Actually the communication between the play caller and quarterback is cut off with 15 seconds on the play clock.
Teams aren't allowed to use artificial crowd noise at all to prevent opposing offenses from communicating with each other.
Au contraire mi Amigo.
But I was wrong on the shutting it down at 15 seconds, they are suppose to turn it off with 20 seconds left on the play clock.
https://fansided.com/2018/12/16/bears-air-raid-siren-noise-rules/
I thought this was illegal as well, since I remember the Falcons and other teams being investigated over pumping artificial crowd noise in and that being sighted as not allowed. The article above doesn't really say why the siren is legal, just that the NFL deemed it legal.
The only thing I can think of as to why this is allowed, is that "artificial Crowd noise", is noise simulating people, an air raid siren, while much louder, isn't simulating "noise made by a crowd".
Really would like to see this practice stopped. Being at any major sporting event these days seems like you are being pummeled constantly by loud music, the air raid sirens have to be even more obnoxious IMO.
Final note. Possibly the Packers noticed this new "advantage" that the Bears were trying to get and in their first Preseason game at Lambeau against the Texans, initiated a "3rd down siren".
https://www.jsonline.com/story/spor...er-shudder-new-horn-lambeau-field/1964506001/