Results of experimental research of acoustic characteristics of acoustic liners for turbofan engines

Abstract

The subject of the research in this article is an acoustic liner designed to reduce aircraft noise in a wide frequency range. The purpose of the study is to determine the acoustic characteristics of an acoustic liner. Tasks: development of a theoretical model of the acoustic liner in the form of tubes of different lengths, calculation of the natural frequencies of each tube of the developed liner, determination of the frequency range in which sound absorption will occur, production of a test sample, and experimental determination of the sound absorption coefficient. An experimental setup is used to determine the sound absorption coefficient of the developed liner, which determines the acoustic characteristics of the acoustic liners using the standing wave method. This study presents the results of: calculation of the natural frequencies of the proposed liner, calculation of the dependence between the amplitude transmission coefficient and the frequency of the exciting force, and measurement of the acoustic characteristics of the proposed acoustic liner. The scientific and practical novelty of the obtained results is as follows: to solve the problem of noise absorption in a wide frequency range, a theoretical model of the acoustic liner, which should consist of tubes of different lengths, was developed. Each tube has its own frequency of oscillation of the air inside the tube. As the path of sound travel inside the tube increases, the natural frequency of the tube decreases. When tubes of different lengths are placed on the test sample, it is possible to obtain an acoustic liner with a significant number of natural frequencies, that is, noise will be absorbed in a wide frequency range. According to the calculation results, the proposed acoustic liner, consisting of seven types of tubes of different lengths, has more than 20 natural frequencies in the frequency range from 500 Hz to 4500 Hz, that is, the frequency range of absorption is quite wide. In order to verify the theoretical calculations with the help of a 3D printer, experimental samples of the proposed acoustic liner, as well as a real acoustic liner used to reduce the noise of the D-18T engine, were made and their experimental studies were carried out. The research results show that the proposed acoustic liner reduces noise in a wide frequency range. In the frequency range from 1300 Hz to 3500 Hz, the sound absorption coefficient exceeds 0.4, and in the range from 1500 Hz to 2600 Hz, the sound absorption coefficient is above 0.5. In the specified frequency ranges, the proposed acoustic liner has significant advantages over the D-18T acoustic liner. By reducing the cross-section of the tubes, or increasing the number of tubes of the experimental sample, it is possible to significantly increase the frequency range of sound absorption.