1.1. Definition (a) "Infrasound is a mechanical disturbance, propagated on an elastic medium, of frequency too low to be capable of exciting the sensation of hearing". (British Standard 661) (b) In this work the upper limit of the infrasonic region will generally be taken as 20Hz. Propagation is in air unless otherwise stated. 1.2. The literature on the effects of infrasound on both man and animals is reviewed in Chapter 2. In Chapter 3 the general requirements and possible sources of infrasound are discussed, the final apparatus is described together with its calibration and performance. Chapters 4 and 5 deal with the methods used to detect both subjective and objective effects evoked by the stimulus and the preliminary experiments performed to test the feasibility of the project. The experiments performed with the system in examining the physiological effects of monaural, binaural and whole body stimulation are discussed in detail in Chapters 6 and 7. The physiological background to the experiments is discussed in Chapter 8. Finally in chapters 9 and 10 the results are discussed and possible mechanisms for the production of the effects postulated.

Room modes are a recognised problem in small critical listening rooms and are known to cause colouration of sound reproduced within them. Investigations on the causes and solutions for this problem have been carried out for some time. Interest in the topic has extended to loudspeaker manufacturers who have mainly concentrated in developing methods for controlling the loudspeaker-room interaction in order to ameliorate low frequency reproduction. Compared to objective work on passive and active control methods, the study of the subjective perception of room resonances has been somewhat neglected. Available publications mostly concern the effects of single resonances, which are perhaps not fully representative of conditions as experienced in real rooms. A study into the subjective perception of room modes is presented. The experimental methodology employs psychoacoustic techniques to study the perception of factors such as modal distribution, and effects of resonances on single tones. Results show that the subjective perception of room modes is strongly affected by temporal issues, and that changes exerted merely on magnitude frequency response are detectable but not likely to remove the effects of resonances for all listeners. Furthermore, it is shown that a reduction of the modal Q-factor, associated with a reduction of decay rates, has a significant effect in decreasing the detection of resonances. Q-factor difference limen were evaluated for three reference decay characteristics corresponding to reference Q-factors of 30, 10 and 1. The limen were 6±2.8, 10±4.1 and 16±5.4 respectively, meaning that detection of changes to modal decay decreases with decreasing decay time. These results may be used to define more perceptually relevant design guidelines for critical listening environments, and indicate target criteria for control techniques used in room correction. The outcomes of this investigation will have repercussions on the design of better rooms for critical listening.

The violin is a highly complex vibrating system, which quite without the aid of science, evolved to a high level of sophistication. Wood, which varies considerably from one sample to another, requires individual attention to be fashioned into the plates of a good violin. It is not therefore surprising that mass-produced instruments are of very poor quality. It is the improvement of these instruments which is the objective of this thesis. After identifying those features of the response upon which the violin's quality most, depend a model is developed and used to answer several questions about violin design. As it is extremely difficult to include the sound post in the model it is suggested that an additional structural element, designed to match the back's impedance, is used to support the post. The amount of. acoustic radiation from the back is shown to be small so that this change does not greatly affect the output level. Finally it is shown that, using such a construction, the low frequency response of the violin may be predicted before assembly. A description of an automated production process in which the violin plates are cut and tested by micro-processor controlled machinery concludes the work.

In theory it should be possible to characterise a vibration source's active and passive properties in an independent way which allows for structure borne sound to be predicted for a source in different installations. When put into practice however, the independent source characterisation approach often results in sur- prisingly poor predictions of source behaviour for the installed condition. The exact cause of the error is currently unknown but is often attributed to the practi- cal difficulties encountered when measuring source properties and hence numerical instabilities resulting from poor quality or unrepresentative data. Here we ad- dress the problem of obtaining independent descriptions of a source's active and passive properties using in-situ measurement approaches. In-situ measurements may be advantageous because the hypothetical quantities required for indepen- dent source characterisation are on the whole unmeasurable and hence elaborate measurements are often required to obtain an approximate source description. It will be shown that the independent blocked force, describing the activity of a vibration source under a blocked condition, can be measured in-situ and that the in-situ blocked force can be used to predict source behaviour in different environ- ments including a free condition. It will also be shown that the in-situ blocked force approach may allow for a transfer path analysis to be performed without dismantling the source-receiver assembly and thus allowing for a significant time saving. To address the characterisation of passive source properties two methods for the in-situ measurement of mobilities are described and investigated. Overall it is shown that active, and possibly passive, properties of vibration sources can be independently characterised whilst a source is installed and that there may be significant benefits in doing so. For example measurements may be faster or easier and the data obtained may be more representative.

Uncertainty in structural dynamics is of growing concern to numerous industries. Significant attention has previously been devoted to the study of frequency response functions, however the uncertainty associated with excitation of structures by structureborne sound sources has received little attention. In this work, the uncertainty in the structure borne sound power transmitted from a vibration source into a receiving structure is considered. A method is presented whereby the uncertainties in the active and dynamic properties of a structure borne sound source and its receiver structure are propagated through to the injected power. Consideration is given to the case where the data describing the source and receiver is incomplete and is therefore termed ‘granular’. An approach for the estimation of the mean and uncertainty of granular variables is developed and it is shown that by estimating the mean and uncertainty of the missing elements the uncertainty propagation approach can be used for a ‘granular’ case. This approach is illustrated using an example in which the free velocity phase data is assumed to be unavailable. Idealised structure borne sound sources are created analytically in order to examine the validity of the presented methods. Good correlation is observed between the estimated uncertainties in the transmitted power and the uncertainties obtained through a Monte Carlo analysis. Insight into the frequency regions where large uncertainties can be expected in the transmitted structure borne sound power is obtained. It is argued that by providing estimates for the uncertainty of a prediction of the transmitted power, an insight into the reliability of the estimate is achieved, allowing engineering decisions to be made with greater confidence.

Spatial impression in both concert halls and reproduced sound has been identified as an important attribute of the listening experience. In this study, the synthesis and objective measurement of spatial impression in reproduced sound is examined. A novel, multichannel spatializing technique for musical synthesis has been developed that entailed the separation of the individual harmonics of a musical note that were spatially distributed over multichannel surround systems. Subjective testing of the techniques revealed that the perceived degree of spatial impression significantly increased as the angular spread of harmonics increased, however, extending the spatial spread beyond 90° did not significantly increase the perception of spatial impression. The concert hall measure of spatial impression, the interaural cross correlation coefficient (IACC) was used to objectively measure the effects of the spatializing techniques. The IACC measurements displayed a strong correlation to the subjective results. Further examination of the IACC measurement indicated the possibility of it’s adaptation to multichannel surround sound in general. A method of adapting IACC to reproduced sound was further developed that involved comparing IACC measurements taken in a concert hall to IACC measurements taken in reproduced versions of the same concert hall. The method was first conducted as a simulation using basic auralisation techniques. Real concert hall measurements and reproduction systems were then employed. Results showed that the method was able to discriminate between the spatial capabilities of a number of different surround sound systems and rank them in a predictable order. The results were further validated by means of a subjective test. In an attempt to sensitise the IACC measurement, the frequency dependency of IACC was investigated by means of a subjective test. The results indicated that a perceptually more accurate indication of spatial impression may be gained by applying a frequency-dependent weighting to IACC measurements. This may be useful in the spatial measurement of both reproduced sound and concert halls.