In this paper boundary element methods (BEM) are mated with near-field acoustical holography (NAH) in order to determine the normal velocity over a large area of a fuselage of a turboprop airplane from a measurement of the pressure (hologram) on a concentric surface in the interior of the aircraft. This work represents the first time NAH has been applied in situ, in-flight. The normal fuselage velocity was successfully reconstructed at the blade passage frequency (BPF) of the propeller and its first two harmonics. This reconstructed velocity reveals structure-borne and airborne sound-transmission paths from the engine to the interior space.
In this paper, global free-field cancellation in the region exterior to a single compact primary source is discussed. The nature of the secondary source or sources required to achieve such cancellation, either globally or in particular regions, is a subject of current interest. In particular, results are presented that relate to global cancellation of a primary monopole’s sound field by the use of a single, multipole secondary source. The secondary multipole component source strengths required to achieve global cancellation can be found in a number of ways. For example, they may be obtained directly from a multipole expansion of the primary monopole sound field, as originally suggested by Kempton. However, improved attenuation can be achieved by choosing the secondary multipole component source strengths to minimize the total sound power radiated by the combination of primary and secondary sources. Closed form solutions for secondary multipole component strengths have been derived for both the latter case and for Kempton’s approach and are presented here along with formulae for the corresponding reductions of sound power and sound pressure. It will also be shown that it is more efficient in some instances to use a secondary multipole than an array of secondary monopoles for global cancellation. However, the primary conclusion of the present work is that useful levels of global far-field cancellation may be obtained by using secondary sources of higher than monopole order placed at a relatively large fraction of a wavelength from the primary source.
In the past, various two- and three-dimensional Cartesian, poroelastic finite element formulations have been proposed and demonstrated. Here an axisymmetric formulation of a poroelastic finite element is presented. The intention of this work was to develop a finite element formulation that could easily and efficiently model axisymmetric sound propagation in circular structures having arbitrary, axially dependent radii, and that are lined or filled with elastic porous sound absorbing materials such as foams. The formulation starts from the Biot equations for an elastic porous material expressed explicitly in axisymmetric form. By following a standard finite element development, a u-U formulation results. Procedures for coupling the axisymmetric elements to an adjacent acoustical domain are described, as are the boundary conditions appropriate for unfaced foams. Calculations described here show that the present formulation yields predictions as accurate as a Cartesian, three-dimensional model in much reduced time. Predictions made using the present model are also compared with measurements of sound transmission through cylindrical foam plugs, and the predicted results are shown to agree well with the measurements. Good agreement was also found in the case of sound transmission through a conical foam plug.
A technique is developed which utilizes numerical models and field pressure information to characterize acoustic fields and identify acoustic sources. The numerical models are based on boundary element numerical procedures. Either pressure, velocity, or passive boundary conditions, in the form of impedance boundary conditions, may be imposed on the numerical model. Alternatively, if no boundary information is known, a boundary condition can be left unspecified. Field pressure data may be specified to overdetermin'e the numerical problem. The problem is solved numerically for the complete sound field from which the acoustic sources may be determined. The model can then be used to identify acoustic intensity paths in the field. The solution can be modified and the model used to evaluate design alternatives. In this investigation the method is tested analytical and verified. In addition, the sensitivity of the method to random and bias error in the input data is demonstrated.
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