From the Economist: Silent skies: Green Aircraft
...The researchers envisage a mid-range aircraft that would carry 215 passengers. Its top speed would be 0.8 Mach—that is, 0.8 times the speed of sound or around 600 miles per hour, slightly slower than the coming generation of airliners. Indeed, it is a slower speed that enables such an aircraft to be so quiet: it would drift in to land rather than powering in, all engines blazing, as today's jets do. Its low approach speed, combined with steep climbs and descents, would make it inaudible outside the airport.
The passenger aircraft is currently a conceptual design. There are many challenges that would have to be overcome before it could become a reality by 2030, as the researchers hope. Not only would passengers have to accept windowless flight, but several technical problems would also need to be overcome, including the need to manufacture pressurised cabins that are not the standard tubular shape.
But it is the concept aircraft's fuel efficiency that is really making aircraft manufacturers take note. The researchers claim that it would use 25% less fuel than current aircraft do. Airliners that are cheaper to run and contribute less to climate change may be more attractive than silent ones. That is why a slightly noisier alternative design by the same researchers that is even more fuel-efficient shows most promise.