A downhole acoustic stimulation tool comprises: a sealed chamber containing a liquid; a pair of electrodes located in the chamber; at least one transducer arranged to generate an acoustic field between the electrodes thereby inducing cavitation in a volume of the liquid between the electrodes; and at least one capacitor configured to apply a pulse voltage across the electrodes when discharged, thereby causing the cavitating volume of liquid to form a plasma which collapses to form a shockwave. The at least one transducer constitutes a first energy source, and the at least one capacitor back and electrodes constitute a second energy source. Alternative forms and arrangements of the first and second energy sources are also disclosed.