Revelation 9:9

Their iron breastplates, which covered the chest and back in John’s day, gave them the appearance of invulnerability (cf. Revelation 9:17). [Note: Robertson, 6:364.] The sound of their activity will be terrifying, like the sound of battle (cf. Joel 2:4-5; 2 Kings 7:6; Jeremiah 47:3). Indeed their sound signifies battle with earth-dwellers.