National Football League teams are racing this season to secure the title of loudest outdoor stadium in the world.

The Seattle Seahawks, who boast that their fans caused a small earthquake after a 2011 touchdown, acclaimed their crowd’s record 136.6-decibel noise level this September after an effort orchestrated by the fan group Volume 12.

Four weeks later, the Kansas City Chiefs — who are still unbeaten — topped the record, in part because of a scream-a-thon organized by the fan group Terrorhead Returns.

“Be LOUD AND PROUD and blow my eardrums out!” one Chiefs fan wrote on Facebook.

The N.F.L. encourages the din.