Several methods can be applied for analyses of the acoustic field in enclosed rooms namely: wave propagation, geometrical or statistical analysis. The paper presents problems related to application of the boundary elements method to modelling of acoustic field parameters. Experimental and numerical studies have been combined for evaluation of acoustic impedance of the material used for the walls of a model room. The experimental studies have been carried out by implementing a multichannel measuring system inside the constructed model of an industrial room. The measuring system allowed simultaneous measurements of the source parameters -the loudspeaker membrane vibration speed, the acoustic pressure values in reception points located inside the model space as well as phase shifts between signals registered in various reception points. The numerical modelling making use of the acoustic pressure values measured inside the analyzed space allowed determination of requested parameters of the surface at the space boundary.
The diffraction phenomenon is described by the Huygens-Fresnel principle. The review of physical laws ruling the bending of sound waves at the edge of the screen allows the effective selection of both acoustical and geometrical parameters of the screen. Sound wave diffraction theories have been developed on the basis of wave optics, when wavelength is small in comparison to the size of the obstacles, which can be also used in acoustics with the same assumptions about geometry of the system. Diffraction can be seen as a result of the interference of waves reaching the point of observation in accordance with the laws of geometrical optics and wave disturbances arising as a result of interaction with the edge of an obstacle. The paper describes a test method using maximum length sequences for determining the intrinsic characteristics of sound diffraction in situ during testing of roadside noise barriers. A scale model experiment has been performed in an anechoic room. Also, a real noise reducing device was tested in free field conditions.
Acoustic eld in enclosed rooms in the low frequency range can be described by the wave model, based on solution of the wave equation. Solution to the wave equation for acoustic eld in the room can be obtained using numerical procedures, e.g. the boundary elements method. Determination of acoustic impedance of the room walls surface material, based on the knowledge of the distribution of acoustic pressure amplitudes in the enclosed space, requires application of the inverse boundary elements method and gathering a proper set of input data. The paper presents the possibilities of analysis of acoustic properties for industrial-type rooms, by using inverse methods in the low frequency range.
The inversion method was used to test vibroacoustic processes in large-size machines used in opencast mines of rock material. When this method is used, the tested machine is replaced with a set of substitute sources, whose acoustic parameters are determined on the basis of sound pressure levels and phase shift angles of acoustic signals, measured with an array of 24 microphones. This article presents test results of a combine unit comprising a crusher and a vibrating sieve, for which an acoustic model of 7 substitute sources was developed with the inversion method.
The problem of identication and localization of sound sources inside industrial rooms, including cases when the density of sound sources is considerable, can be solved by application of inverse methods. The experimental knowledge about the distribution of acoustic eld in the neighborhood of the examined object, followed by the reversal of the model used for sound generation and propagation, leads to evaluation of the object's acoustic power. For the cases when determination of the rooms' acoustical properties is required the combination of the inverse method with the boundary elements method allows the calculation of acoustic impedance for the surface delimiting the examined room. The paper presents the prospects of the inverse method application in the mentioned vibroacoustic problems.
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