When the door is pushed, you hear a whisper that `slide up` to a higher frequency. The oscillator frequency is determined by AF coupling capacitance, C 1 and the value of the resistor connected between the base of IQ and the earth. This resistance, RBG is equal to (Ri + R2) RJ. First, suppose that 51 is closed and R2 have been adjusted to produce a pleasant, low frequency tone. The capacitor C3 charges through R6 until it reaches such a tension that will cause diode Dl to conduct.
When this occurs, the value of RBG is consistent with R4. Thus, because the total decrease resistance RBG, tone output slides upward in frequency. The capacitor C3 continues to charge until the voltage across D2 and D3 causes the diodes to conduct. Then RBG is also paralleled by R5, the total resistance decreases again, and the oscillator frequency increases again.