Fluid volume displacement at the stapes footplate and round window due to air and bone conduction stimulation

Abstract

The fluid volume displacement of the two major in- and outlets of the cochlea, the stapes footplate and the round window, are normally considered equal but with opposite phase. However, other channels, such as the cochlear and vestibular aqueducts, may affect the fluid flow in the cochlea. To test this hypothesis, the volume displacement at the stapes footplate and the round window was measured with a laser Doppler vibrometer with air conduction stimulation in seven fresh human temporal bones and with bone conduction stimulation in eight temporal bones. The volume displacement was computed by measuring at five positions on the footplate and by scanning the motion of the round-window membrane at approximately thirty positions. With air conduction stimulation, the average volume displacement of the two windows was within 5 dB of each other and the phase difference was close to 180 deg within the frequency range 0.1–10 kHz. With bone conduction stimulation and below 1.5 kHz the volume displacement of the round window was about 10 dB greater than at the footplate with a phase difference of 150–200 deg. Above 1.5 kHz this difference rolled off with 6 dB/octave and 100 deg/octave.