A lot of electronic circuits using NE555 timer IC are already published here and this is just another one.Here is the circuit diagram of a police siren based on NE55 timer IC. The circuit uses two NE555 timers ICs and each of them are wired as astable multivibrators.The circuit can be powered from anything between 6 to 15V DC and is fairly loud.By connecting an additional power amplifier at the output you can further increase the loudness.
IC1 is wired as a slow astable multivibrator operating at around 20Hz @ 50% duty cycle and IC2 is wired as fast astable multivibrator operating at around 600Hz.The output of first astable mutivibrator is connected to the control voltage input (pin5) of IC2. This makes the output of IC2 modulated by the output frequency of IC1, giving a siren effect. In simple words, the output frequency of IC2 is controlled by the output of IC1.
The circuit can be assembled on a Perf board.
I used 12V DC for powering the circuit.
Instead of using two NE55 timer ICs, you can also use a single NE556 timer.
NE556 is nothing but two NE555 ICs in one package.
Refer the datasheets of NE555 and NE556 to have a clear idea.
Speaker can be a 64ohm, 500mW one.