The league's policy on crowd noise states the NFL does not try to restrict spontaneous cheering by fans, but it does require the team to "exert proper control" of cheerleaders and mascots and restricts the use of scoreboards and message boards to encourage crowd noise.
It also addresses noise that is under "club control," which includes music and other sound effects inside the stadium. That noise must stop once the play clock begins when the opposing team has the ball.
The league prohibits megaphones, whistles and other artificial noisemakers, and scoreboards or cheerleaders can not be used to start the wave.
Teams are allowed to have four speakers on the field, but they must be located between the goal line and the 20-yard line, and must be pointed away from the team benches on the sidelines. Any noise from those speakers must stop once the play clock starts during a visiting team's possession.
The crowd-noise guidelines state the league does not try to restrict spontaneous crowd noise, but the guidelines do mention the rules passed in 1989 to deal with crowd noise that is so loud the opposing team can not hear its signals. A team can lose timeouts or be assessed 5-yard penalties. Those rules resulted from the noise inside the Kingdome — the Seahawks' former stadium — but are rarely enforced now.