plant for blow moulding hollow plastic bodies , comprising : a plurality of preform blow - moulding dies ; at least an acoustic / electric transducer apparatus adapted to detect the sound issued outwardly by the individual blow - moulding dies during the blow - moulding process , and to convert it into an electric signal , means adapted to process said electric signal and convert it into a signal capable of being compared with a reference signal , means adapted to perform such a comparison of said processed electric signal with said reference signal or level , means adapted to selectively reject said hollow bodies off the production line in accordance with the outcome of said comparison . said acoustic / electric transducer apparatuses are at least partially housed inside the platen or frame that is common to several dies , and are arranged in the vicinity of respective blow - moulding cavities .