The human cochlea and cochlear mechanics under air and bone conduction. a The human cochlea with air and bone conduction pathways. The cochlea is a spiral, fluid-filled organ surrounded by bony structures. It connects the stapes of the middle ear at the oval window (OW). Under air conduction, the sound energy enters the cochlea through the OW, while under bone conduction, the sound energy enters the cochlea mostly through the bony walls. b Vibration patterns of the cochlear walls under bone conduction. At low frequency, the rigid body motion (RBM) dominates; as the stimulating frequency increases, the compressional motion (CPM) becomes significant. The RBM does not change the cochlear volume, while the CPM alters the cochlear inner space. c A sketch of the cochlea and the traveling wave (TW). In the mechanical perspective, the cochlea is simplified as two fluid chambers (scala vestibula, SV and scala tympani, ST) separated by the basilar membrane (BM). SV and ST connect at the apical end of the cochlea, named helicotrema. At the base of the cochlea, the oval window (OW) and stapes abuts SV, and the round window (RW) abuts ST. Under both air and bone conduction stimulations, the traveling wave (TW) emerges at the BM and travels from the basal end toward the apical end. The TW amplitude peaks under pure tone stimulations, and the peak location depends on the sound frequency