An ERJ175 Captain; monitoring the radios and listening to the tug drive; misunderstood the 'Cleared to Start' command and set the parking brake thinking the tow was complete. A tow bar bolt was sheared but the aircraft was not damaged.

Narrative

While operating the flight we had a communication issue with the ground crew. The tug driver pushed the aircraft away from the gate and turned us into the alley way. After the aircraft came to a stop; I believe the ramp crew said to 'set the brake' which I started to comply with. As I had just started to pull the parking brake the tug driver pulled the aircraft forward (the brake was never fully applied). This caused a momentary jerk on the tow bar. The tug driver immediately stopped and we assessed the situation. The tug driver said she was clearing us to start the engine. It is possible I misheard the statement as I was also monitoring the Ramp Control frequency at the same time. Although due to the length in which the aircraft was stopped I believed the push back was complete. From what I was able to gather from talking with the ramp crew this caused a 'bolt' on the tow bar to break. We notified Dispatch and Maintenance Control in order to have a maintenance technician come examine the aircraft. A Company maintenance technician arrived at the aircraft and did not note any defects in the nose gear. After determining that no other issues existed the flight was continued without incident. The noise from the ramp environment transmitting over the tug driver's headsets combined with the radio frequency congestion contributed to the misunderstanding. I generally have the ramp interphone volume turned up higher than the radio frequency volume; however; I need to ensure I do not adjust the radio volume during the push back so that I am able to better hear the ramp crew. I do not recall exactly; but I may have turned the radio volume up in order to hear instructions from the Ramp Controller. In addition; rather than relying solely on headset communication I should also look for the wing walker to confirm the tug driver no longer intends to move the aircraft.