BC550C Simple Audio Peak indicator circuit and explanation

A simple circuit PEAK indicator of foliage of the musical signal. Whenever the signal exceeds the + 4dB, turns on the LED D1.
 It is useful in the console of each channel of sound amplifiers or other final application, we needed.
With the price of the tour, which begins with high level +4 dB (1.25 V RMS). Adjustment for different signal levels, we use a lawnmower capacitor C1. T? circuit can work perfectly with the track, Mic / Line Balance. after C11.

Electronic Part
R1= 10Kohm
R2= 1.2Kohm
R3= 220Kohm
R4-5= 4.7Kohm
 C1= 47uF 25V
C2= 2.2uF 25V
D1= LED RED
Q1-2= BC550C