<jats:sec> <jats:title>Objective:</jats:title> <jats:p>To assess the accuracy of tubo-tympano-aerodynamic-graphy (TTAG) in the estimation of morphological severity of patulous Eustachian tube (PET), and to compare the results with that of sonotubometry in clinical cases and in model experiments.</jats:p> </jats:sec> <jats:sec> <jats:title>Study Design:</jats:title> <jats:p>Retrospective chart review and model experiment.</jats:p> </jats:sec> <jats:sec> <jats:title>Setting:</jats:title> <jats:p>Tertiary referral center.</jats:p> </jats:sec> <jats:sec> <jats:title>Subjects:</jats:title> <jats:p>A retrospective survey of medical records in Sen-En Rifu Hospital identified 26 patients (12 men and 14 women subjects aged 14–78 yr, average 49.4 ± 20.9 yr), comprising of 35 ears with PET. A definite diagnosis of PET was made for all patients based on the diagnostic criteria proposed by the Japan Otological Society.</jats:p> </jats:sec> <jats:sec> <jats:title>Main Outcome Measures:</jats:title> <jats:p>Commercial equipment for sonotubometry and TTAG (JK-05A; RION Co., Ltd., Kokubunji, Tokyo, Japan) were used in the following investigations. Model Experiment: The ET model was made using two truncated syringes with silicone barrels and a narrow connecting tube (0.2, 0.4, 0.58, 0.8, 1.00, 2.00, and 3.00 mm). Clinical Investigation: Morphological evaluation of ET patency was performed by a three-dimensional (3-D) CT (3D Accuitomo; Morita MFG. Corp., Kyoto, Japan) and analyzed by software (Integrated Information System i-VIEW; Morita MFG. Corp.).</jats:p> </jats:sec> <jats:sec> <jats:title>Results:</jats:title> <jats:p>The transmitted sound pressure level gradually increased as the tube caliber was widened up to 3 mm diameter. On the other hand, pressure transmission was suddenly elevated and reached a plateau at 0.58 mm in diameter. The cross-sectional area at the narrowest portion of the ET was significantly correlated with the sound attenuation from nostril to external auditory canal (EAC) (Spearman rank correlation test: <jats:italic toggle="yes">r</jats:italic> = –0.521, <jats:italic toggle="yes">p</jats:italic> < 0.001). In contrast, the cross-sectional area at the narrowest portion of the ET was not correlated with the pressure changes of TTAG (Spearman rank correlation test: <jats:italic toggle="yes">r</jats:italic> = 0.317, <jats:italic toggle="yes">p</jats:italic> = 0.64).</jats:p> </jats:sec> <jats:sec> <jats:title>Conclusion:</jats:title> <jats:p>Sound transmission assessed by sonotubometry could be more useful than pressure transmission assessed by TTAG to predict the morphological severity of PET.</jats:p> </jats:sec>