Standardized methods for measuring sound absorption such as the impedance tube and reverberation chamber methods are limited to normal or diffuse incidence, respectively. Two research axes have been generally followed in the literature to develop alternative techniques, the first one focusing on the measurement part, that is from the two-microphone technique to the use of microphone arrays or pressure-velocity sensors. The second axis focuses on the excitation part with for instance the use of sound field synthesis techniques. Since acoustic impedance and sound absorption coefficient of materials are classically defined under normal and oblique plane wave excitation, synthesizing an “ideal” plane wave using a loudspeaker array would allow measuring these acoustics quantities using a simple microphone pair. In this article, the effect of the different parameters of a loudspeaker array on acoustic plane waves reproduction on a material’s surface is first numerically studied. Then, numerical and experimental results for the estimation of both impedance and absorption coefficients are reported. These results show that sound field synthesis allows to characterize a material for arbitrary incidence angles over a wide frequency range, thus offering an alternative method to standard techniques and an improvement over existing works.
The aim of this work is to continuously provide the acoustic pressure field radiated from nonstationary sources. From the acquisition in the nearfield of the sources of a planar acoustic field which fluctuates in time, the method gives instantaneous sound field with respect to time by convolving wavenumber spectra with impulse response and then inverse Fourier transforming into space for each time step. The quality of reconstruction depends on the impulse response which is composed of investigated parameters as transition frequency and propagation distance. Sampling frequency also affects errors of the practically discrete impulse response used for calculation. To avoid aliasing, the impulse response is low-pass filtered with Chebyshev or Kaiser-Bessel filter. Another approach to implement the impulse response consists of applying an inverse Fourier transform to the theoretical transfer function for propagation. To estimate the performance of each processing method, a simulation test involving several source monopoles driven by nonstationary signals is executed. Some indicators are proposed to assess the accuracy of the temporal signals predicted in a forward plane. The results show that the use of a Kaiser-Bessel filter numerically implemented or that of the inverse Fourier transform can provide the most accurate instantaneous acoustic signals.
This paper discusses cyclic deformation and fatigue behavior of three grades of stainless steel 304L. Effects of pre-straining and mean stress were investigated and it was observed that these parameters influence both deformation and fatigue behaviors. At HCF, significant secondary hardening was observed for two of the materials, leading to a significant change in fatigue behavior at long lives. For two of the materials, pre-strained tests were also conducted. Hardening was observed as a consequence of pre-straining and affected further deformation and fatigue behaviors significantly. Mean stress effects on fatigue lives were as expected, as compressive mean stress lengthened life and tensile mean stress shortened life. The Smith-Watson-Topper (SWT) parameter was used to correlate the experimental data from all test conditions for the three materials and was found to correlate the experimental data reasonably well.
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