System and method for the monitoring of a space surrounded by walls

System and method for monitoring an enclosed space defined by walls, in particular the interior of a motor vehicle, with a sound source for generating sound waves in the space, a sound receiver for receiving the sound waves in the space, and a measurement and evaluation unit for determining the presence of an alarm condition, in particular from the time delay between transmitted and received sound signal, wherein to improve the monitoring, in particular to avoid false alarms, at least one vibration measurement device is additionally provided, by means of which vibrations of the walls enclosing the space can be detected and which is connected to an input of the measurement and evaluation unit for determining the presence of an alarm condition.