Citationjet (C525/C526) - CJ I / II / III / IV
Communication Breakdown; Human-Machine Interface
ATC Issue All Types; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance
Human Factors
There were three inbounds to ZZZ1 led by aircraft X; then aircraft Y; followed by aircraft Z. Aircraft X was 8 miles in front of aircraft Y. Both aircraft were at 4000. I noticed that aircraft X had slowed to approx. 60kts less than aircraft Y. I didn't want too much more compression because they both had 35 flying miles or more to go. I instructed aircraft Y to increase his airspeed to 250kts AND descend to 2600 feet. This would give me a vertical buffer if the speed restriction wasn't complied with in a timely manner. About this same time aircraft Z was descending into my airspace. I had already diverted some of my attention away from the two preceding aircraft because aircraft Z had already busted a previous altitude assignment in the delivering sector and was being dealt with by my colleague.  After initial contact by aircraft Z; I focused my full attention on the preceding two arrivals. It was at this point that I observed aircraft X 4 miles northeast of a 1549' antennae descending through 2700'. The MVA is 2500'. I then saw aircraft Y pass through 2600; 2500; 2400 when I called the acft and asked him to verify he was at 2600 as assigned. He said he was descending to 2000 as I told him to. By then aircraft Y was almost over the obstruction at 2100 feet. I advised him to climb back to 2600 and turn to a 190' heading. Based on the way the pilot responded to my altitude query and knowing that I may have not heard the read back properly because I had already started to focus on a KNOWN altitude violator; I immediately suspected that a read back/hear back error may have been in play here. After listening to the tape; I clearly heard the pilot read back 2000. Because I was responding to the previous Controller via non land line communications reference aircraft Z; I assumed the pilot responded 2600 not 2000. I  also noted that when I gave the 2600' altitude assignment I had paused between the two numbers. By this I mean the  audio reads; 'aircraft Y descend and maintain two thousand aah six hundred'. I am fairly certain that by me fragmenting my speech between the two numbers the pilot stopped listening after he  heard 'two thousand aah'. I am now aware of what those mental hesitations on the frequency can result in.  I will be more cognizant of this and try to eliminate them entirely. I also will stop trying to be two to three steps ahead of the situation when just a step ahead will suffice. Finally; I will focus on ALL read backs before I engage in any off line banter with my peers.
Approach Controller reported a readback hearback issue which resulted in controlled flight towards terrain.