The location and strength of primary noise sources is essential information in controlling interior noise of a vehicle or fuselage. In contrast to passive reactions, such as reflections and absorption, primary noise sources actively radiate sound into enclosures. Therefore, if we eliminate primary noise sources, we can obtain perfect control. The main objective of this research was to identify these primary sources, which can be defined as active sources.
In the past decade, nearfield acoustic holography (NAH) has been used to identify exterior noise sources. Since NAH reconstructs a whole sound field on the basis of the sound pressure measured at a surface, it shows the distribution of noise sources. Moreover, numerical methods have been applied to reconstruct the sound field caused by arbitrarily shaped sources. However, regarding interior noise sources, whether NAH also shows active source distributions has not been verified. In contrast to exterior sound field, reflections are added to interior sound fields; therefore, the sound fields reconstructed by NAH may misrepresent the source distribution.
This study addressed a holographic method that identifies active sound sources in interior sound fields. In the method, active sources are simply identified by eliminating passive reactions from the sound fields reconstructed by NAH. The first step of the method is to estimate surface characteristics, such as surface admittance or impedance. Since these characteristics control passive reactions, they are essential in identifying passive reactions. The second step is to apply acoustic holography to reconstruct the total sound field. The final step is to eliminate the passive reactions from the reconstructed total sound field. The a priori knowledge on surface characteristics is used in the final step.
A technique that estimates surface admittance was also developed in this research. Since a large number of specimens are needed to measure the surface admittance of an entire surface, a technique that estimates in-situ admittance is required for the proposed method. Moreover, taking a specimen from its original surface needs careful attention and skills. The technique introduced in this paper measures the in-situ surface admittance of the area covered by a microphone array. This technique employs BEM-based nearfield acoustic holography to reconstruct surface velocity and pressure. The ratio of the surface velocity to the surface pressure gives the surface admittance.
Errors in the proposed method were reduced by using a regularization process, and remaining errors were predicted. Usually, BEM-based nearfield acoustic holography is incorporated with a regularization process. Higher order mode components are filtered out since they produce enormous errors in predicted acoustic quantities. In this study, an optimal regularization process [B.-K. Kim and J. G.-Ih, J.Acoust.Soc.Am., 100, 3003-3016, 1996] was employed in order to reduce the errors. In addition, we predicted the errors that remain after regularization. The characteristics of the optimal regularization process and the statistics of measured noise were used to predict the errors.
Numerical simulations and experiments were performed to verify the proposed method. Our technique which estimates surface admittance was applied to interior sound fields as well as exterior sound fields. The results proved that this technique can be used as an independent measurement tool. The active sources on surfaces were reconstructed, and they showed correct source locations.