<p>This paper presents a review of anomalous sound event detection(SED) approaches.  SED is becoming more applicable for real-world appliactaions such as security, fire determination or olther emergency alarms. Despite many research outcome previously, further research is required to reduce false positives and improve accurracy.  SED approaches are comprehensively organized by methods covering system pipeline components of acoustic descriptors, classification engine, and decision finalization method.  The review compares multiple approaches that is applied on a specific dataset.   Security relies on anomalous events in order to prevent it one must find these anomalous events.  Audio surveillance has become more efficient as that artificial intelligence has stepped up the game.  Autonomous SED could be used for early detection and prevention.  It is found that the state of the art method viable used in SED using features of log-mel energies in convolutional recurrent neural network(CRNN) with long short term memory(LSTM) with a verification step of thresholding has obtained 93.1% F1 score and 0.1307 ER. It is found that feature extraction of log mel energies are highly reliable method showing promising results on multiple experiments.</p>