While the League does not wish to place restrictions on spontaneous crowd noise or to diminish fan enjoyment in our sport, it is each club's responsibility to exert proper control over cheerleaders and mascots (including noise-making specialists hired exclusively for that purpose), use of scoreboards, message boards, etc. Artificial or manufactured crowd noise in NFL stadiums has increased to the extent that teams have notified the league office that they have experienced difficulty communicating within their bench area as well as on the field.
(1) Club-Controlled Sound: The home club does not have the prerogative to decide if such sound hampers signal calling. While spontaneous crowd noises may be beyond immediate control, noise of any kind (music, horns, gongs, drums, etc.) that is under club control must cease when the play clock (40 or 25 second) is running and the visiting team is in possession of the ball. Flagrant attempts by cheerleaders, mascots or the public-address system to encourage crowd noise for the purpose of disrupting the visiting team's offense while the play clock is running is prohibited. The use of noise meters or such messages as "Noise!," "Let's hear it!," "Raise the Roof," "Let's go Crazy," "Pump it up," "12th Man" are prohibited at any time during the game. These examples are not limited to the foregoing, but also would include similar messages that encourage crowds to make random noise in order to disrupt the opposition. The prohibitions specified in this section also apply during kicking plays.
Exception: Any conventional cheerleader or mascot actions or the use of the scoreboard or message board for acceptable cheers such as "Defense!" and "Push 'em back!" must be stopped when the huddle breaks and/or the offensive team moves to the line of scrimmage.
(2) "Wave": Club-controlled efforts to start the "Wave" cheer, through the use of cheerleaders or message boards — even if the actions are stopped when the visiting team breaks the huddle — are a violation of the crowd noise policy.
(3) Noise-Making Devices: Klaxons, megaphones, bullhorns, whistles and other noisemakers of any kind are not permitted in stadium.
(4) Field-Level Speakers: The number of field-level speakers must be limited to a maximum of four. They must be placed between the goal lines and the 20-yard lines, and be pointed away from the bench area and the playing field. All sound from such speakers must cease when play clock starts for the visiting team's possession.
(5) Mascots: Team mascots must stay behind the six-foot white border at all times during the game (they may be on the field at appropriate times during the pregame and at halftime when players are not on the field), and they are prohibited from engaging in any acts of taunting opposing players, coaches, and game officials. In the event of violations, teams employing the mascots will be subject to significant fines.
(6) Videos: Video clips may be shown during pregame, halftime, postgame, between quarters, and after a score by the home team, as long as the content is in good taste and not derogatory to the visiting team. At no time may such clips be shown while the play clock or game clock is running.
Clubs should be aware of the playing rule adopted in 1989 which establishes a set of procedures, including loss of timeouts or 5-yard penalty on the defense, to handle the problem of crowd noise which prevents the offense from hearing its signals.
Source: The NFL