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Analiza teoretyczna zjawiska koincydencji i częstości krytycznych paneli akustycznych

Majkut L.

Autobusy : technika, eksploatacja, systemy transportowe

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2018

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R. 19, nr 12

549--552

PL

W pracy opisano zjawisko koincydencji oraz podano uproszczone wzory teoretyczne pozwalające na wyznaczenie częstości krytycznej, nierozerwalnie związanej ze zjawiskiem koincydencji. Zrozumienie zjawiska koincydencji oraz znajomość częstotliwości krytycznej są niezbędne przy analizie dynamiki każdego panelu, który jest lub może być pobudzany falą akustyczną. W pracy wyprowadzono zależności pozwalające na wyznaczenie częstości krytycznej w przypadkach gdy panel modelowany jest jako płyta cienka (model Kirchhoffa-Love’a) i płyta gruba (model Mindlina–Reissnera). Zaprezentowano przebiegi zmian wartości częstotliwości krytycznej w funkcji grubości panelu dla różnych materiałów często wykorzystywanych na konstrukcje obudów izolacyjnych maszyn (izolacja od hałasu maszyny).

EN

Critical and coincidence frequencies of panels are important in studying their behaviour under acoustic excitation Expressions for critical and coincidence frequencies of thin and thick homogeous isotropic panels are derived. Characteristics of critical frequency values in functon of panel thickness for five different construction materials are shown.

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Automatic scaling of critical frequency foF2 from ionograms recorded at São José dos Campos, Brazil: a comparison between Autoscala and UDIDA tools

Pezzopane M., Pillat V. G., Fagundes P. G.

Acta Geophysica

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2017

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Vol. 65, no. 1

173--187

EN

This paper considers a dataset of ionograms recorded by the CADI ionosonde installed at São José dos Campos (SJC; 23.2°S, 45.9°W, magnetic latitude 14.1°S), Brazil, to compare two autoscaling systems: Autoscala, developed by the Istituto Nazionale di Geofisica e Vulcanologia, and the UDIDA-scaling, developed by the Universidade do Vale do Paraíba. The analysis, focused on the critical frequency of the F2 layer, foF2, shows that the two systems work differently. The UDIDA-scaling gives always a value of foF2 as output, regardless of the presence of the ionogram trace and its definition, while Autoscala tends to reject ionograms for which the digital information is considered insufficient. As a consequence, the UDIDA-scaling can autoscale more foF2 values than Autoscala, but Autoscala can discard a larger number of ionograms for which the trace is unidentifiable. Discussions are made on the accuracy of the foF2 values given as output, as well as on the main shortcomings characterizing the two systems.

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Statistical Analysis and Modeling of the Local Ionospheric Critical Frequency: A Mid-latitude Single-Station Model for Use in Forecasting

Sapundjiev D., Stankov M. S.

Acta Geophysica

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2016

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Vol. 64, no. 3

810--824

EN

The hourly values of the F-layer critical frequency from the ionospheric sounder in Dourbes (50.1°N, 4.6°E) during the time interval from 1957 to 2010, comprising five solar cycles, were analyzed for the effects of the solar activity. The hourly time series were reduced to hourly monthly medians which in turn were used for fitting a single station foF2 monthly median model. Two functional approaches have been investigated: a statistical approach and a spectral approach. The solar flux F10.7 is used to model the dependence of foF2 on the solar activity and is incorporated into both models by a polynomial expression. The statistical model employs polynomial functions to fit the F-layer critical frequency while the spectral model is based on spectral decomposition of the measured data and offers a better physical interpretation of the fitting parameters. The daytime and nighttime foF2 values calculated by both approaches are compared during high and low solar activity. In general, the statistical model has a slightly lower uncertainty at the expense of the larger number of fitting parameters. However, the spectral approach is superior for modeling the periodic effects and performs better when comparing the results for high and low solar activity. Comparison with the International Reference Ionosphere (IRI 2012) shows that both local models are better at describing the local values of the F-layer critical frequency.

4

Reflection of longitudinal displacement plane waves from the flat boundary of a solid elastic half-space containing a distribution of void pores

Acharya D. P., Mondal A.

International Journal of Applied Mechanics and Engineering

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2008

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Vol. 13, no 3

609-625

EN

This paper investigates the problem of reflection of longitudinal displacement waves in a medium containing a distribution of void pores. It is observed that three types of waves, traveling at three distinct speeds, may exist in an elastic medium with void pores. The critical frequency has been detected. Amplitude ratios have been presented. It is found that the presence of void pores modulates the wave velocities and amplitude ratios to a considerable extent. Numerical calculations have been made and graphs are drawn to exhibit the role of void pores in the problem under consideration.