By generating square-wave output that sweeps down and up in frequency, this circuit can produce force field of high-intensity sound inside car. This circuit can make a car thieve panics and decide to run before anyone catch his action. D1, C1 and R1 produce triangle waveform by shaping a square wave output of IC1.  If using sweeping alarm sound is prohibited by regulation, then C1 can be removed to provide legal two-tone sound. The diodes are general purpose small signal silicon except D2 which is silicon rectifier rated 1 A at 50 PIV. This circuit can handle up to 10W depends on the horn loudspeaker. Here is the schematic diagram of the circuit:

10 W Auto Alarm Siren circuit schematic