Acoustic signals Acoustic detection 2 Acoustics 2 DC bias 2 DC transformers 2 Data statistics 2 Distribution regularities 2 Early-warning method 2 Multiparameters 2 Normal state 2 Operation state 2 Power transformers 2 Transformer 2 Transformer substations 2 acoustic detection 2 data statistics 2 transformer 2 Acoustic Stimulation 1 Acoustic analysis 1 Acoustic emission characteristics 1 Acoustic emission testing 1 Acoustic emissions 1 Acoustic fields 1 Acoustic noise 1 Acoustic propagation 1 Acoustic waves 1 Adult 1 Ambient noise model 1 Amplitude response 1 Amplitude response model 1