Citation X (C750)
Air Traffic Control Fully Certified
Communication Breakdown
Aircraft Equipment Problem Critical
General None Reported / Taken
Aircraft; Procedure; Human Factors
A C750 [was] en route at FL470. The C750 keyed up on frequency with a request during a period of frequency congestion. I answered 1 or 2 other aircraft and then responded to the C750. The C750 calmly requested a descent to FL430 and did not indicate any distress. After being cleared to descend to FL430; the aircraft was handed off and shipped to the next sector. The aircraft's beacon code was indicating an emergency a couple of minutes later and was unresponsive to transmitters from my frequency and descending rapidly. It was determined after the aircraft's safe landing along with rapid depressurization; the aircraft had deteriorated transmission capability. The Tower reported having difficulty hearing the aircraft while it was in the pattern. A timely report from pilots regarding equipment malfunctions.
Enroute Controller noted the rapid descent of an aircraft squawking emergency.  The reporter noted pilots should be more forth coming with information to ATC as to their status.