Commercial Fixed Wing
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 9
Communication Breakdown
ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Weather / Turbulence
Air Traffic Control Provided Assistance; Flight Crew Returned To Clearance
Human Factors
Working sectors XX/YY/ZZ combined with D-side.  Cleared Aircraft X to cross 30 miles west of ZZZ at FL310. Thought I had an accurate readback.  I have not checked the recording yet. There was some frequency congestion with weather deviations and multiple frequencies.  Aircraft X later descended below FL310; when I queried the pilot he thought he had been cleared to FL300; and thought that he had read that back.  With the addition of a D-side; and the new requirement to wear a mask we are not able to social distance 6+ feet; I was required to in this case.  I fully support the safety measures; and appreciate the effort.  I don't know if wearing the mask also caused confusion in delivering the clearances (muffled sound; etc.)?Staff a full time Supervisor (FLM) rather than a CIC.  Open sector(s) more frequently if needed.  With the new mask mandate; perhaps opening sector(s) as opposed to staffing D-sides may allow clear communication without a needed mask.  On the other hand; one bad sneeze can really carry throughout the area; masks would most likely help reduce virus exposure under any of these circumstances; and a common mandate does help keep all under the same safety standard.
Center Controller reported an aircraft descended below what the Controller thought was the assigned altitude.