|“||Think of them as, uh - as a, uh, life preserver. Only instead of water, the thing HRAs protect your from is, um, classified. One day, that classifed not-water might pour in, and... you'll be glad you've got an HRA keeping you afloat. And if you don't have an HRA... don't worry. It'll be, uh, quick and painless.||”|
Hedron Resonance Amplifiers (HRAs) are devices developed by Casper Darling which amplify the resonance originating from Hedron (Polaris). They serve as protection for Bureau agents from hostile resonance, specifically the Hiss.
Most HRA devices are worn as harnesses by FBC agents. They function by amplifying resonance originating from Hedron, an entity contained in the Research Sector of the Oldest House. A Black Rock Prism is required to construct HRAs, as the resonance-blocking properties of Black Rock are also valuable in the protective properties of HRAs.
Bureau agents wearing HRAs are immune to corruption by the Hiss, as they are under the protection of the resonance originating from Hedron. The only individuals in the Oldest House who appear to be capable of surviving the Hiss without HRAs are Jesse Faden and Ahti.
After the discovery of Hedron in Slidescape-36 and its relocation to the Research Sector, Casper Darling began to study its resonance, determining it to be intelligent and benign in nature. Through his exposure to it, he learned of what he believed to be the approaching threat of a hostile, viral resonance which would spread through the Bureau and corrupt its agents. Seeking to prevent this, Darling invented the Hedron Resonance Amplifiers and began distributing them to Bureau agents. He also built a large-scale HRA to protect Hedron's chamber, locking himself inside; his fate after this is unknown.
Zachariah Trench, who had been corrupted by the Hiss in Slidescape-36, saw Darling's distribution of HRAs as proof that Hedron was taking over the Bureau, and subsequently unleashed the Hiss through the Slide Projector. The Hiss corrupted every Bureau agent not wearing an HRA, and also caused HRA production to halt. After Jesse Faden arrived at the Bureau, she and Helen Marshall resumed HRA production in the Research Sector. Hedron was later destroyed by the Hiss when Jesse Faden destroyed Darling's large-scale HRA on Polaris's orders. Polaris then began to use Jesse as the source of her resonance, which kept the Bureau safe by making Jesse the new conduit of the Hedron resonance for the HRAs.