The oscillator shown in the figure generates a 1 kHz tone that is reproduced on the speaker with good power. For supplies up to 6 V, the transistor can be BC558, but for higher voltages, given the power developed, BD136 or TIP32 is recommended, which must be mounted on heat radiators. The circuit frequency can be changed by changing R2 values. Values between 100 k and 1 M ohms can be experienced. Capacitor C1 can also be changed to obtain tones from other frequencies. A piezoelectric tweeter can be used to generate ultrasound in the range of 18 to 25 kHz with the necessary changes in R2 and C1.
