Acoustical inspection method for inspecting the ceramic coating of

Abstract

An acoustic inspection method is disclosed for determining in a catalytic converter monolith substrate whether a ceramic coating was applied in a predetermined amount and uniformly to the surface of exhaust gas passages extending therethrough and whether the ceramic coating is blocking any of the passages. The method includes mounting the substrate in an acoustically sealed structure so as to form a throat communicating a speaker at an entrance end of the coated passages with an empty resonator cavity at an exit end of the coated passages and thereby form a Helmholtz resonator. The speaker is then driven to produce a continuous sound wave through the coated passages into the resonator cavity at a predetermined frequency approximately the natural frequency of the resonator to thereby produce oscillatory sound waves through the coated passages at the same frequency. The substrate is then reversely mounted and the previous step is repeated. The phase angles of the sound waves at the entrance and exit ends of the coated substrate passages are then compared with respect to those of a reference sound wave of the same frequency passed in like manner through a reference substrate known to have the desired quantity and uniformity of coating on the passages and no blockage. Whether or not the passages of the substrate being inspected have the prescribed quantity and uniformity of coating and any blockage is then detected on the basis of the occurrence of a prescribed difference in the phase angles.