http://hdl.handle.net/1842/3420 2013-05-26T09:41:54Z 2013-05-26T09:41:54Z Pal, Saptarshi http://hdl.handle.net/1842/6593 2013-03-22T16:10:30Z 2012-11-28T00:00:00Z

Title: Colouration Analysis of a Midrange Dome using Empirical Mode Decomposition Authors: Pal, Saptarshi Abstract: The midrange driver of a loudspeaker system has the most difficult job – it has to produce the frequencies the human ears are most sensitive to. It is also responsible for handling the major portion of the spectrum comprising the human voice and the fundamental notes emitted by most instruments. Hence it becomes imperative for it to behave well across this entire mid-frequency range. A loudspeaker driver suffers from resonance or ‘ringing’ at various frequencies due to structural limitations. This is the primary cause of artificial colouration of sound or inaccuracy in reproduction of timbre. This thesis attempts to detect and analyse the inherent resonant modes of a high fidelity midrange dome driver used in Linn loudspeaker systems using Empirical Mode Decomposition (EMD). The measurements have been made using MODALYZER™, a software programmed in MATLAB for accurate identification of resonant modes. The method described in this thesis provides a novel approach for accurately detecting colouration or timbral inaccuracies in loudspeakers due to resonance of the transducer components. Limitations of the conventional FFT spectral analysis methods such as spectrogram and Cumulative Spectral Decay have been demonstrated with regard to analysis of colouration.

2012-11-28T00:00:00Z Smith, Graham http://hdl.handle.net/1842/6466 2012-09-27T14:19:58Z 1996-01-01T00:00:00Z

Title: The Measurement of Free Energy by Monte Carlo Computer Simulation Authors: Smith, Graham Abstract: One of the most important problems in statistical mechanics is the measurement of free energies, these being the quantities that determine the direction of chemical reactions and--the concern of this thesis--the location of phase transitions. While Monte Carlo (MC) computer simulation is a well-established and invaluable aid in statistical mechanical calculations, it is well known that, in its most commonly-practised form (where samples are generated from the Boltzmann distribution), it fails if applied directly to the free energy problem. This failure occurs because the measurement of free energies requires a much more extensive exploration of the system's configuration space than do most statistical mechanical calculations: configurations which have a very low Boltzmann probability make a substantial contribution to the free energy, and the important regions of configuration space may be separated by potential barriers. We begin the thesis with an introduction, and then give a review of the very substantial literature that the problem of the MC measurement of free energy has produced, explaining and classifying the various different approaches that have been adopted. We then proceed to present the results of our own investigations. First, we investigate methods in which the configurations of the system are sampled from a distribution other than the Boltzmann distribution, concentrating in particular on a recently developed technique known as the multicanonical ensemble. The principal difficulty in using the multicanonical ensemble is the difficulty of constructing it: implicit in it is at least partial knowledge of the very free energy that we are trying to measure, and so to produce it requires an iterative process. Therefore we study this iterative process, using Bayesian inference to extend the usual method of MC data analysis, and introducing a new MC method in which inferences are made based not on the macrostates visited by the simulation but on the transitions made between them. We present a detailed comparison between the multicanonical ensemble and the traditional method of free energy measurement, thermodynamic integration, and use the former to make a high-accuracy investigation of the critical magnetisation distribution of the 2d Ising model from the scaling region all the way to saturation. We also make some comments on the possibility of going beyond the multicanonical ensemble to `optimal' MC sampling. Second, we investigate an isostructural solid-solid phase transition in a system consisting of hard spheres with a square-well attractive potential. Recent work, which we have confirmed, suggests that this transition exists when the range of the attraction is very small (width of attractive potential/ hard core diameter ~ 0.01). First we study this system using a method of free energy measurement in which the square-well potential is smoothly transformed into that of the Einstein solid. This enables a direct comparison of a multicanonical-like method with thermodynamic integration. Then we perform extensive simulations using a different, purely multicanonical approach, which enables the direct connection of the two coexisting phases. It is found that the measurement of transition probabilities is again advantageous for the generation of the multicanonical ensemble, and can even be used to produce the final estimators. Some of the work presented in this thesis has been published or accepted for publication: the references are G. R. Smith & A. D. Bruce, A Study of the Multicanonical Monte Carlo Method, J. Phys. A. 28, 6623 (1995). [reference details doi:10.1088/0305-4470/28/23/015] G. R. Smith & A. D. Bruce, Multicanonical Monte Carlo Study of a Structural Phase Transition, to be published in Europhys. Lett. [reference details Europhys. Lett. 34, 91 (1996) doi:10.1209/epl/i1996-00421-1] G. R. Smith & A. D. Bruce, Multicanonical Monte Carlo Study of Solid-Solid Phase Coexistence in a Model Colloid, to be published in Phys. Rev. E [reference details Phys. Rev. E 53, 6530–6543 (1996) doi:10.1103/PhysRevE.53.6530]

