Browsing by Subject "Experimental studies"

Now showing 1 - 8 of 8

Open Access
Blue organic light-emitting diodes based on pyrazoline phenyl derivative
(Elsevier, 2012-01-21) Stakhira, P.; Khomyak, S.; Cherpak, V.; Volyniuk, D.; Simokaitiene, J.; Tomkeviciene, A.; Kukhta, N.A.; Grazulevicius, J.V.; Kukhta, A.V.; Sun, X.W.; Demir, Hilmi Volkan; Hotra, Z.; Voznyak, L.
The results of an experimental study of the electroluminescent device made of ITO/CuI/2,6-di-tert.-butyl-4-(2,5-diphenyl-3,4-dihydro-2H-pyrazol-3-yl)- phenol (HPhP)/3,6-Di(9-carbazolyl)-9-(2-ethylhexyl) carbazole (TCz1)/Ca:Al with efficacy up to 10.63 cd/A are presented. HPhP provides blue emission with a peak wavelength at 445 nm. The layer of TCz1 acts as an electron-transporting layer. In the framework of density functional theory (DFT) approach the geometry configuration and energy levels of HPhP are found being in a good agreement with spectral and cyclic voltammogram data.
Open Access
A comparison study on sound defects of Bilkent Amphitheatre in terms of intelligibility of the speech
(2007) Yilmazer, S.
The 4000 seated multi-purpose hall Bilkent Amphitheater is a semi-closed amphitheater with a large volume of 90,000 m3.At the beginning, the amphitheater was designed only for open-air performances. Afterwards it was decided to be roofed with a tensile membrane in order to protect the spectators against atmospheric effects. However, the covered amphitheater was turned into a very resounding and a problematic place with a reverberation time even more than 6 s. when the hall is unoccupied. After that time, some acoustical interventions were implemented, but these have not been sufficient in decreasing the number of problematic areas of echoes and dead spots. In 2000, Akukon Oy Consulting Engineers, Finland, carried out the real-size measurements when the hall was unoccupied, in order to find out the values of different acoustical parameters. In 2004, the computer simulation of the hall for the unoccupied condition was made [2]. In order to test speech intelligibility in the hall, besides the computer simulation and real size measurements, in 2006, articulation testing (AI) was made by using normal hearing listeners in the hall. In this paper, comparison between the measurements and the experimental studies are presented. Results confirm that excessive values of reverberation time cause too much reverberant and liveliness throughout the hall, besides the lack of warmth subjectively. Clarity is undermined by excessive reverberate and the sound becomes blurred. Sound energy are not satisfactory in reflecting the sound sufficiently through the hall and the direct sound becomes insufficient at some locations as back and mid sides.
Open Access
Experimental study of subwavelength focusing by lefthanded metamaterials with a negative refractive index
(2007) Özbay, Ekmel; Aydin, K.
Two dimensional (2D) left-handed metamaterials (LHM) exhibiting negative refraction and subwavelength imaging are studied experimentally. A left-handed transmission band is observed at the frequencies where both dielectric permittivity and magnetic permeability are simultaneously negative. 2D LHM structure is verified to have a negative refractive index by measuring the refraction through a prism-shaped sample. Subwavelength focusing is observed by using a flat LHM superlens.
Open Access
An experimental study on speech interference in public leisure spaces
(Institute of Noise Control Engineering, 2010) Yilmazer, Semiha; Gezginer, Pelin Meriç
Physical comfort requirements of users in the non-acoustical spaces, leisure space, and usability of the space in long-term period should be taken into consideration in terms of sustainability of the space. The aim of this study is to investigate the speech interference of users' in an openplaned public leisure space. Bilkent Rollhouse was chosen as a leisure space because of its variance of activities in an open-plan like bowling, dart, billiards, play station and dining area. In this respect, a questionnaire was prepared to check the perception of sound of different user types in the area. The questionnaires were applied at the most crowded day of the week that is Friday and in between 18:00 to 21:00. The results of the survey were evaluated statistically. In addition to this, the equivalent continuous noise level, Leq, of the leisure space were taken in parallel to the questionnaire application to the users. Measuring of speech intelligibility via articulation index (AI) subjectively was the second phase of this study. Via application of the word list to the normal hearing users, the AI contours were prepared. Finally, objective and subjective measurements were analyzed in terms of speech interference during leisure activity in the space. As a leisure space, the relation between noise annoyance ratings and Leq did not show any significant difference in dart, billiards and play stations activities. However, in bowling activity and dining area, it was found that there is a significant relation between noise annoyance and Leq. This study reveals that users submit ambient noise of the space during leisure activity, they get in a common behaviour psychologically, they adjust their vocal noise that they create and finally these issues lead controlling of ambient noise by them. It means that users give up loudly speaking, they accept repeating or silence, focus on meal and they do not like getting in uncomfortable situation since their main activity is dining. For further study, the number of the samples in not only different leisure spaces, but also measurement points and sampling times should be increased to make clear hypothesis given above, since the study reveals completely different behaviour that is introduced in literature like cocktail party effect phenomenon.
