Experimental investigation on the enhancement of plenum window noise reduction using solid scatterers.

The sound transmission across plenum windows installed with rigid non-resonant cylindrical scatterer arrays was investigated in detail using scale-down model measurements carried out inside a fully anechoic chamber. The arrays have manifested to some extent the acoustical behaviors of virtual sonic crystals. The maximum cross section blockage ratio was 0.6. The effects of plenum window gap, array configuration, and scatterer diameter on the sound transmission characteristics were also examined. Results indicate that the window cavity longitudinal modes and the gap modes control the sound transmission characteristics at low frequencies. The upper bound of this frequency range increases with decreasing gap width. Within this frequency range, the scatterers have negligible effect on the sound transmission. At higher frequencies, the array configurations with scatterer(s) attached to the window walls result in stronger sound reduction. There are relatively higher sound transmission loss improvements around the frequencies where a full bandgap is observed. There are wide bandgaps in various lattice directions, and the present results suggest that they play a role in the broadband improvement of sound reduction.

On sound propagation along an infinite rectangular duct-like structure with a finite slot opening and its modelling.

The sound propagation across a sound leaking section along an infinite rectangular duct-like structure near to the lower order duct eigenfrequencies is investigated numerically in the present study. The sound leakage is achieved by finite length rectangular slots located at a corner of the duct-like structure cross section. The finite-element simulations are performed, in the first place, to gain insights into the modal development inside the structure. A semi-analytical model, which considers the wavy air motions along the slots with oblique sound radiation patterns, is developed. An empirical framework is also proposed to estimate the complex longitudinal wavenumber along the slot using the numerical results and dimensional analysis. The performance of the proposed semi-analytical model, together with the complex wavenumber prediction framework, is tested using two duct-like structures with different cross section aspect ratios. The results show that the present proposed approach gives predictions close to the finite-element simulations. The deviations are well within engineering tolerance.

On the length scale and Strouhal numbers for sound transmission across coupled duct cavities at low Mach number.

The sound transmission across two coupled cavities along a rectangular duct in the presence of a low Mach number flow is examined experimentally in the present study. An effort is also made for a deeper understanding of how the flow, excitation sound frequency, and excitation level influence the sound transmission loss. The results confirm that the high sound transmission loss across the cavities is associated with the strong out-of-phase pressure fluctuations within the cavities. The sound transmission loss deteriorates significantly once the flow speed exceeds a threshold value. A different length scale is proposed. This length scale, together with the threshold flow speed and the peak sound transmission loss frequency, gives a Strouhal number, which is basically independent of the cavity offset for a fixed cavity length. The present finding extends the previous effort of the authors, enabling the prediction of the flow speed limit and operating frequency of the coupled cavities for duct silencing at a low Mach number.

Sound transmission across a narrow sidebranch array duct muffler at low Mach number.

The sound transmission loss across a duct muffler in the form of a linear array of 11 narrow sidebranches is examined experimentally in the present study. The introduction of a low Mach number duct flow deteriorates the broadband acoustical performance of the muffler and strong sound transmission loss dips and sound amplifications are observed at high flow speeds. It is found that a stronger acoustic pressure magnitude inside the sidebranches improves the muffler's performance in the presence of the duct flow. A theoretical analysis using a 2-sidebranch array muffler is conducted and the results indicate the possibility of increasing the sound pressures inside the sidebranches by locating the shorter sidebranch upstream of the longer one. The results of further experiments validate the theoretical deduction. Results also confirm that the muffler with sidebranches arranged in the order of decreasing acoustic impedance magnitude has stronger resilience against aerodynamic disturbance and gives better performance when the upstream excitation level and the duct flow speed are fixed.

Insertion loss of asymmetrical balconies on a building façade.

The acoustical insertion loss of asymmetrical balconies on high-rise buildings was studied experimentally using a 1:3 scaled down model in the present study. Four balcony forms featured by the presence of a full-height side-wall were included. A linear loudspeaker array was adopted as the sound source. The effects of source orientation and balcony elevation angle on the insertion loss and its spectral variation were examined. The position of the full-height side-wall relative to the sound source significantly affects the balcony insertion loss. It is observed that the maximum traffic noise amplification and attenuation are both ∼6 dBA. Results also suggest that the balustrade has no effect on the insertion loss spectral variation pattern, though the insertion loss magnitude could be much reduced without it in the presence of a short side-wall. This short side-wall determines the insertion loss spectral variation pattern. Significant sound amplification is found at frequencies of the odd order transverse modes, the longitudinal modes, and their coupled modes regardless of balcony form and elevation angle. It is also found that the major acoustic mode interactions are basically independent of source orientation for balconies without the short side-wall.

Active cancellation of sound generated by finite length coherent line sources using piston-like secondary source arrays.

