Traffic noise attenuation solutions for naturally ventilated classrooms in South Africa

Traffic noise transmission through the open windows of naturally-ventilated classrooms can reduce speech intelligibility and can negatively impact academic performance. The findings of a numerical study are presented. Software was used to assess effective noise attenuation solutions for naturally-ventilated classrooms exposed to traffic noise. A typical situation in urban schools in Gauteng, South Africa, is considered in which classrooms are ventilated by large open windows in accordance with national building regulations and norms and standards for school design. The aim of the study was to establish a heuristic framework for early design decisions regarding how far from the road a classroom building should be set, and the effective height and position of a solid noise barrier to ensure a suitable ambient noise level inside a classroom with open windows. Efficacy was measured with reference to an indoor ambient sound level of 40 dBA. The findings show that with the insertion of barriers, the required ambient level was achieved for a classroom at least 68 m from the road, if the barrier is at least 3.5 m high. However, it was found that a significant insertion loss (>6 dB) and an improved signal to noise ratio could be achieved for classrooms as close as 17 m from the road with a barrier of at least 2 m high. Though not broadly generalizable, the findings provide a heuristic guide applicable for designing new schools or selecting attenuation interventions in existing city schools that are similar to those used in the study.

Noise Attenuation of a Duct-resonator System Using Coupled Helmholtz Resonator - Thin Flexible Structures

Several studies have been devoted to increasing the attenuation performance of the Helmholtz resonator (HR). One way is by periodic coupling of HRs in a ducting system. In this study, we propose a different approach, where a membrane (or a thin flexible structure in general) is added to the air cavity of a periodic HR array in order to further enhance the attenuation by utilizing the resonance effect of the membrane. It is expected that three attenuation mechanisms will exist in the system that can enhance the overall attenuation, i.e. the resonance mechanism of the HR, the Bragg reflection of the periodic system, and the resonance mechanism of the membrane or thin flexible structure. This study found that the proposed system yields two adjacent attenuation peaks, related to the HR and the membrane respectively. Moreover, extension of the attenuation bandwidth was also observed as a result of the periodic arrangement of HRs. With the same HR parameters, the peak attenuation by the membrane is tunable by changing its material properties. However, such a system does not always produce a wider attenuation bandwidth; the resonance bandwidths of both mechanisms must overlap.

The Role of Legacy Seismic in Enhancing the Image in the Current Seismic Data – Abu Dhabi: Case Study

Acquiring surface seismic data can be challenging in areas of intense human activities, due to presence of infrastructures (roads, houses, rigs), often leaving large gaps in the fold of coverage that can span over several kilometers. Modern interpolation algorithms can interpolate up to a certain extent, but quality of reconstructed seismic data diminishes as the acquisition gap increases. This is where vintage seismic acquisition can aid processing and imaging, especially if previous acquisition did not face the same surface obstacles. In this paper we will present how the legacy seismic survey has helped to fill in the data gaps of the new acquisition and produced improved seismic image. The new acquisition survey is part of the Mega 3D onshore effort undertaken by ADNOC, characterized by dense shot and receiver spacing with focus on full azimuth and broadband. Due to surface infrastructures, data could not be completely acquired leaving sizable gap in the target area. However, a legacy seismic acquisition undertaken in 2014 had access to such gap zones, as infrastructures were not present at the time. Legacy seismic data has been previously processed and imaged, however simple post-imaging merge would not be adequate as two datasets were processed using different workflows and imaging was done using different velocity models. In order to synchronize the two datasets, we have processed them in parallel. Data matching and merging were done before regularization. It has been regularized to radial geometry using 5D Matching Pursuit with Fourier Interpolation (MPFI). This has provided 12 well sampled azimuth sectors that went through surface consistent processing, multiple attenuation, and residual noise attenuation. Near surface model was built using data-driven image-based static (DIBS) while reflection tomography was used to build the anisotropic velocity model. Imaging was done using Pre-Stack Kirchhoff Depth Migration. Processing legacy survey from the beginning has helped to improve signal to noise ratio which assisted with data merging to not degrade the quality of the end image. Building one near surface model allowed both datasets to match well in time domain. Bringing datasets to the same level was an important condition before matching and merging. Amplitude and phase analysis have shown that both surveys are aligned quite well with minimal difference. Only the portion of the legacy survey that covers the gap was used in the regularization, allowing MPFI to reconstruct missing data. Regularized data went through surface multiple attenuation and further noise attenuation as preconditioning for migration. Final image that is created using both datasets has allowed target to be imaged better.

Lessons Learnt Reprocessing a Noisy Onshore 3D Seismic Survey

An 1,100 km2 400-fold seismic survey was acquired over some of the largest sand dunes in UAE in 2007. Large sand dunes generate many challenges for seismic processing due to the irregular data acquisition, large statics caused by the significant difference between the sand and sabkha velocities, and a massive amount of reverberation noise that hides the signal in the data. Occidental and ADNOC Sour Gas reprocessed this survey from January 2019 to August 2020 to overcome the challenges of the strong sand dune noise. For the first time, it was processed through prestack depth migration (PSDM). The primary objectives of the reprocessing project were to get an accurate PSDM volume that tied all of the available well control data; and to derive as accurate seismic amplitudes as possible over the target reservoir interval from near to far offsets to enable elastic inversion for reservoir porosity and net-thickness prediction. Whilst the reprocessing project achieved the project objectives and generated good subsurface images, it did not run as smoothly as hoped, despite being processed by one of the premier multinational processing companies. The extremely large sand dunes, which are present across most of the survey area, created major imaging problems. Key technical lessons learnt during reprocessing included: (1) CRS errors occurred sporadically during acquisition, requiring correction; (2) the sand curve (Liner, 2008) worked well for sand dune static corrections for this data set; (3) near surface statics changed whilst the survey was acquired by up to 6 ms - each shot station needed to be corrected for these statics changes because the shot stations were acquired twice with a symmetric split recording spread; and (4) the contractor's standard post-migration processing sequence (gather flattening, radon, noise attenuation, stack) did not work well for this very noisy data set. Next time we work with similar data and require a high quality result, we know to double the estimated project timeline as every step in the processing sequence takes much longer than expected when the signal-to-noise ratio of the data is very low. The novelty of this work was that we obtained large improvements in the seismic stack by applying offline gather conditioning before calculating trim statics to optimally flatten the very noisy migrated offset vector tile (OVT) gathers, prior to running the final noise attenuation and stacking workflows. Without this offline gather conditioning, the trim statics workflow mostly aligned the noise and damaged the stack.

