B737 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Communication Breakdown; Situational Awareness
ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented
Human Factors
Handed off to final approach controller.  We're assigned 3400 feet but both of us heard 3000 feet.  We descended to 3000 feet on downwind for Runway 30. Upon leveling 3000 feet controller called us and said to climb to 3400 feet terrain alert.  We immediately climbed to 3400 feet.  Weather was VMC and the EGPWS showed we were above all terrain as well as we had everything in sight. The controllers radio was very weak and hard to hear.  We both thought we heard 3000 feet and read that back to controller.  He did not respond with a corrected altitude.  After the event we reported to him his radio was very weak - we had to turn our volume up full.  He changed transmitters and everything returned to normal levels.  Also reported that we read back 3000 feet - he missed it.I should have reported the radio on initial contact - I assumed it was going to improve as we got closer. There were no separation issues and terrain and airport was in sight the whole time.The controller did a great job of trapping the altitude and overall I thought the situation was handled very professionally by all concerned.
[Report narrative contained no additional information.]
B737 flight crew report receiving a low altitude alert from ATC on approach to OAK after misunderstanding altitude clearance.