A residual neural network is proposed to predict the sound absorption of an infinite rigidly-backed porous material from a classical two-microphone measurement above a finite porous sample. The network is trained using the microphones' transfer functions generated by a boundary element model (BEM), with a Delany-Bazley-Miki material model as a boundary condition. The network is validated numerically with BEM simulations and experimentally using two-microphone measurements of a baffled porous absorber of dimensions 60 cm×60 cm and 30 cm×60 cm, subject to various source locations. The results indicate that the network can significantly enhance the predictive capabilities of the classical two-microphone method. The suggested approach shows potential for accurately estimating the sound absorption coefficient of acoustic materials in realistic operational conditions.

The construction industry faces several challenges in adopting sustainable materials for building components. Engineered Wood Products (EWP) are emerging as potential alternatives to traditional materials like hollow clay blocks. This research evaluates an innovative EWP-based façade as a possible replacement for a hollow block wall in terms of acoustic and thermal performance. The study was conducted in a hotel in Guarapuava, Brazil, where acoustical measurements and thermal envelope simulations were performed. The measured Weighted Standardized Façade Level Difference (D2m,nT,w) for the existing hollow block façade was 37 dB, while the simulated data for the proposed wood façade reached 42 dB. Indoor sound insulation between rooms also improved, rising from 46 dB (measured) to 48 dB (simulated) with the EWP façade. From a thermal perspective, the thermal resistance increased from 0.50m2K/W to 1.86 m2K/W, which is more suitable for the Brazilian 1M climate zone where the building is located. This study highlights the potential of using Brazilian pine wood in façade elements.

Acoustic measurements of sources in non-ideal acoustic environments, often the case in industrial product development, issue challenges in source characterization. This study investigates the room-acoustical effects of a bespoke fan test facility on aeroacoustic source characterization via a second-order scheme of spherical harmonics of the half-space. An experimental test of a compact monopole-like sound source reveals the influence of the room's transfer function at low frequencies. Applying the scheme to a benchmark case of a low-pressure axial fan at different loading conditions showcases a satisfactory estimation of sound power and directivity.

The wake topology behind a wall-mounted square cylinder immersed in a turbulent boundary layer is investigated using high-resolution large-eddy simulations (LES). The boundary-layer thickness at the obstacle location is fixed, with a Reynolds number based on cylinder height ℎ and free-stream velocity 𝑢∞ of 10,000 while the aspect ratio (AR), defined as obstacle height divided by its width, ranges from 1 to 4. The mesh resolution is comparable to DNS standards used for similar wall-mounted obstacles, though with relatively lower Reynolds numbers. The effects of AR on wake structures, turbulence production, and transport are analyzed via Reynolds stresses, anisotropy-invariant maps (AIM), and the turbulent kinetic energy (TKE)budget. In particular, the transition from ‘‘dipole’’ to a ‘‘quadrupole’’ wake is extensively examined as AR increases. With increasing AR, the wake shrinks in both the streamwise and spanwise directions, attributed to the occurrence of the base vortices (AR = 3 and 4). This change in the flow structure also affects the size of the positive-production region that extends from the roof and the flank of the obstacle to the wake core. The AIMs confirm three-dimensional wake features, showing TKE redistribution in all directions (Simonsen and Krogstad, 2005). Stronger turbulence production in AR = 3 and 4 cases highlights the role of tip and base vortices behind the cylinder. The overall aim is to refine the dipole-to-quadrupole transition as a function of AR and accounting for the incoming TBL properties. The novelty relies on proposing the momentum-thickness-based Reynolds number Re𝜃 as a discriminant for assessing TBL effects on turbulent wake structures.

The environmental impact of air travel, largely driven by fossil-fuel consumption, remains a critical subject of debate. Addressing this challenge requires immediately adopting sustainable practices to mitigate its environmental footprint. While hydrogen and hybrid-electric propulsion technologies show promise for the future, current efforts focus on Sustainable Aviation Fuels (SAF) as a viable near-term solution to reduce aviation emissions while ensuring compatibility with existing aviation infrastructure. This paper examines the environmental impact of air travel, focusing on the emissions associated with conventional fuel and SAF. Using two methodologies, namely the subsonic fuel flow method (SF2) and an improved version of it, the emissions corrected subsonic fuel flow method (EC-SF2), non-CO2 emissions trends are analyzed along a flight trajectory from Stockholm to Bordeaux. The comparison between the two methods underscores the importance of accurate emission modeling, particularly for SAF correction on emission index. The SF2 method reveals that SAF fuels with higher calorific value than conventional fuel increased total HC and CO emissions while decreasing NOx emissions. Conversely, the EC-SF2 method resulted in a more homogeneous emissions reduction trend. Our proposed methodology, which corrects both fuel flow and emission index based on SAF-specific data, could, therefore, offer a more reliable estimation of emissions behavior for SAF. These findings highlight the sensitivity of emissions modeling on environmental assessment.

