The eardrum is a thin membrane at the end of your ear canal. It is also known as the tympanic membrane.
The eardrum flexes when sound waves coming down the ear canal collide with it. The flexing, which represents sound, is sensed, transported and magnified by the three tiny bones that stretch across the top end of the eustation tube to the round window of the cochlea.
Some problems with hearing are caused by things related to the eardrum. The eardrum may have a hole in it, or it may not have the right amount of flexibility to properly sense the sound waves. A blockage or fluid behind the eardrum in the eustation tube can also keep the eardrum for being as good a sound sensor as it can be.
Audiologists can measure how flexible and responsive the eardrum is to sound, and they can determine whether there might be holes or scars in the membrane or whether a blockage or fluid problem might be involved.