the armature of capacitors C_r. The membrane was charged positively, the same as the radiator circuit.

When the membrane halves at the discriminative contour input oscillated as antiphase, the potential difference doubled; it was picked over the secondary winding of the transformer T2. But when the membrane halves oscillated in phase, in the windings w₁ and w₂ the opposing electromotive force excited. It led to the compensation of the spurious, in this case, effect that might be produced by the longitudinal component of an acoustical wave.

From the receiver’s circuit output, the signal came to a standard amplifier having the potential input. The amplification coefficient of the amplifier was k_y= 30, the input resistance was no less than 3 megohm, the minimal level of the input signal was 50 mkV, the input capacitance was 10 pF, the data-sheet load was 1 kohm, the maximal level of the output signal was 7 V. The effective suppression of the spurious longitudinal component was achieved by the symmetrisation of the primary windings of the transformer T2.

As a display device, we used the cathode-ray oscillograph C1-94 having the resolution up to 10 mHz. The amplification and sweep limits were tested and adjusted in accord with the level of signal and with the task.