Investigation og Nonlinear Acoustical Properties of Perforated Plate Samples and Samples of Aircraft Engine Liners

Abstract: Five samples, three perforated plates and two more realistic samples used in aircraft engine liners have been investigated. All the samples when exposed to high amplitude sound waves show nonlinear acoustic behavior which depends on acoustic velocity at the plate position. Velocity dependent sound absorption and acoustic energy transfer to higher harmonics are the consequence. In the literature there is a lack of systematic experimental approach to determine these energy transfers to other frequencies considering harmonics. In this thesis a one-port model for measurements in impedance tube is chosen with sound pressure at the frequency of excitation and two and three times higher than that as variables. The samples are tested at single tone and two tone excitation involving harmonics at 110Hz, 220Hz and 330Hz with varying particle velocity at the sample position. In the case of realistic samples an excitation frequency of 990Hz was also used because these samples are designed for higher frequencies and higher sound levels. For single tone excitation both low and high sound levels are used to see linear and nonlinear behavior of acoustic properties. A part of investigation for two tone excitation is to analyze acoustical properties when the tones are not exactly harmonics but are slightly different from that, 330Hz combined with 100Hz or 110HZ or 120HZ. The effect of shifting the phase of one of two signals on impedance measurement for three perforated samples has also been studied. Results from experimental tests using these techniques give a good understanding of the acoustical behavior of the samples at both low and high sound levels. Two different methods, the two-microphone method and sound intensity probe (p-u probe) method, are used to obtain the results in order to compare the reliability of both methods.