Several years ago, I read Erik Conway's "High Speed Dreams", and learned about an important feedback loop that operates in the SST world. What happens is, you

Several years ago, I read Erik Conway's "High Speed Dreams", and learned about an important feedback loop that operates in the SST world. What happens is, you design an SST, figure out the noise profile, and are horrified. You pour a ton of research effort into making it quieter. The aero engine industry adopts your awesome quieting technology across the board, subsonic aircraft get much quieter, people's expectations adjust, and for all your effort, your SST is still the noisiest bird in the sky. Because you're this huge outlier, you get a lot of negative regulatory attention, which manifests in the form of flight-path and operating-hour restrictions. You can make SSTs quieter, but they'll never be the quietest. This isn't about sonic booms directly, but it's absolutely relevant, SSTs still have to take off and land, and you can't use engines with those giant fans on the front and hope to perform. The technical solution, if it exists, is to come up with an engine-quieting technology that somehow _only_ works for high-thrust low-bypass SST engines. That will break you out of the loop. AFAIK, no such technology exists.