The bat ultrasounds are picked up by the microphone SPKR1 and go through two stages of amplification at Q1 and Q2.
Separately, a tunable (R12) single frequency is produced by the LM567 oscillator U1. The LM567 is a tone decoder but here its input is grounded and its voltage controlled oscillator is used as a precision oscillator.
The oscillator frequency is given by
f = 1/(1.1*C4*(R10 + R12))
= 1/(1.1*0.0022*10-6*(4.7 + [0-50])*103)
= [88-7.5] khz
The two signals are mixed at Q3 to produce both a signal in the audio range and higher frequencies that are the