For the sound to reach the inner ear the vibrations in the air have to convert into vibrations in the cochlear fluids. There is a resistance in this stage that is called impedance, it is the resistance between air and fluid. This resistance results in a mismatch or reduces the transmission of sound. The tympanic membrane along with the ossicles try to overcome this mismatch or reduction between air and fluids and therefore the middle ear serves as a transformer. This is the function of an ossicles chain.