Eustachian Tube Dysfunction (ETD) primarily refers to an absent or inadequate ability to open the eustachian tube. The term Serous Otitis Media (SOM) generally referring to an accumulation of fluid within the middle ear space, in absence of signs indicating acute infection. Commonly, this can result in a conductive hearing loss due to restriction of tympanic membrane mobility. Negative Middle Ear Pressure (NMEP) is often a precursor to the development of SOM, and has it's own effect on the acoustic properties of the middle ear, also resulting in conductive hearing loss.