This thesis describes experimental researches in ultrasonic and viscoelastic relaxation in pure liquids, mixtures and a polymer solution. The measurements were made over a range of temperatures and pressures in the frequency range 5 to 78 MHz. Density, steady flow viscosity, real part and imaginary part of the shear impedance were measured on five liquids. Four of the liquids obeyed the Time-Temperature Superposition principle and could be described by the Barlow, Erginsav and Lamb (B.E.L) model. The fifth liquid did not obey the Time-Temperature Superposition principle and deviated from the model. This was attributed to the distribution of relaxation times widening as the temperature is decreased. The shear compliance for all five liquids varied linearly with temperature at atmospheric pressure. At higher pressures it was the shear modulus that varied linearly with pressure. Both low and high frequency longitudinal velocities were found to vary linearly with temperature at atmospheric pressure, while at higher pressures only the low frequency velocity varied linearly with pressure, the high frequency velocity results were too scattered. Values of the bulk moduli obtained from the longitudinal moduli and shear moduli were normalised and the variation with reduced I frequency was found to be of the same shape as for the shear moduli but displaced along the reduced frequency axis. Therefore the shear and bulk relaxation properties have a common origin. From this displacement and the values of G(w) and the relaxing part of the bulk modulus K(O) the ratio of volume to shear viscosity was calculated for three liquids at atmospheric pressure and two liquids at higher pressures. The ratio Tlv/T, varied from 2.7 to 4.2 at atmospheric pressure with slightly lower values at higher pressures but the difference was not statistically significant. Measurements of density and velocity were made on solutions of a surfactant at elevated pressures. The density measurements were fitted to the linear secant modulus equation. Investigation of the shear properties of a gel-soap solution showed that it was thixotropic. Finally, a conformational analysis at pressures above atmospheric pressure showed that the relaxation frequency was not significantly changed by increasing the pressure.

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This project concerns quantifying the diffuseness of sound reflections from surfaces by means of a diffusion coefficient. Afthough it is now acknowledged that diffuse reflections are important in determining sound fields within rooms, no standardised diffusion coefficient currently exists. Definition of a universal coefficient would permit comparison of different surfaces and aid the understanding of diffusion. It would also benefit diffuser designers and room acoustic computer modellers. Previously proposed diffusion parameters for room surfaces are investigated and new ones developed. One approach is to parameterise the uniformity of the scattered energy measured as a polar response; a number of such parameters have been previously published. These are appraised using measured and predicted 2D and 3D polar responses for a diverse range of sample surfaces. The situations in which the parameters succeed and fail are discussed and it is demonstrated that none is ideal. A new polar response coefficient, superior to those previously published, is presented. This satisfies many criteria of the ideal diffusion measure and is likely to be standardised by the Audio Engineering Society. It is shown that the application of all polar response diffusion parameters is, however, limited. Two recently proposed alternative approaches to evaluating a scattering coefficient, which involve measuring the invariance of the energy reflected from a surface to its orientation, are discussed. One of these is a free field technique and the other requires reverberant conditions. Practical analysis shows that the reverberation chamber method is superior. It is likely to be standardised by ISO. An empirical reverberation chamber technique is also investigated, as is the possibility of quantifying the diffusion efficacy of surfaces from their effect on sound field diffuseness. Both of these approaches require further research. It is concluded that to provide maAmum benefit, the choice of diffusion coefficient is application dependent.

