Home HomeFinn Jacobsen, Technical University of Denmark, (Denmark)
The sound intensity method is one of the most significant developments of the past twenty years in the technology of noise control enginnering. The method is now generally recognised as a 'mature technology', which is reflected in the fact that several international and national standards for sound power determination using sound intensity and for instruments for such measurements have been issued in the past few years. The paper summarises the basic theory and gives an overview of the state of the art with particular emphasis on recent developments in the field. It describes and discusses the sources of error in measurement of sound intensity and the resulting limitations imposed on various applications of such measurements. Finally, some unresolved problems are mentioned, and the possibility of improving the instrumentation is discussed.
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Monica Carfagni, Universita di Firenze (Italy)
Marco Pierini, Universita di Firenze (Italy)
Experimental testing was performed to investigate the validity of ISO Standard 9614-1 for determining the sound power of noise sources using sound intensity at discrete points. Among the conditions varied during testing were environmental geometry, source position, background noise, and measurement surface. On the basis of this investigation and extensive industrial testing, a critical revision of the standard has been written with the intent to eliminate the ambiguity and repetitiveness encountered in the original version.
sv970339.pdf (Scanned) TOPOrjan Johansson, Lulea University of Technology (Sweden)
3D-intensity measurements may be used for source localisation. There is, however, a complex relation between the near field intensity estimate and the sound pressure in the far field. Another complication is that the intensity field near a complex structure may be a combination of direct radiation and interference from coherent secondary sources. This is of particular interest when measuring in narrow bands because almost no frequency smearing effect occurs. In this paper, the multivariate technique partial least squares (PLS) regression, is applied for evaluating and interpreting intensity measurements in narrow bands in relation to different reference signals, such as sound pressure in the far field. The PLS method allows correlation in both the frequency and spatial domains. The method is tested and validated experimentally, by studies of a complex but controlled source model with partially correlated sources, in anechoic condition. It is concluded that a decomposition of the intensity vector field in so called principal components enhances the identification of inherent noise sources. The possibility of modelling the intensity vector field in relation to different reference signals reveals a better understanding of the relationship between the partly coherent near field and a specific position in the far field.
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Christophe Picard, Laboratoire d'Etudes Aerodynamiques (France)
J.C. Patrat, LEA (France)
A. Duran, LEA (France)
H.S. Na, KHRC (Korea)
J.C. Rebillat, LEA (France)
A well-known unidirectional probe to which a rotating motion is imparted can measure the three components of the sound intensity vector. Such a sequential measurement procedure produces some scattering and bias errors when the acoustic field is unsteady. During the measurement period, the magnitude of the physical phenomenon is supposed to vary but not its geometric properties. This leads to a simple calculation of these errors and an optimal estimation of the sound power level of the source which is used to correct the active and the reactive sound intensity vectors. For a random steady field, the same method of correction reduces the scattering of the measurements. Theoretical and experimental studies under laboratory conditions verify the effect of the correction even in a reactive field. We also show that in the case of noise emission by a short-duration supersonic jet, the same scattering reduction thus allows a more accurate localization of the intensity vectors.
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Sung-Chon Kang, Korea Advanced Institute of Science and Technology (Korea)
Jeong-Guon Ih, Korea Advanced Institute of Science and Technology (Korea)
The problems in the identification and localization of noise sources using the sound intensity method are dealt with for the reactive field. For these purposes, a three-dimensional model structure resembling the engine room of the passenger car is considered. The model contains complicated noise sources distributed within the small space, which includes the narrow, connected, reflecting planes constructed with rigid boxes. In addition, a small clearance exists between the structure and the reflecting bottom. The sources are actually hanging in the space over this rigid bottom. For this model, the near field acoustic intensity is calculated scanning over the upper plane opposite to the bottom by using the acoustic boundary element method. The effects of relative source phases, frequencies, and locations are investigated. It is observed that the application of sound intensity method without proper care in this situation can yield the detection of fake sources. Therefore, the sound intensity scanning over the engine room upper, with its hood open, may indicate the false positions or components as noise sources. The field reactivity has to be checked and the care should be attended in this type of measurement using the sound intensity methods.
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