Recent Developments of Noise Attenuation Using Acoustic Barriers for a Specific Edge Geometry

The aim of this research is to provide a better prediction for noise attenuation using thin rigid barriers. In particular, the paper presents an analysis on four methods of computing the noise attenuation using acoustic barriers: Maekawa-Tatge formulation, Kurze and Anderson algorithm, Menounou formulation, and the general prediction method (GPM-ISO 9613). Accordingly, to improve the GPM, the prediction computation of noise attenuation was optimized for an acoustic barrier by considering new effects, such as attenuation due to geometrical divergence, ground absorption-reflections, and atmospheric absorption. The new method, modified GPM (MGPM), was tested for the optimization of an y-shape edge geometry of the noise barrier and a closed agreement with the experimental data was found in the published literature. The specific y-shape edge geometry of the noise barrier contributes to the attenuation due to the diffraction phenomena. This aspect is based on the Kirchhoff diffraction theory that contains the Huygens-Fresnel theory, which is applied to a semi-infinite acoustic barrier. The new method MGPM of predicting the noise attenuation using acoustic barriers takes into consideration the next phenomena: The effect of the relative position of the receiver, the effect of the proximity of the source or receiver to the midplane of the barrier, the effect of the proximity of the receiver to the shadow boundary, the effect of ground absorption-reflections, the effect of atmospheric absorption, and the meteorological effect due to downwind. The conclusion of the paper reveals the optimization of the method for computing the noise attenuation using acoustic barriers, including the necessary corrections for ISO-9613 and the Sound PLAN software, as well as the optimization on a case study of a specific geometry of the edge barrier.

Further assessment of a time domain impedance boundary condition for the numerical simulation of noise-absorbing materials

In regard to the mitigation of environmental noise across major industry sectors, the present study focuses on the numerical prediction of passive noise reduction devices. Here, it is further explored how the noise attenuation induced by locally reacting noise absorbing materials (also called acoustic liners) can be simulated using a time domain highly accurate Computational AeroAcoustics (CAA) method. To this end, it is assessed how a classical Time Domain Impedance Boundary Condition (TDIBC) can effectively model acoustic liners of practical interest, including when the latter are exposed to realistic conditions (grazing flow and noise excitation). The investigation consists in numerically reproducing two experimental campaigns initially performed at NASA Langley Research Center. Two different materials are considered (honeycomb superimposed with perforate or wiremesh resistive face-sheet), each being characterized by a specific noise attenuation behaviour ( e.g. dependency on the flow conditions and/or noise excitation). Each material is tested under various flow conditions ( e.g. grazing flow of Mach up to 0.5) and/or noise source excitation ( e.g. multiple tones of level up to 140 dB each). The results demonstrate the ability of the underlying CAA/TDIBC approach to simulate realistic acoustic liners in non-trivial configurations, with enough physical accuracy ( e.g. correct capture of the noise attenuation characteristics) and numerical robustness ( e.g. absence of instabilities). The study also reveals that, independent from the CAA/TDIBC approach itself, some specific pre-processing tasks (e.g. impedance eduction and subsequent TDIBC calibration) may play a bigger role than expected, in practice.

Noise Attenuation Effects on Speech Recognition of Cochlear Implant Users Inside Helicopters

BACKGROUND: The speech recognition levels of cochlear implant (CI) users are still incompatible with ICAO hearing requirements for civil aviation pilots testing in the noisy background condition of the helicopter cockpit. In this study, we evaluated noise attenuation effects on speech recognition in the same background condition.METHODS: The study involved the evaluation of 12 Portuguese-speaking CI users with post-lingual deafness and with a pure tone average up to 35 dB HL between 500 and 2000 Hz and up to 50 dB at 3000 Hz on at least one of the ears, and of three normal hearing pilots (controls). We performed speech recognition tests using sentences, numbers, and disyllables for all participants through the VHF radio. The assessment took place inside a helicopter with engine on, using three setups: 1) with headset without the active noise cancellation; 2) activating the noise cancellation system of the headset itself; and 3) connecting the speech processor directly to the helicopter radio system.RESULTS: The headset active noise-cancellation improved only the recognition of sentences. The direct connection system compared to the headset without anti-noise attenuation significantly improved all the recognition tests. The median for numbers was 90%, but the best score for disyllables recognition was 56%.DISCUSSION: The noise attenuation resources proposed in this study improved the CI users speech recognition when exposed to the noisy helicopter cockpit. However, speech recognition of CI users still did not meet the standards of ICAO, which requires at least 80% for understanding disyllables in the speech in noise test.Caldeira JMA, Goffi-Gomez MVS, Imamura R, Bento RF. Noise attenuation effects on speech recognition of cochlear implant users inside helicopters. Aerosp Med Hum Perform. 2021; 92(11):880-885.