I am aware that T1, T2, and T3 are act like common emitter amplifiers. Input is fed to the base and the collector current is β times base current. Therefore there is a amplification of approx β times. Also, RV1 can change the detect frequency of this circuit by changing its resistance. T4 acts like a buffer for matching of the output stage. The transistor amplifies this small current to allow a larger current to flow through from its collector to its emitter. This collector current is large enough to make LED works.
I still have a question about how to calculate the detective frequency of the circuit (for example, when RV1 is at its largest value, 220K, how to calculate the high pass frequency). Also, how the negative feedback of T1 and T2 works on this circuit.