ACOUSTIC REFLECTIONS DURING RHINOMETRY - SPATIAL-RESOLUTION AND SOUNDLOSS

The accuracy of the acoustic reflections method for the evaluation of human nasal airway geometry is determined by the physical limitations of the technique and also by the in vivo deviations from the assumptio ns of the technique. The present study 1) examines the sound loss caus ed by nonrigidity of the nasal mucosa and viscous loss caused by compl ex geometry and its influence on the estimation of the acoustic area-d istance function; 2) examines the optimal relation between sampling fr equency and low-pass filtering, and 3) evaluates advantages of breathi ng He-Ga during the measurements on accuracy Measurements made in eigh t plastic models, with cavities exactly identical to the ''living'' na sal cavities, revealed only minor effects of nonrigidity of the nasal mucosa. This was confirmed by an electrical analog model, based on las er vibrometry admittance measurements of the nasal mucosa, which indic ated that the error in the acoustic measurements caused by wall motion is insignificant. The complex geometry of the nasal cavity per se (i. e., departure from circular) showed no significant effects on the meas urements. Low-pass filtering of the signal is necessary to cut off cro ss modes arising in the nasal cavity. Computer simulations and measure ments in models showed that the sampling frequency should be approxima tely four times the low-pass filtering frequency (i.e., twice the Nyqu ist frequency) to avoid influence on the result. No advantage was foun d for the the use of He-O-2 vs. air in the nasal cavity.