Abstract: Aircraft nacelle acoustic liner is a key mean of aircraft noise reduction. The success of its design depends strongly on the development of experimental technology of acoustic performance verification. The acoustic design of nacelle acoustic liner is generally divided into two stages, each focusing on different research purposes and verification objects. At the preliminary design stage, the acoustic impedance design is generally verified by the impedance eduction technology with proper designed experimental installation; at the detailed design stage, the overall acoustic performance is verified by the use of acoustic mode decomposition technology. The comparative study in this paper summarizes the acoustic liner impedance eduction technology based on the in-situ method and the direct extraction method, and the acoustic modal measurement and control technology based on the phase control array, as well as the application of the two in the design of the acoustic liner of the engine intake and exhaust ducts. The research results show that the in-situ method has higher accuracy at low frequencies, and the accuracy of the both methods are decreased in the high frequency range. The difference in the non-dimensional acoustic resistance and the non-dimensional acoustic reactance measured from the two methods in the mid-frequency range is less than 0.2, both showing an acceptable accuracy and good applicability. The rotating acoustic mode generation technology based on designated phase control and the developed generator can produce a modal resolution of more than 10 dB, which can be used to test the noise reduction effect of the acoustic liner on multiple target modes. To further study the main mechanism of increasing the noise reduction of seamless acoustic liners, a modal generator was built and used in the experiment to emit separate and pure acoustic modes in sequence, and a comparative test was carried out on the two types of acoustic liner test pieces. Compared with the seamed acoustic liner made by the previous generation of molding technology, the seamless acoustic liner developed by the new generation technology has significantly improved the noise reduction effect at the multi-acoustic mode control and target frequencies, which improves the overall noise reduction effect of the seamless acoustic liner about 5.2 dB. Through research, the article has established the key and validated the technology of acoustic performance test for nacelle acoustic liner.

Key words: nacelle, liner, noise control, acoustic impedance, acoustic modes, impedance eduction