An audiologist is a specialist, assessing the functions of the middle and inner ear.
The middle ear is the air filled cavity between the ear drum and the fluid filled inner ear. Air gets into this cavity via the eustachian tube which is a tube that runs from the ear to the back of the throat. It is this structure that allows you to "pop" your ears when going up or down in an aeroplane or up and down a hill. It basically equalises the pressure between the trapped air in the middle ear and the atmosphere.
The sound is transmitted from the ear drum through to the inner ear across this middle ear by three bones called the ossicles.
The inner ear has two sections. The cochlear is a spiral shaped structure like a sea shell, that allows us to hear. Vibrations (sound) get transmitted into the cochlear and nerve cells around the spiral get stimulated depending on the pitch of the sound. Different nerve cells get stimulated by different sounds - and the information is transmitted along the eighth cranial nerve to the brain.
Elsewhere in the inner ear the organs of balance - the semicircular canals, the saccule and utricle. The saccule and utricle are set at right angles to each other and are merely tell us whether we are standing upright, lying down, lying on our side, upside down etc the semicircular canals are three canals all at 90° to each other. These can tell us whether we are rotating in any particular direction. It is thanks to these organs of balance that we can tell what position body is in and how we are moving, even with our eyes shut.
When all of these structures are working well, we don't even notice the tremendous job that they do. However if any start misreporting to the brain, it has tremendous effect on us. Either we cannot hear, or we feel that we are moving when we are not and our eyes tell us we aren't. When this happens, it makes us feel very sick and is often called vertigo.
An audiologist specialises in examining and testing these different functions of the middle and inner ear. When patients have a problem with hearing or balance, an audiologist by using specialist tests, can isolate where the problem is so that treatment strategies can be developed.