With this circuit we can create a altered sound of siren. The oscillator IC1a-b is constituted by two gates NAND, oscillating in very low frequency. This oscillation drive the IC2, that is a electronic switch, which opens and closes in the rythm of oscillation. When closes the switch then the voltage of supply charge the capacitor C3 through the resistance R2. When switch IC2 opens, the C3 discharge through the R3. The voltage with which is charge the C3, regulates the VCO of IC3, that is PLL, so that we have in the exit his one altered sound. You can experiment with the prices of resistances and capacitors in order to you change the sound of siren. The R1, C1 check the time of operation, the R4, C2 check the frequency of produced sound and the R3, C3 check the change of sound.
Source : http://users.otenet.gr/~athsam/electronic_siren.htm
Sam Electronic Circuits 10/01