Noise propagation within ducts is of practical concern in many areas of industrial processes where a fluid has to be transported in piping systems. The paper presents experimental data and visualization of flow in the vicinity of an abrupt change in cross-section of a circular duct and on obstacles inside where the acoustic wave generates nonlinear separated flow and vortex fields.For noise produced by flow wave of low Mach number, laminar and turbulent flows are studied using experimental sound intensity (SI) and laser particle image velocimetry (PIV) technique adopted to acoustics (A-PIV). The emphasis is put on the development and application of these methods for better understanding of noise generation inside the acoustic ducts with different cross-sections. The intensity distribution inside duct is produced by the action of the sum of modal pressures on the sum of modal particle velocities. However, acoustic field is extremely complicated because pressures in non-propagating (cut-off) modes cooperate with particle velocities in propagating modes, and vice versa. The discrete frequency sound is strongly influenced by the transmission of higher order modes in the duct. By understanding the mechanism of energy in the sound channels and pipes we can find the best solution to noise abatement technology.In the paper, numerous methods of visualization illustrate the vortex flow as an acoustic velocity or sound intensity stream which can be presented graphically. Diffraction and scattering phenomena occurring inside and around the open-end of the acoustic duct are shown.

Many of theoretical research of the acoustics fields provides useful information about pressure fields, but none currently offers a full mapping of the acoustic energy flow (vectorial effects) in front and back of any scattering systems working in 3D real environmental conditions. Interference, diffraction and scattering of waves made the real field very complex and difficult to the theoretical modelling. This is one of the reasons why the experimental investigations of acoustic fields using sound intensity (SI) technique are so effective and serviceable methods. The visualization of acoustic energy flow in real-life acoustic 3D space fields can explain many particulars energetic effects (perturbations and vortex flow, effects of scattering in direct and near field, etc.), concerning the areas in which it is difficult to make numerical modeling and analysis with the CFD-FSI-CAA simulation methods. The sound intensity image represents a more accurate and efficient information compare to the spatial sound fields modelled. The article presents the application of SI technique to graphic presentation of spatial distribution the acoustic energy flow over the barriers of various geometrical shapes structures located in a three-dimensional space. As the results of research, the graphic analysis of the sound intensity flux in 2D and 3D space is show. Visualisation of research results is shown in the form of intensity streamlines in space and as a shape of flow wave or isosurface in three-dimensional space. Numerous examples illustrate the application of the SI measurement for practical problems at the vibroacoustical diagnostic and noise abatement, as well as to the validation of results of CFD/CAA numerical modelling. The differences, if appearance, mainly result from the fact that theoretical forecasting uses far too big simplifications or that it is impossible to obtain proper data on real physical features of the tested area and structures.

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The shedding process and acoustic ow formation in the region of outlet of a cylinder duct has been investigated by means of the particle image velocimetry (PIV) technique. The research work on the behavior of the sound eld at outlets of waveguides addresses issues of importance to industrial applications. The obtained results can be used in a number of industrial applications involving pipes, ducts and tubing. In our investigation the acoustics ow elds are measured with a phase-locked PIV system with high sampling rate and large internal memory block using DAVIS v. 8.11 software. A cross-correlation method, in conjunction with the FFT analysis, is used as a vector processing algorithm. A series of PIV vector maps was used to evaluate the acoustic velocity eld at the open end of circular tube throughout the acoustic cycle for frequencies of 700 Hz and 1400 Hz. Six phase steps were assumed for one frequency cycle and the characteristic signal was extracted from the velocity data step and shown graphically.

Flow fields could be of great interest in the study of sound propagation in aeroengines. For ducts with rigid boundaries, the fluid-resonant category may contribute significantly to unwanted noise. An understanding of the multi-modal propagation of acoustic waves in ducts is of practical interest for use in the control of noise in, for example, aero-engines, automotive exhaust and heating or ventilation systems. The purpose of our experiments was to test the acoustic energy transmission of duct modes based on studies carried out by the sound intensity technique. Sound intensity patterns in circular duct are discussed of modal energy analysis with particular reference to proper orthogonal decomposition and dynamic mode decomposition. The authors try to justify some advantages of the sound intensity experimental research in this area. In the paper, the wide-band sound signal propagated from source approximated with loudspeaker in hard-walled duct is imaged using a sound intensity -based approach. For a simple duct geometry, the sound intensity field is examined visually and by performing a modal decomposition greater insight into the acoustic structures is obtained. The image of sound intensity fields below and above "cut-off" frequency region are found to compare acoustic modes which might resonate in duct.