The active cancellation of the sound generated by finite length coherent line sources is investigated numerically in the present study. A secondary source consists of a central circular core enclosed within an annulus is proposed. It is demonstrated that this source type can produce a much more directional secondary sound field than a single circular piston, even its size is much smaller than the latter if its two parts are vibrating out-of-phase with the right magnitude ratio. This property gives rise to more effective spanwise sound reduction. An example of a secondary source array for broadband noise control is also provided. Though the control performance becomes weaker as frequency increases, the noise reduction within the central region of the receiver plane remains significant in all the cases included in the present study.

The effects of neighborhood views containing multiple environmental features on road traffic noise perception at dwellings.

The importance of non-acoustical factors including the type of visual environment on human noise perception becomes increasingly recognized. In order to reveal the relationships between long-term noise annoyance and different types of neighborhood views, 2033 questionnaire responses were collected for studying the effect of perceptions of different combinations of views of sea, urban river, greenery, and/or noise barrier on the annoyance responses from residents living in high-rise apartments in Hong Kong. The collected responses were employed to formulate a multivariate model to predict the probability of invoking a high annoyance response from residents. Results showed that views of sea, urban river, or greenery could lower the probability, while views of noise barrier could increase the probability. Views of greenery had a stronger noise moderation capability than views of sea or urban river. The presence of an interaction effect between views of water and views of noise barrier exerted a negative influence on the noise annoyance moderation capability. The probability due to exposure to an environment containing views of noise barriers and urban rivers would be even higher than that due to exposure to an environment containing views of noise barriers alone.

Vortex sound radiation in a flow duct with a dipole source and a flexible wall of finite length.

The noise attenuation of fan-ducted noise at low blade-passage frequency remains a challenge. The present study investigates the noise reduction mechanism of a tensioned membrane housing device that directly controls the sound radiation from the doublet which is enclosed in an infinitely long duct with a point vortex. The time dependent sound radiation mechanism and the vibro-acoustics coupling mechanism of the systems are studied by adopting the potential theory and matched asymptotic expansion technique. The silencing performance of such a passive approach depends on the amplitude and phase of the sound field created by the doublet and the acoustic pressure induced by the membrane oscillation in order to achieve sound cancellation. Results show that the response of membrane vibration is strongly associated with the flow field induced by the grazing uniform flow and also the fluid loading generated by the inviscid vortex. The geometrical property of the cavity and the mechanical properties of the flexible membranes play important roles in controlling the performance of the proposed device.

Geometrical parameter combinations that correlate with early interaural cross-correlation coefficients in a performance hall.

The previous binaural data of the authors measured inside two multi-purpose performance halls are re-analyzed using regression in this study. It is done in an attempt to establish a framework that can improve the prediction of early interaural cross-correlation coefficients (IACCs), but with as little measurement effort and parameters as possible. The results show that regression models consist of linear combinations of polynomials of geometrical parameters, when used together with the measurement schemes suggested previously by the authors, are sufficient for predicting the IACCs to within engineering tolerance. The predictions are better than those obtained previously by the neural network approach of the authors. The relative importance of the geometrical parameters in the prediction of IACCs is also investigated.

AAV1/2-mediated BDNF gene therapy in a transgenic rat model of Huntington's disease.

Reduced expression and disrupted corticostriatal transportation of brain-derived neurotrophic factor (BDNF) is proposed to contribute to the selective vulnerability of medium spiny striatal projection neurons (MSNs) in Huntington's disease (HD). We have previously demonstrated that BDNF overexpression in the quinolinic acid lesioned rat striatum attenuates motor impairment and reduces the extent of MSN cell loss. To further investigate the potential therapeutic properties of BDNF for HD, the current study examines the effect of bilateral AAV1/2-mediated BDNF expression in the striatum of a transgenic rat model of HD. Transfer of the BDNF gene to striatal neurons using an AAV1/2 serotype vector enhanced BDNF protein levels in the striatum. Bilateral BDNF expression attenuated the impairment of both motor and cognitive function when compared with AAV1/2-vehicle- or YFP-treated transgenic HD rats. Interestingly, a gender effect was apparent with female transgenic HD rats exhibiting less functional impairment than males. Quantification of NeuN and DARRP32 immunoreactivity and striatal volume revealed limited disease phenotype between wild type and transgenic HD animals. However, AAV1/2-BDNF-treated transgenic HD rats showed evidence of greater striatal volume and increased NeuN+ cell numbers compared with wild-type vehicle- and AAV1/2-vehicle- or YFP-treated transgenic HD rats. We propose BDNF holds considerable therapeutic potential for alleviating behavioral dysfunction and neuronal degeneration in HD, with further work required to examine the role of BDNF-TrkB signaling and the preservation of axonal and synaptic function.

