Sounds entering the auditory canal impact the tympanic membrane (TM), or ear drum as it is commonly known, causing it to vibrate. The TM is very small, only 9 mm in diameter. It is constructed in three layers. The outer or lateral layer is skin continuous with that of the auditory canal. The inner or medial layer of the TM is mucous membrane that is continuous with the tympanic cavity of the middle ear. In between the outer and inner layers of the TM is a layer of fibrous tissue (lamina propria) that gives the TM its stiffness and tension. The tension of TM is adjusted by the tensor tympani muscle, the larger of the two muscles located in the middle ear (the other being the stapedius muscle). This very small muscle is attached to the manubrium (handle) of the malleus (hammer) bone.
The vibration of the TM is transformed into mechanical vibrations by the ossicular chain creating a leverage effect that increases the efficiency of the transmission of sound energy and increases the sound pressure at the oval window located on the cochlea by approximately 30 decibels. This increase in sound pressure is due to the transfer of pressure from the larger area of the TM to the much smaller area of the oval window located. The increase is needed to overcome the natural resistance (impedance) to the transmission of the sound vibrations by the fluids contained in the inner ear.
The tympanic membrane is susceptible to damage caused by perforation, infection, etc. When this occurs, the transmission of sound is not necessarily ended, but is markedly diminished. Care should be exercised when putting objects such as Q-tips, long fingernails, paperclips, etc. into the auditory canal.