An at least partially implantable hearing aid has an input transducer (26) for capturing audio signals from ambient sound, an audio signal processing unit (32) for processing the captured audio signals, an actuator (20) for stimulating a patient's hearing, and a driver unit (44) for driving the actuator. The actuator has a rigid housing (64) closed on at least one side by a vibration diaphragm (66) that is driven by a piezoelectric transducer (68, 88). The housing is designed to float in the cochlea in direct contact with the cochlear liquids in order to couple vibration energy from the diaphragm directly into the cochlear liquids.