Insertion losses of balconies on a building façade and the underlying wave interactions.

This study used scale model experiments to investigate the insertion losses of balconies on a building façade in the presence of ground reflections. The experiments measured both A-weighted broad- and narrowband insertion loss spectra. The underlying wave interactions/interferences and their couplings with and without reflections from the balcony ceilings were also examined in detail, and these findings were related to the dimensions and elevations of the balconies. The findings indicate that the ground and ceiling reflections and their interferences with the direct sound play very important roles in shaping the frequency characteristics of the insertion losses. Strong sound attenuation can be attained with a carefully designed geometry and acoustical properties of the balcony and the balcony ceiling.

Redirection of doublecortin-positive cell migration by over-expression of the chemokines MCP-1, MIP-1α and GRO-α in the adult rat brain.

Inflammation-induced chemoattraction plays a major role in adult subventricular zone (SVZ)-derived precursor cell migration following neural cell loss, in particular through the release of chemokines by activated microglia and macrophages. We previously demonstrated that monocyte chemotactic protein-1 (MCP-1) (chemokine (c-c motif) ligand (CCL)2), macrophage inflammatory protein-1α (MIP-1α) (CCL3) and growth regulatory protein-α (GRO-α) (chemokine (c-x-c motif) ligand (CXCL)1) are up-regulated following neural cell loss in the adult striatum and act as potent chemoattractants for SVZ-derived precursor cells in vitro. Based on these observations, the current study aimed to examine the individual effect of MCP-1, MIP-1α and GRO-α on the migration of adult SVZ-derived neural precursor cells in vivo. To address this without the confounding effects of injury-induced chemotactic cues, adeno-associated viral (AAV)2-mediated in vivo gene transfer was used to ectopically express either MCP-1, MIP-1α or GRO-α, or the control red fluorescent protein (RFP) in the normal adult rat striatum. The extent of doublecortin (Dcx)-positive cell recruitment from the SVZ into the striatal parenchyma was then determined at 4 and 8weeks following AAV2 injection. Ectopic expression either of MCP-1 or MIP-1α in the normal adult rat brain significantly increased the number of Dcx-positive cells and the extent of their migration into the striatum at both 4 and 8weeks after vector injection but did not promote either precursor cell proliferation or neural differentiation. In contrast, while over-expression of GRO-α 4weeks after vector injection induced a significant increase in Dcx-positive cell migration compared to control, this effect was reduced to control levels by 8weeks post injection. Further, direct comparison between MCP-1, MIP-1α and GRO-α at both 4 and 8weeks post vector injection indicated that GRO-α may have a reduced effect in inducing Dcx-positive cell migration when compared to MCP-1. Combined, these results confirm that over-expression of the chemokines MCP-1, MIP-1α and GRO-α can override cues directing precursor cell migration along the rostral migratory stream (RMS) and provides a mechanism by which neural precursor cell migration can be redirected into a non-neurogenic region. Differences in the migratory effect observed between individual chemokine may be due to ligand-binding affinity and/or receptor expression on SVZ-derived precursor cells.

Neural network predictions of acoustical parameters in multi-purpose performance halls.

A detailed binaural sound measurement was carried out in two multi-purpose performance halls of different seating capacities and designs in Hong Kong in the present study. The effectiveness of using neural network in the predictions of the acoustical properties using a limited number of measurement points was examined. The root-mean-square deviation from measurements, statistical parameter distribution matching, and the results of a t-test for vanishing mean difference between simulations and measurements were adopted as the evaluation criteria for the neural network performance. The audience locations relative to the sound source were used as the inputs to the neural network. Results show that the neural network training scheme using nine uniformly located measurement points in each specific hall area is the best choice regardless of the hall setting and design. It is also found that the neural network prediction of hall spaciousness does not require a large amount of training data, but the accuracy of the reverberance related parameter predictions increases with increasing volume of training data.

Plenum window insertion loss in the presence of a line source--a scale model study.

The acoustical insertion losses of plenum windows installed on a building facade in the presence of a non-parallel line source are studied by using a 1:4 scaled down model in a semi-anechoic chamber in the present investigation. Two types of insertion losses, weighted by the normalized traffic noise spectrum (from the 100 Hz to 5000 Hz one-third octave bands), are defined with different references. The first one is for the case where the orientation of the building facade relative to the line source is fixed. The reference case is the opened window having the same orientation angle as the plenum window. The maximum and minimum insertion losses under this condition across the orientations tested are found to be around 14 dB and 5 dB, respectively. The other is the opposite situation where such orientation is allowed to change because of practical purposes and the reference for this condition is the opened window with its width span parallel to the line source. The corresponding maximum and minimum insertion losses are found to be around 18 dB and 8 dB, respectively. There are evidences showing that the lower order plenum acoustic modes are responsible for the relatively high low frequency insertion loss.

