[Role of the maxillary sinus as a resonant cavity]

Patients with a paranasal disorder often manifest voice change. Yet, computer simulation of these nasal sounds is difficult using a nasal tract model without any branching cavity. In other words, acoustic property of the nasal tract is influenced by a coupling with the paranasal sinuses. If the transfer function of the sinus acts as a Helmholtz resonator, the resonance frequency, or "zero" point, of the sinus would be present on the acoustic spectrum of the nasal cavity. This study was designed to prove the validity of this hypothesis. The sweep tone was given from the subjects' epipharyngeal space. The tone passed through their nasal space and radiated from the anterior nostrils. In 13 cases without nasal or paranasal disorders, the tones obtained at the nostrils were analyzed with Fast Fourier Transformation (FFT) and were compared between two conditions of the ostia of the maxillary sinuses, obstructed and opened with epinephrine. The resonance frequencies of the maxillary sinuses ranged from 1 to 2 kHz and varied considerably among individuals. This variation may be due to a difference in the maxillary sinus volume and in the diameter and length of the natural maxillary ostium. In past reports, in which the resonance frequency of the sinus was measured using a compound model or computed simulation, the maxillary sinus resonated below 1 kHz. In these reports the ostium of the maxillary sinus was regarded as a straight pipe. However, the examination of 29 cadavers revealed that the radius of the ostium differs according to its depth. The radius in the depth halfway from the edge was narrower than that of the edge. The way of evaluating a shape of the ostium is different between the present and the past studies, thus possibly resulted in discrepancy of the resonance frequency.