Volume 126, Issue 6, December 2009
Index of content:
- ADVANCED-DEGREE DISSERTATION ABSTRACTS
Design and evaluation of digital signal processing algorithms for acoustic feedback and echo cancellation126(2009); http://dx.doi.org/10.1121/1.3257646View Description Hide Description
This thesis deals with several open problems in acoustic echo cancellation and acoustic feedback control. Our main goal has been to develop solutions that provide a high performance and sound quality, and behave in a robust way in realistic conditions. This can be achieved by departing from the traditional ad-hoc methods, and instead deriving theoretically well-founded solutions, based on results from parameter estimation and system identification. In the development of these solutions, the computational efficiency has permanently been taken into account as a design constraint, in that the complexity increase compared to the state-of-the-art solutions should not exceed 50% of the original complexity.Full text available at http://hdl.handle.net/1979/2599
Time domain normal mode analysis of underwater acoustic wave propagation for a single layered acoustic channel in two dimensional Cartesian coordinates126(2009); http://dx.doi.org/10.1121/1.3270468View Description Hide Description
A new analytical time domain Normal Mode solution in a single layer acoustic waveguide in Cartesian coordinates is presented in the thesis. The method is based on the separation of variables technique for the two dimensional time domain wave equation. After separation, Sturm-Liouville type ordinary differential equation for depth dependency and inhomogeneous Klein-Gordon type partial differential equation for time and range dependency are obtained. Whenever the ordinary differential equation is solved in a classical manner with the application of Dirichlet boundary condition on the upper and lower boundaries of the waveguide, Inhomogeneous Klein-Gordon type partial differential equation is solved by using the Green function technique in time domain. Then the exact solution satisfying the causality principle is constructed for the acoustic pressure. The main advantage of the proposed causal solution is not necessary to use the Fourier Transformation to obtain time domain response of arbitrary acoustical source signals having the monochromatic and Gaussian pulse type time dependency in the waveguide. The excellent agreement for the comparisons of the range dependent transmission losses is observed between the proposed solution and the KRAKEN program.
126(2009); http://dx.doi.org/10.1121/1.3270469View Description Hide Description
One of the salient features of the human cochlea is the incredible dynamic range it possesses—the loudest bearable sound is 10,000,000 times greater than the softest detectable sound; this is in part due to an active process. More than twelve thousand hairlike cells known as outer hair cells are believed to expand and contract in time to amplify cochlear motions. However, the cochlea's response is more than just the sum of its parts: the local properties of outer hair cells can have unexpected consequences for the global behavior of the system. One such consequence is the existence of otoacoustic emissions(OAEs), sounds that (sometimes spontaneously!) propagate out of the cochlea to be detected in the ear canal. In this doctoral thesis, a classical, lumped-element model is used to study the cochlea and to simulate click-evoked and spontaneous OAEs. The original parameter values describing the microscopic structures of the cochlea are re-tuned to match several key features of the cochlear response in humans. The frequency domain model is also recast in a formulation known as state space; this permits the calculation of linear instabilities given random perturbations in the cochlea which are predicted to produce spontaneous OAEs.Full text available at http://eprints.soton.ac.uk/64535/
The influence of structural-acoustic coupling on the dynamic behaviour of a one- dimensional vibro-acoustic system126(2009); http://dx.doi.org/10.1121/1.3270472View Description Hide Description
The aim of this thesis is to investigate the structural-acoustic coupling effects on the dynamic behavior of a vibro-acoustic system under passive/active control. The simplest model of a vibro-acoustic system one can consider is a one-dimensional acoustic cavity driven by a single-degree-of-freedom (SDOF) structure. This simple model is used to demonstrate the physical characteristics of the coupling phenomenon. This simple analytical model can provide various degrees of structural-acoustic coupling, which are dependent upon (i) the structural-acoustic stiffness ratio, (ii) structural-acoustic natural frequency ratio, (iii) structural damping, and (iv) acoustic damping. In this case, although the geometric coupling factor is not included because the SDOF structure has a single mode, 80 percent of the factors that determine the degree of coupling can be accounted for by the simple analytical model. The coupling mechanism, in the simple vibro-acoustic system, is investigated using the mobility-impedance approach. In order to provide the threshold of the degree of coupling, a coupling factor is calculated in terms of non-dimensional structural-acoustic parameters. Vibro-acoustic responses are represented by the acoustic potential energy in the cavity and the kinetic energy of the structure coupled to the acoustic cavity.Full text available at http://eprints.soton.ac.uk/65676/
A methodology for developing high damping materials with application to noise reduction of railway track126(2009); http://dx.doi.org/10.1121/1.3270474View Description Hide Description
A methodology is developed that allows a material to be formulated for a particular damping application where temperature-dependence has to be taken into account. This is applied to a tuned absorber system used for damping the vibration of a railway track. This is required to be effective over a temperature range -20 to 40 deg C. The time-temperature superposition principle is used to convert frequency-dependence to temperature-dependence for a notional material with constant loss factor. This is used in the prediction of decay rates and thereby noise reduction. In addition, a weighted noise reduction is studied by using measured rail temperature distributions. Two types of viscoelastic material, butyl and EPDM rubbers with various amount of fillers and plasticisers, are investigated. The properties of both rubbers have been measured over the range of temperatures for frequencies 300-3000 Hz. For butyl, the best combination of filler and plasticiser gives temperature weighted noise reductions up to 5.9 dB(A). The best EPDM compound gives a temperature-weighted noise reduction up to 6.2 dB(A). Comparing these two rubbers, EPDM is more suitable for low temperatures below 10 deg and butyl is more suitable for higher temperatures above 10 deg.