In Sirens
In a siren, the stator is the crucial part which actually cuts off and reopens air flow pushed by the blades of the chopper as the blades move past holes in the stator known as ports, generating the siren's tone(s). One can determine the port ratio of a siren by looking at the number of ports in each row on a stator. A siren with only one row of ports is called a single tone siren. A siren with two rows of ports is known as a dual tone siren.
Read more about this topic: Stator