Technology, This is a police alarm circuit. The two 555 timer ICs used as astable multivibrators. The frequency is controlled by the pin 5 of the IC. Actually, the sound is genereted by combination of sound from both IC 555. One timer generated low frequency and the another generated high frequency. By varying VR1 you can set how fast the siren changes from high freq. to low freq. VR2 sets the siren frequency.