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We describe a new implantable hearing-aid output device, a piezoelectric bellows round-window driver (PBRD), which is expected to be unaffected by external magnetic fields. The core elements of the PBRD are a piezoelectric element and a gold-coated copper bellows of low stiffness that transmits piezoelectric displacements, without significant attenuation, to the round window (RW). We analyzed structural and mechanical vibrations when confirming bellows transmission efficiency using a finite element model simulation. The PBRD was bench-tested under no-load conditions to determine its frequency response characteristics. We compared the outputs of the PBRD and a commercial floating-mass transducer in situ in human cadaveric temporal bones with responses measured using a laser Doppler vibrometer. PBRD outputs were excellent at both low (0.1-0.7 kHz) and high (2-16 kHz) frequencies; thus, the PBRD has the potential to compensate for conductive and/or sensorineural hearing loss. The frequency-response performance of the PBRD is better than that of conventional RW drivers (actuators or transducers).