In Brazil, normal strength concrete (NSC) and steel bars are commonly used to construct residential buildings with concrete walls. However, recent discussions have highlighted the environmental impact of these traditional methods, particularly in terms of carbon footprint and the use of virgin materials. In response, alternative materials such as lightweight recycled coarse aggregates are promising. While Eco-design often focuses on structural and material aspects, acoustic efficiency is often neglected. This paper focuses on the airborne sound insulation of a single-story house in a Brazilian housing program constructed using a concrete wall system made of NSC. An accurate experimental validation was carried out in this house, recording a measured and simulated Weighted Standardized Level Difference (D_{nT,w}) of 50 dB for a partition wall between two living rooms, outperforming the national standard requirement of 40 dB. With the simulation data validated, an innovative assessment using reliability methods was proposed to analyze the sensitivity of the problem in terms of the main variables, using three alternative wall materials: NSC, Lightweight Concrete (LC) and Recycled Lightweight Concrete (RLC). A sensitivity analysis using Sobol's indices identified wall thickness, density, and Young's modulus as the most critical input parameters to the model. Monte Carlo Simulation (MCS) was used to calculate the failure probability of each assumed thickness, and Reliability-Based Design Optimization (RBDO) was used to determine the optimum thickness to meet the standard requirement. Through RBDO, we determined optimal wall thicknesses for the concrete mixes studied, aiming at a sound reduction index (R_w) of 43 dB, following the Brazilian standard ABNT NBR 15575-4 and ISO 12354-1. The optimal thicknesses were 7.3 cm for NSC, 11.15 cm for LC, and 10.27 cm for RLC. Innovative acoustic-carbon efficiency indices are proposed to evaluate the studied materials: the Carbon-Acoustic Efficiency Index (CAEi) and the Recyclability-Acoustic-Carbon Efficiency Index (RACEi), which are used for a comprehensive assessment of both sustainability and acoustic performance between materials. Using these indices, the study shows that RLC can definitively replace NSC for airborne sound insulation, offering additional environmental benefits such as recyclability.

Axial cooling fans operating downstream of obstructions are often exposed to distorted inlet flow conditions, consequently impacting acoustic performance. Herein the impact of upstream obstructions with simple geometries on the psychoacoustics of an axial fan with rotating ring is investigated. Different loading conditions and inlet shroud lengths are considered, while a reference non-obstructed installation is also recorded. Results showcase significant increase in loudness for obstructed installations at intermediate and free discharge loading conditions irrespective of inlet shroud lengths tested. Sharpness, fluctuation strength and roughness demonstrate no sensitivity to loading conditions for obstructed installations, while tonality levels depend on inlet shroud length.

Low-speed axial fans for engineering cooling applications are often operating in the vicinity of humans. Over the last decades, an effort to mitigate fan noise has been observed. Novel fan designs with substantial noise abatement and little to no expense on aerodynamic performance have been achieved. However, the case of unfavorable installation conditions, namely fans immersed in non-ideal inlet flows, still complicates the optimization of fan designs with regard to acoustic performance. In this study, the acoustic performance alteration of a low-speed axial fan with a rotating ring is documented. Different inlet geometries are tested, while the fan aerodynamic performance is also monitored. The acoustic performance of the fan, including sound power and sound quality metrics, is discussed for three operating points. Furthermore, a parallel fan setup is tested to study the emergence of acoustic interference. Results show marginal gains in aerodynamic performance for elongated inlet geometries. On the contrary, acoustic performance and sound quality are negatively affected, particularly at high loading. Moreover, the inlet configuration with an elongated straight duct demonstrates weak coupling to loading conditions concerning sound power and roughness. An overall consistent scaling of aerodynamic and acoustic performance is observed for the parallel fan system when compared to the single fan case, irrespective of inlet geometry. Sound quality estimates of two incoherent sound sources agree well with measured values of the parallel fan system, apart from fluctuation strength, which is overestimated at stall conditions.

This paper introduces the boostlet transform to analyze and reconstruct spatiotemporal acoustic fields measured in 2D space-time. The transform builds upon the insight that sparse multi-scale representations learned from natural wavefields perform geometric transformations that preserve the dispersion relation. The boostlet transform decomposes a spatiotemporal wavefield using a collection of wavelet-like functions parametrized by dilations, hyperbolic rotations, and translations in space-time. From a physical viewpoint, boostlets encompass global and localized waveforms with variable band-limited frequency and phase-speed content. We show transform applications of wavefront segmentation and sparse reconstruction of room impulse responses. In particular, we find that boostlet decompositions excel at representing localized wavefront phenomena typical of the early part of such room recordings. At the same time, plane waves perform equally as well as or better than boostlets in the late part.

This study investigates an analytical framework to parametrize transitions of acoustic waves in space-time. The work considers a linear microphone array and a point source in a linear, non-dispersive medium between two parallel, rigid wall surfaces. The proposed framework parametrizes spatiotemporal waveforms into frequency/phase-speed representations, leveraging the physical attributes of the boostlet transform. The parametrization suggests that a superposition of plane waves boosted along a hyperbola in the Fourier domain resembles the Green's function solution in this two-wall problem. This hyperbola is centered on the frequency axis, and its apex can be related to the distance between the walls. In particular, we find a resemblance between the Gaussian-parametrized waveforms and the Green's function solution. The motivation for this work originates from deriving a framework that easily parametrizes transitions between free and reverberant spaces, offering versatility in modeling local and global spatiotemporal wave propagation phenomena.