The Resonance Blaster is a sonic weapon of Decepticon design. It was recovered by Soundwave after he defeated Wheeljack in single combat. Knock Out used it to retrieve the second Omega Key from Bumblebee and Arcee.
It's current whereabouts are unknown, but since it seemed to be Soundwave's key weapon of choice it's reasonable to assume that he last had possession of it and if so, it would be trapped in the Shadowzone with Soundwave.
- This was the only relic from the four "Iacon" episodes to not appear in Season 3. It seems to have been misplaced or forgotten.