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Published Articles

The Volume 6, No 2, June 2001

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Force Measurement in Vibration Testing - Applications to Modal Identification

Ulrich Fuellekrug and Michael Sinapius


This paper first concentrates on the fundamentals of the vibration behaviour of a structure under base excitation. The equations for the structural accelerations and the dynamic forces at the interface are presented and discussed. The principles of modal identification for structures under base excitation including the determination of the effective and modal masses are explained. Then the practical measurement of the interface forces, consisting of the resultant six-degree-of-freedom components, is described and the utilised force measurement devices are presented. The application of the modal identification method is demonstrated on a laboratory test structure as well as on a more complex structure. Examples for typical measurement data and the extracted modal parameters are shown.

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The Improvement of the Dynamic Behaviour of a Spindle Motor for a CD-ROM Drive

Jhy-Horng Wang and Wei-Chih Huang


The demand for CD-ROM drives with higher rotation speeds has increased in recent years. As a result, it is becoming more and more difficult to design a spindle motor to perform within a certain vibration level limits. In this work, the effect of different design variables on the forced vibration of a spindle motor was investigated in order to design an optimal new spindle. The target of the optimisation was to design a new spindle motor with a minimum axial vibration level at the periphery of the turntable or disk. The theoretical results show that the axial vibration of the proposed new spindle can be significantly reduced. The theoretical result was verified by experiments.

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Experimental Study of Upstream Fan Broadband Noise Radiated by a Turbofan Model

Serge Lewy


Broadband fan noise has become the major component of the sound radiated by high bypass ratio turbofans because of the past progress in the reduction of the tones. New analysis is presented of the tests performed in the framework of the European project FANPAC to obtain more information on the generation and radiation of broadband noise. It is explained first how to isolate this component from the tones in the far-field acoustic spectra measured upstream of the fan. Next, the overall sound pressure directivities and sound power levels are reduced for the basic configuration. The sound power increases as the blade tip velocity to the fifth power, and by 1.6 dB per degree increase in blade angle of attack. Other configurations are then considered. It is shown that flow distortions, either due to the removal of the turbulence control screen or to a droop inlet, only have a minor effect on broadband noise, increasing its level by about 1 dB. Finally, an acoustical lining on the duct wall is used to effectively absorb the multiple pure tones at supersonic tip speeds. It can be inferred that the mechanisms of broadband noise generation are similar at both subsonic and supersonic flows.

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Modification of the Diffracted Sound Field by Some Noise Barrier Edge Designs

Jorge P. Arenas, Ana M. Monsalve


The use of noise control by barriers has become a common measure for environmental protection. An efficient sound barrier must shield the receiver from the predominant portion of the sound energy radiated from the source which is directed toward the receiver. The acoustic field in the shadow region of a barrier, when transmission through the barrier is negligible is due to diffraction at the edge alone. The aim of this paper is to present some results of acoustical scale modelling experiments covering some noise barrier edge designs. A summary of the details of the theory of diffraction by a single edge is presented as well. The top edges were modified by means of the addition of a horizontal cap in order to compare the results of the diffracted sound field with a non-modified equivalent barrier. Conventional barriers used in traffic-noise control can be improved in some cases, producing a significant increase in insertion loss.

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Near-field Acoustical Holography without the Errors and Limitations Caused by the Use of Spatial DFT

Rolf Steiner, Jorgen Hald


The present paper describes a Statistically Optimal Near-field Acoustical Holography (SONAH) method, which avoids the errors caused by the use of spatial DFT/FFT in conventional planar Near-field Acoustical Holography (NAH). The SONAH method performs a statistically optimal sound field reconstruction, the plane-to-plane calculations being performed in the spatial domain instead of the spatial frequency domain. Furthermore the method allows an arbitrary positioning of the measurement points in the hologram plane. This allows for optimisation of the microphone positions, for example the use of a sparse measurement grid. For a small 6 x 6 array, it is shown by means of simulations that the new algorithm performs better than the DFT-based NAH method.

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Perspective of the Sound Quality Approach Applied to Noise Control in Earth Moving Machines

Giovanni Brambilla, Eleonora Carletti and Francesca Pedrielli


The paper describes two experiments aimed at evaluating the feasibility and potential of the ?sound quality? approach in improving the design of noise reduction treatments for Earth-Moving Machines (EMMs). In both experiments, binaural noise recordings taken in EMMs operating at stationary idle conditions have been used and subsequent laboratory listening tests have been carried out. In the first experiment, the recordings taken in three EMMs at different engine speeds were considered and presented by headphones to 35 naive subjects using the paired comparison method. In the second experiment, one of the previous recordings has been modified to get six sound stimuli, all having equal overall unweighted Leq levels but with different spectral shapes. The results confirm that subjective preference for a sound stimulus is influenced both by the overall sound energy and by its distribution in the frequency domain. Thus, sound energy-oriented parameters turn out to be not sufficient to characterise the auditory perception in EMMs. To investigate this aspect deeply, a principal component analysis has been performed on the sound stimuli. The results show that the most important factors which explain the subjective preference include the 400-5000 Hz frequency range and low engine order harmonics. In addition, loudness and PSIL appear to be promising for assistance with choosing suitable noise treatments more related to auditory perception.

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Index Method of Assessment of the Acoustical Climate in Industrial Rooms

Zbigniew Engel and Janusz Piechowicz


Noise is one of the main problems inside industrial rooms. Therefore minimising the noise levels is essential for the creation of the proper acoustical climate for working conditions. The problem of the assessment of the acoustical conditions in industrial rooms has considerable practical significance. One solution of the problem is suitable analysis of the model of the acoustical field inside industrial rooms. Such a model needs to take into account room shape, engineering processes, work place locations and layout and the interrelation of various noise sources. Index method of the assessment of the acoustics of industrial rooms is connected with the modernisation of the ways of the identification and estimation of occupational risks caused by excessive noise. It is the activity directed towards the protection of work environment and the possibility of improvement of work conditions.

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An Experimental Investigation of the Vibration Characteristics of a Piping System

E.M.A. Rabeih


In this paper, an experimental investigation of the vibration characteristics of a piping system is presented. A PVC simple plane water-fully-filled piping system has been considered. Preliminary experiments have been carried out to obtain the dynamic modulus of elasticity of the pipe material using vibration measurements. The main experimental work has been carried out to obtain the resonance frequencies of the piping system and corresponding modal damping ratios from a set of frequency response functions (FRF) of the piping structure and fluid pressure measurements. The experimental results for the resonance frequencies have been compared with the theoretical values that were obtained using a finite element technique. The experimental results have been shown to be in good agreement with the theoretical values. The effect of the fluid on the modal damping ratios has been investigated by comparing the modal damping ratios of the empty piping system with the fully-filled one.

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