When a material is placed under stress, small changes within the specimen release ultrasonic energy in the form of stress waves. The change may, for example, be a dislocation movement or the advancement of a crack tip. These ultrasonic pulses are termed Acoustic Emission and may be detected at the material surface by ultrasonic transducers. The detected pulse shape is related to the generating source, to the material geometry through which the pulse propagates and to the response of the ultrasonic transducer used to detect the waves. Work has been carried out to measure both the effect of wave propagation and to calibrate the response of ultrasonic transducers. Three types of ultrasonic wave may exist in a material with a non-zero shear modulus; these are longitudinal waves, shear waves and surface or Rayleigh waves. In a large number of specimen geometries, the surface wave has the largest amplitude. The response of a transducer to this wave is therefore very important. Most transducers respond to the out of plane motion of a material surface carrying ultrasonic waves. Therefore, to successfully calibrate a transducer, some absolute measurement of the out of plane motion due to surface waves must be made. An interferometer has been designed and constructed for this purpose. The calibration of ultrasonic transducers has enabled some development work to be carried oLt on high-fidelity piezoelectric transducers and on piezomagnetic transducers. It is not always possible to measure an ultrasonic pulse directly with a calibrated interferometric detector and therefore to enable a wider range of propagation problems to be investigated, various methods of ultrasonic pulse generation have been studied. These artificial sources of acoustic emission have included brittle fracture, laser impact and stimulation by piezoelectric transducers. This work has enabled theoretical calculations on pulse propagation to be verified.

The application of an active control force on a thin-walled acoustic boundary can modify the motional dynamics, and so influence the impedance presented to incident waves. This impedance determines transmission of acoustic energy, reflection of acoustic waves from the boundary and absorption of incident energy. This thesis studies control systems that generate control forces for the active control of surface acoustic impedance. The proposed systems rely on measurement of the acoustic pressure and surface velocity of the boundary. The systems can use adaptive digital signal processing, which offers significant advantages over non-adaptive techniques. The active control of the specific acoustic impedance of a loudspeaker that terminates a waveguide for axially propagating plane waves provides a motivating problem. Theoretical analysis establishes the control of specific acoustic impedance of a simple compliantly-suspended piston by a control force. Operational constraints of a physical piston define theoretical operating limits for controlled specific acoustic impedances. The control systems use either feedback or feed-forward techniques for which theoretical treatment reveals restrictions on the range of controlled specific acoustic impedance. A novel result is that conventional implementations of the control systems can be unstable for certain desired impedances unless feedback cancellation is used. Digital feedback techniques are less effective for broader frequency bandwidth where feed-forward techniques may work. Theoretical analysis produces solutions that confirm the feasibility of these control techniques for the active control of specific acoustic impedance. Potential errors in the implementation of the systems have predictable effects on the controlled specific acoustic impedance. Experimental results support the theoretical work presented in this thesis, demonstrating active control of specific acoustic impedance for normally incident acoustic plane waves. An adaptive digital feed-forward control system creates desired specific acoustic impedances for band-limited noise and transient signals.

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.

No description available.

No description available.

A detailed theoretical analysis is presented for calculating the surface acoustic wave (SAW) reflection coefficient of thin metallic layers. Based on this analysis, directions of propagation are classified as symmetric or asymmetric. An augmented scalar transmission line circuit model which contains a new lumped network element that accounts for asymmetry is introduced to describe SAW reflection and transmission through a strip. The resulting network model is used to analyze grating and transducer structures. Computed results based on this new network model are in excellent agreement with measured data, not only on devices oriented along symmetric directions, but also on devices which exhibit directivity due to asymmetric orientations. A simple procedure, based on physical arguments, is outlined for the identification of high directivity orientations. An algebraic construction is given which demonstrates that the coupling-of-modes (COM) modelling of gratings and transducers is derivable from the new network model. Approximate explicit analytical expressions, in terms of the network model, are given for the COM model parameters. The properties of pseudo-surface-waves are re-examined and a new high-velocity pseudo-surface acoustic wave (HVPSAW) is described. It is shown that this mode, not referenced in the SAW device literature, has a low attenuation along certain directions, and is thus very attractive for high-frequency low-loss SAW devices.

No description available.

Computational Fluid Dynamics (CFD) are used to investigate two speech phenomena. The first phenomenon is the English bilabial plosive /pa/. Simulations are compared with microphone recordings and high speed video recordings to study the penetration rate and strength of the jet associated with the plosive /pa/. It is found that the dynamics in the first 10ms of the plosive are critical to penetration rate, and the static simulation was not able to capture this effect. However, the simulation is able to replicate the penetration rate after the initial 10ms. The second speech phenomenon is the English fricative /sh/. Here, the goal is to simulate the sound created during /sh/ to understand the flow mechanisms involved with the creation of this sound and to investigate the simulation design required to predict the sound adequately. A variety of simulation methods are tested, and the results are compared with previously published experimental results. It is found that all Reynolds-Averaged Navier-Stokes (RANS) simulations give bad results, and 2D Large Eddy Simulations (LES) also have poor results. The 3D LES simulations show the most promise, but still do not produce a closely matching spectra. It is found that the acoustic analogy matches the direct measurements fairly well in 3D simulations. The studies of /pa/ and /sh/ are compared and contrasted with each other. From the findings of the studies, and using theoretical considerations, arguments are made concerning which CFD methods are appropriate for speech research. The two studies are also considered for their direct applications to the field and future research directions which might be followed.

A Baroque violin was initially made. It was then incrementally converted to a Romantic (modern) setup by replacing the short neck with a longer, more slender neck and adding a longer ebony fingerboard, a heavier bassbar and soundpost. This increased the total mass from 386 to 440 g. Several different Baroque and modern configurations, with baroque and modern style bows, were used for acoustical measurements and playing tests with professional violinists. Chladni patterns were similar in both versions and also when the bridge was placed below the soundholes. The Baroque version gave higher body mode frequencies than the Romantic. Placing the bridge below the soundholes lowered the frequency of the 800 Hz resonance to 600 Hz. Saunders Loudness Tests showed a response that varied strongly over the body resonances. For the transition from Baroque to Romantic setup, hand bowing showed an increase of 1 dB and machine bowing about 5 dB. The compliance of the body added to the air lowered the main air resonance by 5 Hz, equivalent to adding about 130 cc to the 2000 cc air volume. The top plate stiffness measured at the bridge feet was about 10 kN/m higher at the treble foot than at the bass foot, for all locations of the soundpost outside the treble foot. The stiffness at the bass foot remained constant. This was reversed when the soundpost was placed between the two feet: the stiffness at the treble foot was then lower than at the bass foot. The rocking and bounce frequencies of the bridge were lowered from 3000 and 6000 Hz respectively to about 2.5 and 3 kHz when fitted to the violin. Thinning the bridge waist lowered the rocking frequency. Recordings of performances on the violin were made for many combinations of physical state (baroque or romantic), type of string and bow, position of bridge, and others. Long-term average spectra for these recordings are compared here, and an online appendix includes these recordings in a way that allows them to be readily compared: www.phys.ul1sw.edu.au/music/people/mclennanappendix.html

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2006. / Source: Dissertation Abstracts International, Volume: 68-02, Section: B, page: 1018. Adviser: Richard L. Weaver. Includes bibliographical references (leaves ) Available on microfilm from Pro Quest Information and Learning.