Um, noise mitigation is still an issue at existing gas plants (e.g., [0-3]) and often only partially dealt with by local regulations, and this group are claiming their engine is substantially different, supersonic vs subsonic, and crossing that threshold usually involves large noise generation effects.

[0] https://aercoustics.com/blog/noise-vibration-gas-turbine-cog...

[1] https://www.powermag.com/major-noise-sources-and-mitigation-...

[2] https://www.turbomachinerymag.com/view/noise-levels-at-energ...

[3] https://turbinelogic.com/enc/cog-gt-units/regulatory-safety-...