The invention relates to an acoustic sensor system (1) for detecting a defect (2) of a pipeline wall (3), having: at least one transmitter unit (4) which is configured to emit ultrasound in the direction of a pipeline wall (3) and detect an ultrasound echo reflected by the pipeline wall (3); and a control unit (5) which is connected to the at least one transmitter unit (4) for signaling purposes and which is configured to detect a defect (2) of the pipeline wall (3) using a present change in the ultrasound echo. The invention additionally relates to an in-line inspection device comprising the sensor system (1), to a method for detecting a defect (2) in a pipeline wall (3), to a computer program, to a data carrier signal, and to a data storage unit.