From Halopedia, the Halo wiki
During the Battle for Zeta Halo, the Banished set up at least forty propaganda towers on the fragmented section of the Ring, which were operated by Glibnub, an Unggoy communications officer. Glibnub constantly broadcasted taunting and demoralizing messages to the surviving UNSC personnel. These messages typically contained unverifiable statements, half-truths, or outright lies (such as the Master Chief merely killed "four Grunts in an Escharum suit"), and occasionally the Unggoy officer would simply air some personal grievance instead, such as him being upset over the towers being destroyed.
John-117 has the option of destroying these towers, weakening the Banished's influence in the area and earning valor. Destroying a tower earns the player 10 valor points. There are a total of 40 towers on the map. Destroying the first tower rewards the Please Shut Up achievement, while destroying all 40 of them rewards the player with the Off the Air achievement.
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