1996-01-01T00:00:00Z Young, Ross Donaldson http://hdl.handle.net/1842/6272 2012-08-08T14:44:29Z 2012-08-08T00:00:00Z

Title: Measurements of B± Meson production at LHCb and characterisation of hybrid photon detectors Authors: Young, Ross Donaldson Abstract: LHCb is an experiment designed to make precision measurements of Charge- Parity violation in the B meson system. We report a measurement of the B± crosssection and production asymmetry, using B± → J/u K± decays collected at the LHCb detector in 2010 and 2011. Using 27.6 pb-1 of pp collisions at a centre-of-mass energy 7 TeV, we obtain a B± cross-section of [41.6 ± 0.6 (stat.) ± 3.0 (sys.) ± 4.2 (lumi.)] μb in the rapidity region 2 to 4.5. Using 371.1 pb-1 of pp collisions at a centre-of-mass energy 7 TeV, we obtain a B± production asymmetry of [-2.09 ± 1.20 ± 0.8 (CP) ]% in the same rapidity region. The Ring Imaging Cherenkov system of LHCb uses Hybrid photon detectors (HPDs) for single photon detection. This thesis summarises the use of ion feedback measurements as indicators of HPD vacuum quality.

2012-08-08T00:00:00Z Thompson, Alasdair Graham http://hdl.handle.net/1842/6248 2012-08-08T13:57:42Z 2012-06-22T00:00:00Z

Title: Lattice models of pattern formation in bacterial dynamics Authors: Thompson, Alasdair Graham Abstract: In this thesis I study a model of self propelled particles exhibiting run-and tumble dynamics on lattice. This non-Brownian diffusion is characterised by a random walk with a finite persistence length between changes of direction, and is inspired by the motion of bacteria such as Escherichia coli. By defining a class of models with multiple species of particle and transmutation between species we can recreate such dynamics. These models admit exact analytical results whilst also forming a counterpart to previous continuum models of run-and- tumble dynamics. I solve the externally driven non-interacting and zero-range versions of the model exactly and utilise a field theoretic approach to derive the continuum fluctuating hydrodynamics for more general interactions. I make contact with prior approaches to run-and-tumble dynamics of lattice and determine the steady state and linear stability for a class of crowding interactions, where the jump rate decreases as density increases. In addition to its interest from the perspective of nonequilibrium statistical mechanics, this lattice model constitutes an efficient tool to simulate a class of interacting run-and-tumble models relevant to bacterial motion. Pattern formation in bacterial colonies is confirmed to be able to stem solely from the interplay between a diffusivity that depends on the local bacterial density and regulated division of the cells, in particular without the need for any explicit chemotaxis. This simple and generic mechanism thus provides a null hypothesis for pattern formation in bacterial colonies which has to be falsified before appealing to more elaborate alternatives. Most of the literature on bacterial motility relies on models with instantaneous tumbles. As I show, however, the finite tumble duration can play a major role in the patterning process. Finally a connection is made to some real experimental results and the population ecology of multiple species of bacteria competing for the same resources is considered.

2012-06-22T00:00:00Z