Open Access
A local search heuristic with self-tuning parameter for permutation flow-shop scheduling problem
(IEEE, 2009) Dengiz, B.; Alabaş-Uslu, Ç.; Sabuncuoğlu, İhsan
In this paper, a new local search metaheuristic is proposed for the permutation flow-shop scheduling problem. In general, metaheuristics are widely used to solve this problem due to its NP-completeness. Although these heuristics are quite effective to solve the problem, they suffer from the need to optimize parameters. The proposed heuristic, named STLS, has a single self-tuning parameter which is calculated and updated dynamically based on both the response surface information of the problem field and the performance measure of the method throughout the search process. Especially, application simplicity of the algorithm is attractive for the users. Results of the experimental study show that STLS generates high quality solutions and outperforms the basic tabu search, simulated annealing, and record-to-record travel algorithms which are well-known local search based metaheuristics.
Open Access
Mel-cepstral methods for image feature extraction
(IEEE, 2010) Çakır, Serdar; Çetin, A. Enis
A feature extraction method based on two-dimensional (2D) mel-cepstrum is introduced. The concept of one-dimensional (1D) mel-cepstrum which is widely used in speech recognition is extended to 2D in this article. Feature matrices resulting from the 2D mel-cepstrum, Fourier LDA, 2D PCA and original image matrices are converted to feature vectors and individually applied to a Support Vector Machine (SVM) classification engine for comparison. The AR face database, ORL database, Yale database and FRGC version 2 database are used in experimental studies, which indicate that recognition rates obtained by the 2D mel-cepstrum method is superior to the recognition rates obtained using Fourier LDA, 2D PCA and ordinary image matrix based face recognition. This indicates that 2D mel-cepstral analysis can be used in image feature extraction problems. © 2010 IEEE.
Open Access
Observation of biexcitons in the presence of trions generated via sequential absorption of multiple photons in colloidal quantum dot solids
(IEEE, 2012) Cihan, Ahmet Fatih; Hernandez-Martinez Pedro L.; Kelestemur, Yusuf; Demir, Hilmi Volkan
Multi exciton generation (MEG) and multi exciton recombination (MER) in semiconductor quantum dots (QDs) have recently attracted significant scientific interest as a possible means to improve device efficiencies [1-5]. Convenient bandgap tunability, easy colloidal synthesis, and solution-based processability of these QDs make them further attractive for such device applications using MEG and MER. For example, recent theoretical and experimental studies have shown that MEG enables >100% peak external quantum efficiency where the generated multi excitons (MEs) are collected in a simple QD solar cell structure [1]. Furthermore, MEG has also been shown in QD photodetectors exhibiting substantially increased photocurrent levels [2]. Another promising application for MEs is the use of QDs as an alternative gain medium based on MER for lasers. Although MEG is very promising and supported with quite persuasive reports, there are still some debatable issues that need to be clarified. One of the issues that have generated great debates in the field has been the confusion of MER with the recombination of trions, which takes place in photocharged QDs. To utilize MEG and MER in practical devices such as QD solar cells and QD lasing devices, these phenomena need to be well understood. Here, we showed distinct spectrally-resolved temporal behavior of biexciton (BX), single exciton (X) and trion radiative recombinations in near unity quantum yield (QY) quasi-type II CdSe/CdS core/shell nanocrystal QDs. Upon sequential absorption of multiple photons, the extraction of Xs, BXs, and trions were achieved using time correlated single photon counting (TCSPC) measurements performed on low concentration thin film samples of these QDs at different emission wavelengths. The QDs were embedded in PMMA medium to obtain homogeneous samples and avoid Förster-type nonradiative energy transfer (NRET) between them. Here to extract Xs, BXs, and trions, we devised a new analysis approach for the time decays of the QDs that allowed us to attribute the physical events to their corresponding time decay terms, which were further verified with their excitation intensity dependencies [6]. © 2012 IEEE.
Open Access
Two-dimensional Mellin and mel-cepstrum for image feature extraction
(Springer, Dordrecht, 2010) Çakır, Serdar; Çetin, A. Enis
An image feature extraction method based on two-dimensional (2D)Mellin cepstrum is introduced. The concept of one-dimensional (1D) melcepstrum which is widely used in speech recognition is extended to two-dimensions both using the ordinary 2D Fourier Transform and the Mellin transform in this article. The resultant feature matrices are applied to two different classifiers (Common Matrix Approach and Support Vector Machine) to test the performance of the melcepstrum and Mellincepstrum based features. Experimental studies indicate that recognition rates obtained by the 2D melcepstrum based method are superior to the recognition rates obtained using 2D PCA and ordinary image matrix based face recognition in both classifiers. © 2011 Springer Science+Business Media B.V.