Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE credits
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
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
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.
2011. , 73 p.