Effect of polymeric properties on the operation of gel-type audio transducers.

A novel design of a moving-coil transducer coupled with a low-hardness elastomer called “the gel surround” is presented in this thesis. This device is termed a “gel-type audio transducer”. The gel-type audio transducer has been developed to overcome the problems that conventional loudspeakers have suffered - that is, the problem with size of the audio device against the quality of sound at low frequency range. Therefore the research work presented herein aims to develop the “gel-type audio transducer” as a next-generation audio transducer for miniaturized woofers. The gel-type audio transducer consists of the magnetic and coil-drive plate assembly, and these parts are coupled by the gel surround. The transducer is driven by the electromagnetic conversion mechanism (a moving-coil transducer) and its output driving force can be greatly enhanced by applying the novel mechanism of the gel surround especially at low frequency range, resulting in the enhanced acoustic efficiency. The transducer can be attached to a stiff and light panel with both the optimized impedance matching and minimised wave collisions. The performance of the gel-type audio transducer is greatly influenced by the mass of the magnetic assembly and compliance of the “gel surround”. But as the size of the magnet and its weight have to be kept minimal for a miniaturisation of the device, the focus of the research is on the effect of the of the gel surround. As a result, the effect of the gel surround, made of the RTV (room-temperature vulcanising) silicone elastomer, TPE (thermoplastic elastomer), and the silicone foam, on generation of the output driving force, the energy transfer from the transducer to a panel to which the transducer is attached, and sound radiation from the vibrating panel, was investigated. This effect was studied by COMSOL multiphysics (FE analysis) and thereby, the simulated results were verified by experiments such as the laser scanning measurement, DMA (dynamic mechanical analyzer), and the acoustic test. Successful development of prototypes of the gel-type audio transducers, with an enhanced acoustic efficiency at reduced size and weight, was achieved. Implementation of the transducers into consumer applications was also demonstrated with their commercial values.

Micromechanical insight into the undrained instability of granular materials: deformation characteristics of geomaterials.

There is no agreement between experimental researchers whether the point where a granular material responds with a large change of stresses, strains or excess pore water pressure given a prescribed small input of some of the same variables defines a straight line or a curve in the stress space. This line, known as the instability line, may also vary in shape and position if the onset of instability is measured from drained or undrained triaxial tests. Failure of granular materials, which might be preceded by the onset of instability, is a subject that the geotechnical engineers have to deal with in the daily practice, and generally speaking it is associated to different phenomena observed not only in laboratory tests but also in the field. Examples of this are the liquefaction of loose sands subjected to undrained loading conditions and the diffuse instability under drained loading conditions. This research presents results of DEM simulations of undrained triaxial tests with the aim of studying the influence of stress history and relative density on the onset of instability in granular materials. Micro-mechanical analysis including the evolution of coordination numbers and fabric tensors is performed aiming to gain further insight on the particle-scale interactions that underlie the occurrence of this instability. In addition to provide a greater understanding, the results presented here may be useful as input for macro-scale constitutive models that enable the prediction of the onset of instability in boundary value problems.