1,721,076 research outputs found
Sound radiation from perforated plates
Full text linkPerforated plates are quite often used as a means of engineering noise control to reduce
the sound radiated by structures. However, there appears to be a lack of representative
models to determine the sound radiation from a perforated plate. The aim of this thesis is
to develop such a model that can be used to give quantitative guidance corresponding to the
design and effectiveness of this noise control measure.
Following an assessment of various models for the radiation efficiency of an unbaffled
plate, Laulagnet’s model is implemented. Results are calculated and compared with those
for baffled plates. From this, simple empirical formulae are developed and give a very good
agreement with the analytical result. Laulagnet’s model is then modified to include the
effect of perforation in terms of a continuously distributed surface impedance to represent
the holes. This produces a model for the sound radiation from a perforated unbaffled plate.
It is found that the radiation efficiency reduces as the perforation ratio increases or as the
hole size reduces. An approximate formula for the effect of perforation is proposed which
shows a good agreement with the analytical calculation up to half the critical frequency. This
could be used for an engineering application to predict the noise reduction due to perforation.
The calculation for guided-guided boundary conditions shows that the radiation efficiency
of an unbaffled plate is not sensitive to the edge conditions. It is also shown that perforation
changes the plate bending stiffness and mass and hence increases the plate vibration.
The situation is also considered in which a perforated unbaffled plate is located close
to a reflecting rigid surface. This is established by modifying the Green’s function in the
perforated unbaffled model to include an imaginary source to represent the reflected sound.
The result shows that the presence of the rigid surface reduces the radiation efficiency at low
frequencies.
The limitation of the assumption of a continuous acoustic impedance is investigated
using a model of discrete sources. The perforated plate is discretised into elementary sources
representing the plate and also the holes. It is found that the uniform surface impedance is
only valid if the hole distance is less than an acoustic wavelength for a vibrating rectangular
piston and less than half an acoustic wavelength for a rectangular plate in bending vibration.
Otherwise, the array of holes is no longer effective to reduce the sound radiation.
Experimental validation is conducted using a reciprocity technique. A good agreement is
achieved between the measured results and the theoretical calculation for both the unbaffled
perforated plate and the perforated plate near a rigid surface
Use of a reciprocity technique to measure the radiation efficiency of a vibrating structure
Full text linkThe reciprocity principle is well-known and has many applications in acoustics and vibro-acoustics. This paper discusses a reciprocity measurement method to determine the radiation efficiency of a vibrating structure. The method comprises two steps: (i) measurements of the acceleration response of the structure induced by a sound field in a reverberation chamber and (ii) measurements of the spatially-averaged squared transfer mobility of the structure. The approach is more flexible than a direct method and has the advantage that no shaker is required to excite the structure in the acoustic measurements. To demonstrate the applicability of this method, experiments were conducted on rectangular flat plates, on two components of a railway track test-rig and on three different built-up structures. For the plates and the railway rig components, comparisons are also made with theoretical models. It is shown that the measured results for each arrangement obtained using this reciprocity method provide good agreement with conventional direct measurements and with theoretical modelling. However, in most of the examples presented, the direct method has been found to be less practical and sometimes even less accurate than the reciprocal one, mostly due to the structure-shaker connection and to the inherent uncertainty of acoustic intensity measurement
Measurement, Modeling, And Optimization Of Sound Absorption Performance Of Kenaf Fibers For Building Applications
No full textThe use of natural fibers in the buildings and construction industries as a sustainable and biodegradable product with the aim of noise pollution control has attracted the attention of many researchers. This work aims to study the acoustic behavior of porous absorbers made of natural Kenaf fibers. To this end, samples of sound absorber were fabricated with thicknesses of 10–40 mm at two different bulk densities of 150 and 200 kg/m3, and their sound absorption coefficient (SAC) was determined by standing wave sound impedance tube at different air gap cavities. A hybrid numerical-mathematical model was also proposed to investigate the acoustic behavior of the samples. To this end, a code was developed to simulate the 3D virtual structure of samples, and flow resistivity was calculated by numerically solving the flow of air in the structures. Tortuosity and two characteristic lengths were obtained using an inverse method programmed in MATLAB®. These parameters were then imported into the Johnson-Champoux-Allard (JCA) model to predict SACs at different frequencies. Afterward, considering the cost and sound absorption average (SAA), samples were optimized using factorial design. Consequently, the acoustic behavior of the optimized acoustic panels was investigated in the reverberation room in terms of reverberation time and random absorption coefficient. Moreover, in order to provide aesthetically and artistically pleasing appearance, the samples were covered with spacer fabrics, and their sound absorption behavior was also studied. The results revealed the promising sound absorption performance of Kenaf fibers. It was found that the SAC at low, mid, and high frequencies increases significantly with increasing the bulk density. The average of SACs for frequencies above 1250 Hz for samples of 40 mm thickness was found to be 0.95, while these values for samples of 30 and 20 mm thickness were respectively 0.85 and 0.7. The introduction of the air gap was found to improve the SAC at low-frequency bands and shift the peak of absorption toward low frequencies. Very good consistency was observed between the predicted and experimental data. The results of the statistical analysis suggested a thickness of 33 mm and a bulk density of 150 kg/m3 for the optimized panels. The results showed that the mean of SAC increased from 0.68 to 0.72 after covering the optimized panels with spacer fabrics
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Radiation efficiency of beam-stiffened plate: experimental setup and preliminary results
No full textBeam stiffening technique has been widely used in engineering application to minimize the vibration of structures, to increase its loading capacity and to prevent the structure from buckling. However, study on the sound radiation from the beam stiffened plate is still lacking. This paper mainly presents the experimental setup to measure the radiation efficiency of three-beam and the five-beam stiffened plates. The treatment for the test plate is discussed. Reciprocity technique was employed to measure the sound power. In general, the measured results for the acoustic power show good agreement with those from the proposed mathematical model. Measured radiation efficiencies overestimate the simulation results of high frequencies due to error in the mobility measurement
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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