Four research topics on new acoustic liner concepts and modal propagation measurement

Four different studies of acoustic liners and measurement techniques are presented in this thesis. The titles of these investigations are as follows:The possible use of bulk absorbers in engine nacelle linersThe effect of transwall sound propagation in the honeycomb core of an acoustic linerThe potential of piezzo-electric actuators to control the impedance of single layer linersThe development of the multi modal acoustic measurement technique in the flow duct A wide range of previously published work on physical and prediction models for bulk absorbers was studied. An empirical correlation of the impedance established by M. E. Delany and E. N. Bazley was chosen for the impedance computation. With this method, the impedance of bulk absorbers was computed with several configurations. Then, the sound suppression data for bulk absorbers in a fan-exhaust duct of a high bypass turbofan engine were calculated and compared to the corresponding suppression data for conventional liners.New liner samples were designed and fabricated to investigate the effects of the transwall sound propagation in the honeycomb core of a single layer acoustic liner. A method for impedance measurement with grazing incident sound was specifically developed for this investigation. This method is based on the two-dimensional wave propagation theory. Experiments were conducted to observe the validity of the new measurement method.The potential use of a piezzo-electric actuator device for active control of the acoustic impedance of a single layer acoustic liner was also studied. The absorption coefficient of the device was measured in the wave tube with phase differences between the incident sound and the device excitation signal. Results showed that the absorption coefficient could be improved by adjusting the phase and the amplitude of the excitation signal for the piezoelectric actuator.Finally, a multi modal measurement technique with 16 transducers was developed. This technique will be used for acoustic measurements in a flow duct apparatus. The method was tested in a prototype duct which had the same cross section as the flow duct. The sudden change of the energy flux at cut-on frequency and the energy reduction by the sound absorber were observed. As these results agreed with what was expected from theory, it was concluded that the method was properly established.