Abstract: An acoustic method and apparatus detects or characterizes a medium in a structure which may be a container, such as a pipeline for transportation of oil, gas, or hydrocarbon condensate. A pulse of broadband acoustic energy is emitted towards the structure by a first transducer. A return signal is generating by a second transducer from acoustic energy returned from the structure in response to the emission of acoustic energy. A return signal spectrum representing acoustic spectral components of the acoustic energy returned from the structure is derived from the return signal, and the medium is detected or characterized by applying a return signal processing medium detection or characterization algorithm to the return signal spectrum.