Sound propagation in and low frequency noise absorption by helium-filled porous material

Low-frequency noise is difficult to deal with by traditional porous material due to its inherent high acoustic impedance. This study seeks to extend the effective range of sound absorption to lower frequencies by filling a low density gas, such as helium, in the porous material. Compared with conventional air-filled absorption material, the helium-filled porous material has a much reduced characteristic impedance; hence, a good impedance matching with pure air becomes more feasible at low frequencies. The acoustic properties of a series of helium-filled porous materials are investigated with a specially designed test rig. The characteristic of the sound propagation in a helium-filled porous material is established and validated experimentally. Based on the measured acoustic properties, the sound absorption performance of a helium-filled absorber (HA) of finite thickness is studied numerically as well as experimentally. For a random incidence field, the HA is found to perform much better than the air-filled absorber at low frequencies. The main advantage of HA lies in the middle range of oblique incidence angles where wave refraction in the absorber enhances sound absorption. The advantage of HA as duct lining is demonstrated both numerically and experimentally. © 2009 Acoustical Society of America.published_or_final_versio

Optical properties of a novel yellow fluorescent dopant for use in organic LEDs

In this paper, the optical properties of a novel organic, 2,8-di(t-butyl)-5,11-di[4(t-butyl) phenyl]-6,12-diphenylnaphthacene (tetra(t-butyl)rubrene) have been investigated. Our results show that there are two peaks in the photoluminescence (PL) spectra of tetra(t-butyl)rubrene (TBRb) which are also confirmed in the electroluminescence (EL) spectra. Photo-quenching of the PL intensity is observed when the irradiation time increases. It is shown that oxidation is the dominant reason for photo-quenching. The absolute refractive index and absorption coefficient have also been determined and the results correlate well with the PL results. The results show that TBRb can be a good dopant to achieve the Förster energy transfer and to assist light emission. The optical properties of TBRb are similar to those of rubrene; however, the PL of TBRb is much stronger than that of rubrene. Finally, although crystalline organics have been commonly reported by heating the sample, we report crystallization of TBRb at low temperature <230 K when the TBRb film is in an amorphous form before cooling. © Springer-Verlag 2004.postprin

Reactive control of subsonic axial fan noise in a duct

published_or_final_versio

Optical Properties of a Novel Dye in Yellow Florescent Organic LEDs

In this paper, time-resolved PL, lifetime and crystallization of a novel organic, 2,8-di(t-butyl)-5,11-di[4(t-butyl)phenyl]-6,12-diphenylnaphthacene, which is considered to be potential candidate to substitute the conventionally used 5,6,11,12-tetraphenylnaphthacene for a higher efficient yellow-color organic LED, were investigated. The PL intensity will reduce when the irradiation time increases because of photo-quenching. It is shown that oxidation is the dominant reason for photo-quenching. In addition, TBRb has two lifetimes. The short one is /spl sim/5 ns and the long one is /spl sim/31 ns. They are considerably contributed by the short PL peak and long PL peak respectively. Furthermore, we first report, to the best knowledge, crystallization of TBRb at low temperature of 11 K and with irradiation.published_or_final_versio

Sound propagation in and low frequency noise absorption by helium-filled porous material

Author name used in this publication: Y. S. Choy2009-2010 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

Hybrid noise control in a duct using a light micro-perforated plate

Author name used in this publication: X. N. WangAuthor name used in this publication: Y. S. Choy2012-2013 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

Noise suppression of a dipole source by tensioned membrane with side-branch cavities

Author name used in this publication: Y. S. Choy2012-2013 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

Perception of urban park soundscape

2011-2012 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

A Note on Encodings of Phylogenetic Networks of Bounded Level

Driven by the need for better models that allow one to shed light into the question how life's diversity has evolved, phylogenetic networks have now joined phylogenetic trees in the center of phylogenetics research. Like phylogenetic trees, such networks canonically induce collections of phylogenetic trees, clusters, and triplets, respectively. Thus it is not surprising that many network approaches aim to reconstruct a phylogenetic network from such collections. Related to the well-studied perfect phylogeny problem, the following question is of fundamental importance in this context: When does one of the above collections encode (i.e. uniquely describe) the network that induces it? In this note, we present a complete answer to this question for the special case of a level-1 (phylogenetic) network by characterizing those level-1 networks for which an encoding in terms of one (or equivalently all) of the above collections exists. Given that this type of network forms the first layer of the rich hierarchy of level-k networks, k a non-negative integer, it is natural to wonder whether our arguments could be extended to members of that hierarchy for higher values for k. By giving examples, we show that this is not the case