Statistics and control of waves in disordered media

Fundamental concepts in the quasi-one-dimensional geometry of disordered wires and random waveguides in which ideas of scaling and the transmission matrix were first introduced are reviewed. We discuss the use of the transmission matrix to describe the scaling, fluctuations, delay time, density of states, and control of waves propagating through and within disordered systems. Microwave measurements, random matrix theory calculations, and computer simulations are employed to study the statistics of transmission and focusing in single samples and the scaling of the probability distribution of transmission and transmittance in random ensembles. Finally, we explore the disposition of the energy density of transmission eigenchannels inside random media.Comment: 28 Pages, 18 Figures (Review

Transmission statistics and focusing in single disordered samples

We show in microwave experiments and random matrix calculations that in samples with a large number of channels the statistics of transmission for different incident channels relative to the average transmission is determined by a single parameter, the participation number of the eigenvalues of the transmission matrix, M. Its inverse, M-1, is equal to the variance of relative total transmission of the sample, while the contrast in maximal focusing is equal to M. The distribution of relative total transmission changes from Gaussian to negative exponential over the range in which M-1 changes from 0 to 1. This provides a framework for transmission and imaging in single samples.Comment: 9 pages, 4 figure

Acoustic properties of the porous material in a car cabin model

This paper predicts the acoustic properties of the porous material in a car cabin model by using an appropriate experimental method, and it verifies the estimated acoustic properties by conducting the FEM (Finite Element Method) analysis. A simplified vibro-acoustic system imitating a car cabin is set up. The car cabin is made of six rigid walls, and a flexible panel is mounted on the front firewall position. The porous material is applied to the inner surface of the panel and modifies the coupling between the panel and the cabin air cavity. The panel is mechanically excited by using an electromagnetic shaker, which is imitating the structure-borne noise. The radiated noise is recorded by using pressure microphones at the different locations inside the car cabin. Based on the model proposed, the effect of the porous material on the acoustic properties is investigated by using the Sound Pressure Level (SPL) at the microphone locations. Finally, the experimentally acquired acoustic properties of the porous material are compared with the numerical analysis of FEM. The simulation results show that the proposed model agrees well with the experiment data. The noise propagating inside the car cabin is predicted to be of similar level in both the experimental method and in the numerical analysis

Experimental and numerical investigation of enclosed cavity noise in the presence of interior trim materials

This paper presents a rigid walled car cabin model to predict the acoustic effects of vehicle interior trim materials. The car cavity is made of a rigid walled enclosure system with one flexible wall on the front firewall position, where the interior trim materials are applied to the inner surface of the front firewall to modify the coupling between the flexible wall and the cabin air cavity. The car cabin is acoustically excited by using a single point source positioned at one corner of the inner air cavity to imitate the airborne noise. The propagated noise is measured by using pressure microphones at different locations inside the car cabin: one near the driver's ear position and another one near the passenger's ear position. An acoustic FE (Finite Element) model is also developed to investigate and predict the effects of interior trim materials on the car cabin noise level. Finally, the simulation results are compared with the experimentally acquired acoustic effects of the interior trim materials. The predicted acoustic response results show that the simulation agrees well with the experiment data, both with and without the interior trim materials. The noise propagating inside the car cabin is reduced by a similar ratio in both the experimental method and in the numerical analysis. The selected interior trim materials are starting to absorb noise at frequencies above 500 Hz, but they do not reduce the low-frequency noise effectively

The effects of porous materials on the noise inside a box cavity

Porous materials are usually attached to the vehicle body metal panels to reduce the radiation of the noise into the vehicle cabin. This study focuses on investigating the effects of porous materials on the noise level due to the structure-borne noise excitation. Because of the com-plexity of the analysis of noise in a full vehicle cabin, a box cavity is used. The box has five rigid walls and one flexible aluminium plate, which make a coupled plate and cavity enclo-sure system. The pressure microphones are used for measuring the Sound Pressure Level (SPL) at different measurement locations in the box cavity with and without porous materi-als. A Computer Aided Engineering (CAE) model is also developed to simulate the experi-ment setup using the Finite Element Method (FEM). Good agreement has been obtained be-tween the experimental results and the simulation values for the acoustical response at dif-ferent locations inside the box cavity. Furthermore, the influences of porous materials on the vibration reduction of the plate and the noise reduction of the cavity are also characterised. The experimentally validated developed model is used to characterise the effects of damping and porous treatments in reducing the structure-borne noise

Acoustic measurement of a 3D printed micro-perforated panel combined with a porous material

The sound absorption coefficient of a micro-perforated panel (MPP) backed by a porous sound absorbing material is investigated in this paper. The feasibility to fabricate an MPP acoustic absorber using 3D printing technology is presented. The test specimens are made of polymer material, and they are printed with different perforation ratios. A porous sound absorbing material is added to the MPP samples to form an acoustic absorber structure. Their sound absorption coefficient is experimentally measured by using the impedance tube method. The results obtained are theoretically validated by using the transfer matrix method (TMM). The MPP is modelled using Maa's method, and the equivalent fluid model is used to calculate the acoustic property of the porous sound absorbing material layer. The results show that the sound absorption coefficient of a 3D printed MPP backed by a porous sound absorbing material agrees fairly well with the theoretical model. By adjusting the perforation ratio, MPP acoustic absorbers with high absorption peaks can be implemented. The results in this paper provide a new approach for fabricating MPP sound absorbers for acoustic applications

Access to primary health care services for Indigenous peoples: a framework synthesis

Published online: 30 September 2016Background: Indigenous peoples often find it difficult to access appropriate mainstream primary health care services. Securing access to primary health care services requires more than just services that are situated within easy reach. Ensuring the accessibility of health care for Indigenous peoples who are often faced with a vast array of additional barriers including experiences of discrimination and racism, can be complex. This framework synthesis aimed to identify issues that hindered Indigenous peoples from accessing primary health care and then explore how, if at all, these were addressed by Indigenous health care services. Methods: To be included in this framework synthesis papers must have presented findings focused on access to (factors relating to Indigenous peoples, their families and their communities) or accessibility of Indigenous primary health care services. Findings were imported into NVivo and a framework analysis undertaken whereby findings were coded to and then thematically analysed using Levesque and colleague’s accessibility framework. Results: Issues relating to the cultural and social determinants of health such as unemployment and low levels of education influenced whether Indigenous patients, their families and communities were able to access health care. Indigenous health care services addressed these issues in a number of ways including the provision of transport to and from appointments, a reduction in health care costs for people on low incomes and close consultation with, if not the direct involvement of, community members in identifying and then addressing health care needs. Conclusions: Indigenous health care services appear to be best placed to overcome both the social and cultural determinants of health which hamper Indigenous peoples from accessing health care. Findings of this synthesis also suggest that Levesque and colleague’s accessibility framework should be broadened to include factors related to the health care system such as funding.Carol Davy, Stephen Harfield, Alexa McArthur, Zachary Munn and Alex Brow