Crafting the Feedback Resonator upgrade in Subnautica 2 improves your existing Sonic Resonator gadget so that it fires a projectile. Why would you need this? Well, as you may have realised, the Angel Comb in the Alien Ruins has small viral flowers that like to close when you get close.
The only way to destroy them and cleanse the area is by firing from a distance, something your regular Resonator cannot do. To get this upgrade, you're going to need the Tadpole Depth module, but otherwise you can grab it whenever.
Subnautica 2 Feedback Resonator location
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