Super King Air 350
Air Traffic Control Fully Certified
Communication Breakdown
ATC Issue All Types; Conflict NMAC; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
General None Reported / Taken
Human Factors
A Phenom was descending on the arrival. The aircraft entered the sector at FL190; when the aircraft entered my airspace; I gave the pilot a decent to 140. The pilot hesitated in his response and read back descending to one zero. The transmission was weak and I missed the bad altitude. The Phenom had southbound traffic a B350 at 130 with a restriction to cross the intersection at 040. When I saw the Data Block of the Phenom drop below 140 I queried the pilot and asked to verify level at 140. I think I was stepped on; or the pilot didn't respond; and I asked again to verify level at 140. This time the pilot said no; [they] were descending to ten. I told the pilot of the Phenom to stop his descent but by this time separation had been lost. I'm not sure what I can say; the only thing I can recommend is to be ultra vigilant in making sure that the proper read back is gotten for every transmission.
Enroute Controller described a NMAC situation when he failed to note an incorrect descent clearance resulting in the conflict.