Ultrasonic peening uses a 20-30 kHz piezoelectric transducer that amplifies its motion via a sonotrode in contact with a bundle of carbide or ceramic balls. The balls oscillate at high frequency and bombard the target surface with controlled energy, with no projection and no conventional media consumption.
The ultrasonic peening advantage is twofold: no projection (safety, cleanliness, no media recovery) and excellent energy control on fine geometries or sensitive materials. The residual stresses generated are comparable to conventional shot peening, often with lower residual roughness.
Reference: AMS 2546 (aerospace ultrasonic peening) and specific OEM specifications.