The normal modes of an enclosed sound field introduce spatial, time and frequency domain artefacts to signals reproduced in such an environment, such that undesirable colouration of these signals may be perceived. Modal density with respect to frequency is minimal at low frequencies, and in small rooms this means that widely spaced discrete modes dominate sound reproduction up to frequencies of the order of one hundred Hertz. The removal of unwanted colouration is not straightforward; conventional passive absorptive treatments offer poor performance at low frequencies, and where their use is attempted costs (in terms of space consumption) may be prohibitive. This Thesis presents a series of investigations into the active control of low frequency acoustic resonance, using both adaptive digital filters in feedforward and feedback configurations, and fixed feedforward controllers. The adaptive filters are based around the active control of acoustic impedance, using hardware available to the project as the product of previous work at Salford University. The application of the technique to the control of modes in a three dimensional environment is however novel. The fixed feedforward controllers use a novel application of an analytical modal decomposition of an enclosed soundfield as the basis for a digital IIR acoustic model. This model is utilised in order to manipulate the locations of z-plane poles and change the behaviour of the sound field. These two techniques are applied to a number of control tasks in one- and threedimensional test environments, using numerical models and practical hardware implementations. The tasks include pressure cancellation, and more usefully the control of frequency domain Q-factor and corresponding modal decay times. It is shown that active impedance methods are superior in the duct; the fixed feedforward controllers suffer from the combined effect of the finite source impedance of practical control loudspeakers with changing and strongly modal radiation loads. In the room, both techniques are shown to be capable of useful reductions in modal Q-factor and decay time. Fixed methods offer control over a defined spatial volume, and adaptive techniques may be further developed by the refinement of the control hardware.

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The two main attributes of seating in auditoria have been investigated. Tle first is random incidence absorption. The second is the low-frequency selective attenuation which seating can impart to sound travelling over it at grazing incidence: the so-called "seat dip" effect. It was found that there was a need for a more accurate laboratory measurement method to predict auditorium seat absorption. The traditional method tended to overpredict the absorption of the exposed front and sides of seating blocks. A new method was studied which involves the use of barriers to obtain realistic measurements of front and side absorption. The new method was validated by comparing measurements of seats made in a reverberation chamber with in-situ absorption data for the same seats, calculated from reverberation time measurements in ten auditoria with and without the seats present. The accuracy of the new method was found to be satisfactory in all cases, although a severe lack of diffusion in two of the halls hindered the validation process. The important physical factors affecting seat dip attenuation were investigated by measurements in a concert hall and on scale model seats. A scheme for reducing the attenuation with resonant absorbers was evaluated, and a simple theoretical model developed. 'Ibe subjective significance of the effect was established with a panel of ten subjects and a fully simulated auditorium sound field. The absolute threshold of perception of the seat dip effect was found to be 7.1 ± 0.6 dB attenuation in the 200 Hz octave band of the early field. It was found that seat dip attenuation might be made less audible in a hall by: (i) supplying early energy along paths remote from the seating, (ii) increasing the vertical angle of incidence of the direct sound and (iii) installing resonant absorbers in the floor between seat rows.

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This thesis focuses on physical measurements of the sound radiated by stringed musical instruments. The radiation efficiency, defined as the ratio of acoustical power output to mechanical power input, was measured to study the acoustical behaviour of the instruments between 80 Hz and 2000 Hz. The research used spherical-harmonic decomposition to determine the power output from monopole and dipole sources. On classical guitars, monopole power produced by the low-frequency resonance triplet provided the greatest contribution to the power output below 300 Hz. At higher frequencies, where the body modes have more complex shapes, dipole sources dominated the total power output. As the dipole contribution to the power output increases, the radiation efficiency of the instrument decreases. The research demonstrated that the resonance frequencies of the body modes of the instruments do not correspond with either a large or small value of radiation efficiency. Instead it is the mode shape that determines the radiation efficiency. Modes with similar-sized anti-nodal areas, of opposite phase, were found to be less efficient than modes which had unequal-sized anti-nodal areas. Measurements of the in-plane velocity of these modes, made with a 3D scanning laser vibrometer, showed that the less-efficient modes had greater values of in-plane velocity. The largest values of radiation efficiency for classical guitars occurred between 200 Hz and 600 Hz. The upper frequency limit of this range was determined by the resonance frequency of a particular mode. This was confirmed by experiments on a purpose-built guitar in which the cross-grain stiffness could be adjusted. Experiments on classical guitars, steel-string guitars and violins produced characteristically different radiation efficiencies.

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