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Acoustic emission (AE) signals induced from an alternating current (ac) NbTi superconducting coil were studied to observe the relationship between AE signals and the superconducting states of the winding of the ac coil, around the time of quenching initiation during operation at a commercial frequency of 50 Hz. The envelopes of the AE signals induced at quenching initiation were analyzed using information from operating currents and voltages in the coil. The AE signals rose and increased in accordance with the appearance and enhancement of the normal components for most cases in a series of quenching tests, where the quenching was initiated at the coil ends. In the case that the quenching was initiated by a different cause, the AE waveform had a different pattern, in which the AE signals started like a impulsive signal having slow decay, and normal zone propagation was not initiated at the moment when the AE signal started. Those differences are compared and discussed.