Phase transitions in PbZn1/3Nb2/3O3-7%PbTiO3 (PZN-7%PT) relaxor ferroelectric crystals have been investigated by the acoustic emission (AE) method. Two types of transitions have been detected (i) associated with the thermally induced (on cooling) formation of intrinsic polar nanoregions (PNRs) and (ii) related to the electric-fieldinduced macroscopic phase transitions. An AE signal found near 740 K corresponds to the Bums temperature, below which the PNRs begin to form. Further cooling reveals an AE signal at 500 K, which is ascribed to the shift of PNRs along their polar direction relative to the surrounding non-polar matrix in the framework of the "phase-shifted condensed soft mode" model. Monitoring of the AE response of PZN- 7%PT crystals under an increasing [001] dc electricfield has allowed a unique in situ observation of a sequence of phase transitions, from the zero-field rhombohedral via MA monoclinic (similar to 3 kV/cm) and Mc monoclinic (similar to 16 kV/cm) to the tetragonal (similar to 19 kV/cm) phase, respectively. It is shown that AE is a very sensitive method for detecting both the PNRs kinetics and low distortion phase transitions in relaxor ferroelectrics.