Journal of Computational Acoustics

The variational theory of complex rays (VTCR) is a wave-based predictive numerical tool for medium-frequency problems. In order to describe the dynamic field variables within the substructures, this approach uses wave shape functions which are exact solutions of the governing differential equation. The discretized parameters are the number of substructures (h) and the number of wavebands (p) which describe the amplitude portraits. Its capability to produce an accurate solution with only a few degrees of freedom and the absence of pollution error make the VTCR a suitable numerical strategy for the analysis of vibration problems in the medium-frequency range. This approach has been developed for structural and acoustic vibration problems. In this paper, an error indicator which characterizes the accuracy of the solution is introduced and is used to define an adaptive version of the VTCR. Numerical illustrations are given.

Internal tides can cause density and sound speed fluctuations in the ocean, and the thickness of the mixing layer can fluctuate in short periods of time. This phenomenon affects underwater sound propagation and induces transmission loss variations. In the sea area northeast of Taiwan, regular internal tide activity is observed. The topography and sediment there are also complex, which makes the effect on transmission loss by internal tides even more complicated. This paper studies how transmission loss variation is affected by internal tides in the sea area northeast of Taiwan. The hydrographic fields with influence from the internal tides are provided by an experiment survey on the 4th of September, 2008 and a 3-D tide model. The transmission loss comparison between different thicknesses of the mixing layer in the internal tide fields is shown. Furthermore, the correlation coefficient is calculated and shows that layer depth and transmission loss have a medium to high negative correlation in the upper water column during the summer spring-tide period, and a low to medium negative correlation during the summer neap-tide period.

Coir fiber from coconut husk is an important agricultural waste in Malaysia. The porous structure of fiber makes it an eligible material for acoustical absorption. In previous studies at Universiti Kebangsaan Malaysia, single layer coir fiber showed low acoustical absorption in medium and low frequencies; e.g. absorption coefficient of a 20 mm industrial prepared coir fiber sample was below 0.3 for frequencies less than 1.5 kHz. Current research was initiated to improve the shortcoming by mixing industrial prepared coir fiber with air gap layers. Analyzes were based on two approaches, namely; Delany–Bazley with acoustic transmission analysis (ATA) and Allard elastic model with transfer matrix analysis. Experimental measurements were conducted in impedance tube to validate the analytical results. Outcomes described that Delany–Bazley-ATA technique was an approximate solution showing overall absorption path without giving any accurate information about the peaks and resonances. Allard method took the elasticity of material into account and transfer matrices were able to characterize the whole structure as a combination of single layers. Results were close to experimental values and predicted the path and resonances very well. Further analyzes were conducted by Allard method and derived that having a fiber layer backed by an air gap was better than leaving the same gap in between that layer. The explanation was that, in the first arrangement, the sound field impacted a solid layer with higher thickness and followed a longer transmission path which caused a higher acoustical absorption. Furthermore, it was noticed that a reasonable thickness of backing air gap improved the overall sound absorption. Increase in the gap thickness produced more peaks and moved them to lower frequencies which caused better absorption in low frequencies. The cause of this phenomenon was increase in impedance of the panel due to rise in the air-gap thickness. This moved the acoustical resonances to lower frequencies and improved the sound absorption in this frequency span. Finally, it was concluded that other acoustic absorption techniques such as adding perforated plate may be combined with coir fiber-air gap structure to improve the low frequency acoustic absorption coefficient without any need to highly increase the air gap thickness.

In recent years the development of free field radiation conditions in the time domain has become a topic of intensive research. Perfectly matched layer (PML) approaches for the frequency domain are well known. In the time domain, on the other hand, they suffer in many cases from highly increased complexity and instabilities. In this paper, we introduce a PML for the conservation equations of linear acoustics. The used formulation requires three auxiliary variables in 3D and circumvents thereby convolution integrals and higher order time derivatives. Furthermore, we prove the weak stability of the proposed formulation and show their good absorption properties by means of numerical examples.

Triangulation is commonly used for source localization and most triangulation applications are based on intersection of the bearing direction to locate a source on a two-dimensional plane. In this paper, two new mathematical models (a basic model and an improved one) that expands the traditional triangulation concept to three-dimensional space are developed to locate multiple incoherent sound sources. The basic model uses four microphones and concentrates on solving a set of three quadratic equations simultaneously. The improved model requires more than four microphones and uses the solution from the basic model, as well as analyzing the intersection of bearing angles. Redundancy checks on the time differences of arrival are added to further reduce the source localization error in the improved model. Moreover, the input data are pre-processed and de-noised through filtering and windowing to enhance the effective signal to noise ratio. Various sound sources are tested, including transient, impulsive, continuous, broad-band, and narrow-band sounds. Numerical simulations and experimental validation using the real world sound sources are conducted. The impacts of the source direction/source detection range on the accuracy of source localization results are examined and discussed.