Determining the onset of acoustic emissions (AE) signal is very time consuming if the onset is picked manually, so it is important to read the arrival times of P phase automatically. Automatic onset detection and picking algorithm has been proposed by applying the spectro-ratio on time–frequency sub-band. Powers of frequency sub-bands are determined by spectrogram as a time–frequency representation. Adaptive thresholds are calculated for one of these sub-bands to check if there is a P-wave arrival in the segment or not. To verify this check another test is done using the spectro-ratio. The application of this algorithm on the AE signals from the pencil-breaking test shows a reasonable result.