The purpose of the siren is to get the attention of others nearby when a crime or other emergency is happening.
This siren has a 5:6 frequency ratio, also known as a untempered minor third. Having two clashing tones simultaneously makes the siren more noticeable.
This has been improved from other designs, having fewer parts for ease of assembly, using an electric motor rather than a hand crank ed siren, and by having 2 tones instead of 1.
Here is a bill of materials for building it yourself:
-2 X 3mm dia X 10 mm length fine thread socket cap screws ~ $0.66 total
-2 X 3mm dia split lock washers ~ $0.50 total
-13.6 V 6290 RPM Johnson electric motor part number: 9167AJ ~ $3.95
-12 V 4 amp AC-DC power supply ~ $2.99
-2mm hex wrench ~ $0.30
-Stator ~ $5
-Rotor ~ $6
Total Cost ~ $19