The function of the middle-ear air spaces as tuned resonators

The middle-ear air-spaces (MEAS) not only act as an air reservoir and provide a buffering effect against negative intratympanic pressures caused by tubal dysfunction, but more fundamentally constitute a resonant cavity whose impedance is appropriately tuned to the frequency response of the auditory system of each individual species. When this ideal air volume is altered, either by pathology or by surgery, the response characteristics of the middle ear, and consequently, of the entire auditory system are modified. This concept was tested by measuring the compliance of artificial membranes coupled to closed rigid air spaces of varied volumes, and also by obtaining compliance-pressure functions (tympanograms) of guinea pig ears when the volume of their MEAS was surgically modified. A direct relationship between changes in the middle ear air volume and measurements of tympanic compliance was demonstrated in both models and real ears. This effect has implications on the sensitivity and frequency response characteristics of ears in which this volume has been significantly altered by either pathological processes or by surgical procedures.