OAEs are routinely used to test for normal hearing in newborn babies.
Hearing relies on a chain of events all working together.
Sound reaches the eardrum and transmits vibration in the inner ear. The inner ear spreads out the different rates of vibration which compose the sound - over the string of cells which have the job of translating the vibrations into nerve impulses. Thousands of nerves carry the 'digested' sound signal to the lower levels of the brain where its features are recognised as sound elements. If the sound elements are correctly recognised then we eventually understand the meaning behind the sounds. A physical problem arises when the inner ear tries to perform its sound analysis.