Siren test

We are within a kilometre of a large gasholder, full of liquified natural gas, and if that were to explode, it would create rather a large bang. If there is a danger of the gasholder going pop, a siren will sound. Every 6 months, the siren is tested. In order to hear it we have to open a window or door - precisely the opposite to what you're supposed to do in an emergency.