EXPERIMENTAL MODAL ANALYSIS OF A PALM TREE LOG UNDER RADIAL VIBRATIONAL EXCITATION

   Trees may be subject to rot-inducing agents that degrade the strength of the material making their trunk, and decrease the quality of their crop. Several techniques, both non-destructive and destructive, are available for assessing the extent of damage caused by rot in a tree trunk. The present work presents the results of a preliminary study conducted on a palm tree trunk for isolating a specific mode from its response to a vibrational excitation, namely the so-called “ovalling” mode. This latter is cross-sectional and in a circular cylinder manifests itself relatively locally, i.e. has little dependence on the lateral extension of the cylinder. An experimental modal analysis is made on a piece of a date palm tree trunk when set into vibration through a radial mechanical excitation, and the response is collected at points along a circumference on the trunk. The value of the resonance frequency of the ovalling mode was found to be somehow variable, probably resulting from some coupling phenomena between various modes of vibration due to the inhomogeneity, anisotropy and fibre-like structure of the trunk wood. As rot usually affects markedly the strength of the trunk wood, the frequency of the ovalling mode, which depends on the strength of the material, can be used for estimating the severity of rot attack in the trunk. A numerical simulation is also made to a cylinder as a simplified representation of a tree trunk

Diffraction by a hard half-plane: Useful approximations to an exact formulation

in this paper, the problem of diffraction of a spherical wave by a hard half-plane is considered. The starting point is the Biot-Tolstoy theory of diffraction of a spherical wave by a fluid wedge with hard boundaries. In this theory, the field at a point in the fluid is composed eventually of a geometrical part: i.e., a direct component, one or two components due to the reflections on the sides of the hard wedge, and a diffracted component due exclusively to the presence of the edge of the wedge. The mathematical expression of this latter component has originally been given in an explicit closed form for the case of a unit momentum wave incidence, but Medwin has further developed its expression for the more useful case of a Dirac delta point excitation. The expression of this form is given in the time domain, but it is quite difficult to find exactly its Fourier transform for studying the frequency behaviour of the diffracted field. It is thus the aim of this paper to present various useful approximations of the exact expression. Among the approximations treated, three are most accurate for engineering purposes, and one of them is proposed. for its simplicity, as appropriate for most occurring practical situations. (C) 2002 Elsevier Science Ltd

Sound scattering by a hard half-plane: Experimental evidence of the edge-diffracted wave

In this short note, some experimental results are presented on the diffraction of a spherical way by a hard half-plane. This study was conducted with the aim to give evidence to the existence of the edge-diffracted wave. The sound source used in this experimental study is a condenser microphone operating in a reverse way. The wave emitted by a sound source propagates in space and hits a thin aluminium sheet with a straight edge, considered as an idealization of the hard half-plane. The resulting impulse response includes among others a wave diffracted by the edge of the half-plane, which is compared to its theoretical prediction. This latter is calculated from the exact Biot and Tolstoy solution to the problem of diffraction of a spherical wave by a hard wedge. Relatively satisfactory agreement is found between theory and experiment. (C) 2002 Published by Elsevier Science Ltd

Study on the relationship between some room acoustical descriptors

The results of a preliminary investigation into the theoretical evaluation and study of the relationship between some room acoustical descriptors used for the subjective assessment of performance halls are considered. The concern is about three parameters, namely, the inter-aural cross-correlation coefficient (IACC), the early lateral energy fraction (ELEF) or a related measure, the spaciousness S, and the initial time-delay gap (ITDG). To this end, the impulse response (JR) for a hard rectangular room with a side balcony on each lateral wall is calculated. This room configuration is considered a coarse approximation of a small performance hall. The theoretical model used for this calculation represents a combination of the image sources method for the wave reflections at the hard surfaces and an exact model accounting for the diffraction of waves at the wedges of the balconies, the latter being even extended to the second order of multiple diffractions. Furthermore, and in view of a more realistic determination of the IACC, the thus obtained impulse response is convolved with the head-related transfer function (HRTF) for both ears as measured using a dummy head (KEMAR) in an anechoic environment. In this respect the directional characteristics of the different components of the impulse response are accounted for in the IACC, but to a lesser degree than in the ELEF, whereas the ITDG is independent of direction. It is found that simple relations may be established between these parameters, which may be useful for room acoustical assessments or for estimating one of these parameters, when inaccessible, by knowing the value of any of the other ones

An acoustical technique for determining the loss factor of solid materials

A technique is presented here for evaluating the loss factor of a solid material element. This technique is commonly used in room acoustics to evaluate the reverberation time (RT) of rooms, and is therefore reviewed in this paper for applications in solid materials. The reverberation time is a quantity that describes the rate of decay of the sound level in the room, and is inversely proportional to the amount of sound absorption in the room. The loss factor is a measure of the proportion of vibrational energy that is dissipated during one cycle of vibration. Hence, interest is focused in this study on the damping characteristics of the material in terms of the loss factor, which is inversely proportional to RT. The reverberation time may be assessed relatively easily and rapidly from the impulse response of the test specimen as measured with the help of a smart technique. The example of a wood beam-like specimen with artificial defects in the form of voids is presented