Acoustic anechoic layers with singly periodic array of scatterers: Computational methods, absorption mechanisms, and optimal design

The acoustic properties of anechoic layers with a singly periodic array of cylindrical scatterers are investigated. A method combined plane wave expansion and finite element analysis is extended for out-of-plane incidence. The reflection characteristics of the anechoic layers with cavities and locally resonant scatterers are discussed. The backing is a steel plate followed by an air half space. Under this approximate zero transmission backing condition, the reflection reduction is induced by the absorption enhancement. The absorption mechanism is explained by the scattering/absorption cross section of the isolated scatterer. Three types of resonant modes which can induce efficient absorption are revealed. Due to the fact that the frequencies of the resonant modes are related to the size of the scatterers, anechoic layers with scatterers of mixed size can broaden the absorption band. A genetic optimization algorithm is adopted to design the anechoic layer with scatterers of mixed size at a desired frequency band from 2 kHz to l0 kHz for normal incidence, and the influence of the incident angle is also discussed.

Receptivity of hypersonic boundary layer due to fast-slow acoustics interaction

The objective of receptivity is to investigate the mechanisms by which external disturbances generate unsta- ble waves. In hypersonic boundary layers, a new receptivity process is revealed, which is that fast and slow acoustics through nonlinear interaction can excite the second mode near the lower-branch of the second mode. They can generate a sum-frequency disturbance though nonlinear interaction, which can excite the second mode. This receptivity process is generated by the nonlinear interaction and the nonparal- lel nature of the boundary layer. The receptivity coefficient is sensitive to the wavenumber difference between the sumfrequency disturbance and the lower-branch second mode. When the wavenumber difference is zero, the receptivity coefficient is maximum. The receptivity coefficient decreases with the increase of the wavenumber difference. It is also found that the evolution of the sum-frequency disturbance grows linearly in the beginning. It indicates that the forced term generated by the sum-frequency disturbance resonates with the second mode.

Acoustic reflection of rib-stiffened double plates coated with a viscoelastic layer

To predict sound-absorbing performance of anechoic materials,the acoustic reflection problem of a viscoelastic layer backed with periodically rib-stiffened infinite double plates is studied in this paper.The reason why structural theories of plates are not applicable to viscoelastic plates is explained through comparing dispersion and attenuation curves of flexural waves with those of Lamb waves.As a result,（visco-）elastic theory is adopted to deal with（visco-）elastic plates,and ribs are treated by structural theories of plates.The coupling between ribs and plates are solved by Hull＇s method,and solution of the reflected field is obtained.The accuracy of present method is validated by comparing with the results by the structural theories of plates.The influence of a backing on the acoustic reflection of the viscoelatic layer is analyzed by computing reflection coefficients.Performances of different viscoelastic materials are evaluated by the average reflection coefficients.The computational results show that,influence of a backing on the acoustic reflection cannot be suppressed by the viscoelastic materials in low frequencies.The resonance is determined by the coupling of the fluid layer and the double plates.And ribs,which are coupled with the double plates,mainly reduce the acoustic reflection.

The high frequency acoustic radiation from the boundary layer of an axisymmetric body

The mechanism of acoustic radiation from the boundary layer of an axisymmetric body is analyzed, and its sound pressure spectrum is predicted. It is shown that the acoustic radiation results from the transition region and the turbulent boundary layer; and that the acoustic radiation from transition region is predominant at low frequencies; while the turbulent boundary layer has the decisive effect on acoustic radiation at high frequencies. The calculated values are in good agreement with the experimental data.

A study of computations for acoustic velocities of arbitrarily dipping layers

Through deriving expressions relating the dip-angle ( m) of the lower boundary of a layer to the acoustic velocity (υm) of the layer and other pre-determinable parameters, υm and m can be taken as simultaneously iterative variables while solving Shah 's equations . Consequently the previous method of computing υm and m presented by ZHANG S . is improved [1] , and the accuracy of solutions increased greatly.

A study of measuring acoustic velocities of thin and hard sea-bed layers

It has been known that the error of measuring acoustic veloicities of thin sediment layers by the well-known T2-X2 approach is usually untolerable, and that this approach is unavailable in the case where sea-bed is hard because no echo from any subsurface below sea-bottom can be received. Therefore applying the ray-parameter method to thin layers and the refraction method to hard layers need to be considered in an acoustic velocity measurement system composed of a sound source and a towed hydrophone streamer. Some problems of practical importance about the applications of the two methods, such as echo-data processing procedures and error estimations in measuring acoustic veloicities, are discussed, and the effectiveness of theoretical analyses has been verified through computer simulations.

Geoacoustic model and acoustic reflection properties of fluid mud layer in Changjiang Estuary and Hangzhou Bay

A generalized geoacoustic model of fluid mud layer in Chanaiiang Estuary and Hangzhou Bay has been derived from a large amount of in-situ measurements of bulk density (p) profiles of the lay6rs and of lab measurements of acoustic velocities (c) and attenuation coefficients (o) of the fluid mud samples with different values of p for four frequencies of 100 kHz, 150 kHz, 500 kHz, 1500 kHz. The main features of the geoacoustic model can be expressed as follows: from the upper boundary, the bulk density of the fiuid mud increases linearly with depth z, however there is a gradient change (knee) when p is about 12.5 kN/m', then p increases linearly to a value about 15.0 kN/m'. After p more than 15.0, the fluid mud layer quickly transform into an ooze layer. In the fluid mud layer, the acoustic velocity c can be regarded as constant since its variation with z less than 1.5%, and a minimum vaue of c ekists when p is about 13.5 kN/m'. The variations of β with p and with frequency f are linear. Based on the geo-acoustic model and the ray theory, simulations of sound refiection from the fluid mud layers have been made, and some significallt results obtained, from which the bulk density profiles of fluld mud layers can be derived inversely.