The transmission of infrasound within the human ear is not well understood. To investigate infrasound propagation through the middle and inner ear, velocities of the stapes and round window membrane were measured to very low frequencies (down to 0.9 Hz from 2000 Hz) in fresh cadaveric human specimens. Results from ear-canal sound stimulation responses show that below 200 Hz, the middle ear impedance is dominated by its stiffness term, limiting sound transmission to the inner ear. During air-conduction, normal ears have approximately equal volume velocities at the oval (stapes) and round windows, known as a two-window system. However, perturbing the impedance of the inner ear with a superior canal dehiscence (SCD), a pathological opening of the bone surrounding the semicircular canal, breaks down this simple two-window system. SCD changes the volume velocity flow in the inner ear, particularly at low frequencies. The experimental findings and model predictions in this study demonstrate that low-frequency auditory and vestibular sound transmission can be affected by a change in the inner-ear impedance due to a SCD.