Frequency response of second order low pass filter
A series resistor from input signal followed by a shunt capacitor reduces the gain of high frequency inputs, and it's called a low pass filter (passes low frequencies).
This circuit has two such filters (so it's called a second order filter), except that the capacitor of the second filter is a feedback from the output.
Here, the rheostat is used as a variable voltage divider feedback as the frequency is dependent on the gain, which is in turn dependent on ratio of the voltage divider.
From calculations, it's found that gain should be 1.586 for butterworth response. Hence ratio of resistors of voltage divider should be 58.6℅.
