a, c Spectra of the stapes velocity when driven by IMEHD-A with tonal stimuli of 990 or 2002 Hz (0.1 V), respectively. b, d Spectra of the stapes velocity before implantation when acoustically driven by our earphone with stimuli of 990 or 2002 Hz (1 V), respectively. The stimulus levels elicit similar levels of stapes response from the IMEHD and the earphone. The response to the stimulus frequency is labeled fo. Harmonic responses are at 2, 3 or 4 fo.