Aeroacoustics of T-junction merging flow.

This paper reports a numerical study of the aeroacoustics of merging flow at T-junction. The primary focus is to elucidate the acoustic generation by the flow unsteadiness. The study is conducted by performing direct aeroacoustic simulation approach, which solves the unsteady compressible Navier-Stokes equations and the perfect gas equation of state simultaneously using the conservation element and solution element method. For practical flows, the Reynolds number based on duct width is usually quite high (>10(5)). In order to properly account for the effects of flow turbulence, a large eddy simulation methodology together with a wall modeling derived from the classical logarithm wall law is adopted. The numerical simulations are performed in two dimensions and the acoustic generation physics at different ratios of side-branch to main duct flow velocities VR (=0.5,0.67,1.0,2.0) are studied. Both the levels of unsteady interactions of merging flow structures and the efficiency of acoustic generation are observed to increase with VR. Based on Curle's analogy, the major acoustic source is found to be the fluctuating wall pressure induced by the flow unsteadiness occurred in the downstream branch. A scaling between the wall fluctuating force and the efficiency of the acoustic generation is also derived.

Narrow sidebranch arrays for low frequency duct noise control.

The present study investigates the sound transmission loss across a section of an infinitely long duct where one or more narrow sidebranch tubes are installed flushed with the duct wall. The finite-element method is used to compute the wave propagation characteristics, and a simplified theoretical analysis is carried out at the same time to explain the wave mechanism at frequencies of high sound reduction. Results show that the high sound transmission loss at a particular frequency is due to the concerted actions of three consecutive sidebranch tubes with the most upstream one in the resonant state. The expansion chamber effect of the setup also plays a role in enhancing sound attenuation at non-resonance frequencies. Broadband performance of the device can be greatly enhanced by appropriate arrangements of tube lengths and/or by coupling arrays on the two sides of the duct.

Full scale model investigation on the acoustical protection of a balcony-like façade device (L).

The acoustical insertion losses produced by a balcony-like structure in front of a window are examined experimentally. The results suggest that the balcony ceiling is the most appropriate location for the installation of artificial sound absorption for the purpose of improving the broadband insertion loss, while the side walls are found to be the second best. Results also indicate that the acoustic modes of the balcony opening and the balcony cavity resonance in a direction normal to the window could have a great impact on the one-third octave band insertion losses. The maximum broadband road traffic noise insertion loss achieved is about 7 dB.

Vortex sound in the presence of a low Mach number flow across a drum-like silencer.

The sound generated by a vortex propagating across a two-dimensional duct section with flexible walls (membranes) in an infinitely long rigid duct conveying a flow is investigated numerically using the matched asymptotic expansion technique and the potential theory. The effects of the initial vortex position, the mechanical properties of the flexible walls, and the mean flow on the sound generation are examined in detail. Results show that the presence of a vortex inside a uniform mean flow can strengthen or attenuate the sound generation, depending on the phase of the membrane vibration when the vortex starts vigorous interaction with the membranes and the strength of the mean flow. The results tend to imply that there is a higher chance of sound amplification when a vortex stream is moving closer to the lighter membrane under a relatively strong mean flow or when the mean flow is weak. The chances of sound amplification or attenuation are equal otherwise.

Can surrounding greenery reduce noise annoyance at home?

Annoyance has been identified as the most important psychological impact arisen from noise. Besides socioeconomic status, residing neighborhood characteristics such as greenery has been shown to be able to reduce noise annoyance. To study the effects of these potential annoyance modifiers, nine hundred and ninety-two responses were collected through face-to-face interviews via questionnaire surveys. Among them, six-hundred and eighty-eight responses were collected together with adequate dwelling information which enabled a more accurate prediction of home noise levels. All these data were analyzed using an ordered logit model. Results indicate that greenery perception exerts considerable influence on noise annoyance rated at home. Wetland parks and garden parks are shown to be able to reduce noise annoyance to a greater degree than grassy hills. And the effects of the perceived amount of greenery on noise annoyance reduction at home differ according to the setting of greenery to which participant perceived from individual home.

On sound transmission loss across a Helmholtz resonator in a low Mach number flow duct.

A simplified physical model, which described the generation of sound by a Helmholtz resonator upon flow excitation at its neck, was developed in the present investigation to study the drop in sound power transmission loss across such a resonator mounted on the wall of a duct conveying a low Mach number mean flow. Experiments were derived to validate the model. Mitigation methods derived according to the model were also tested experimentally. Results showed that the simplified model gave predictions which agreed with experimental observations. The proposed mitigation methods were also proved to be effective for building services application. It was also found that the sound intensity generated by the flow excited resonator scaled with approximately the ninth power of the flow velocity inside the duct.