> Not a noise standard, which would make sense. A speed limit.
Is enforcing a perceived noise, or even a measured decibel limit on the ground even realistic? It would be difficult to investigate reported infractions unless they're consistently coming from a scheduled route, because the noise level on the ground depends on more than just the plane. "Supersonic jets that don't have a lot of headroom on the noise limits in a dry, flat desert shouldn't fly supersonic through this area because the the land underneath is shaped like a parabola and it's very humid." is much more difficult for a pilot to manage and a regulation body to enforce than "Don't fly faster than this airspeed." especially when the effect was the same back when the bill was introduced.
Is enforcing a perceived noise, or even a measured decibel limit on the ground even realistic? It would be difficult to investigate reported infractions unless they're consistently coming from a scheduled route, because the noise level on the ground depends on more than just the plane. "Supersonic jets that don't have a lot of headroom on the noise limits in a dry, flat desert shouldn't fly supersonic through this area because the the land underneath is shaped like a parabola and it's very humid." is much more difficult for a pilot to manage and a regulation body to enforce than "Don't fly faster than this airspeed." especially when the effect was the same back when the bill was introduced.