A sum of simple and complex motions on the eardrum and manubrium in gerbil

Based on comparisons of ear canal and scala vestibuli pressures the gerbil middle ear transmits sound with a gain of ∼25 dB that is almost flat from 2 to 40 kHz, and with a delay-like phase corresponding to a 25–30 μs delay. How the middle ear is able to transmit sound with such high temporal and amplitude fidelity is not known, and is particularly mysterious given the complex motion the ossicles and tympanic membrane (TM) are known to undergo. To explore this question, we looked at the velocities of the manubrium and along a line on the TM. The TM motion was complex, and could be approximated as the combination of a wave-like motion and an in-and-out piston-like motion. The manubrium underwent bending at some stimulus frequencies and therefore its motion was not like a rigid body. It had a complex motion with frequency fine structure that seemed likely to be derived from resonances on the drum-like TM.