INVESTIGATION OF ACOUSTIC CHARACTERISTICS OF LINER SAMPLES IN INTERFEROMETERS WITH DIFFERENT DUCT DIAMETER
The investigation of acoustic characteristic of sound-absorbing liner samples in normal incidence interferometers with different diameters of a duct is carried out. The object of the investigation is the test of the technique for reliable identification of the liner acoustic characteristics, taking into account the propagation of azimuthal mode in the duct, for further application of the technique to measuring the large-scale liners by portable interferometer. The basis of the verification is the independence of the impedance of locally reacting liner on the sample diameter when the normal waves incident. The fundamentals of the determination of the liner sample impedance based on 2- and 4-microphones measurements in the interferometer are briefly described. The design of a new interferometer with a duct diameter of 50 mm is presented. The frequency range of the created interferometer is investigated. Single-layer and double-layer liner samples with diameter of 30 and 50 mm were developed and produced by additive technology. The processing of the measurement results was carried out using the transfer function method based on 2 microphones, and the modal decomposition method based on 4 microphones. For all investigated liner samples, a good qualitative matching of the impedances between the corresponding samples with diameter of 30 and 50 mm is observed. The good quantitative matching is also observed for the most part of the frequency range. Some quantitative discrepancies are due to the difficulty of reproducing the absolute uniformity of the experimental conditions in two different interferometers.